use std::{
    collections::{BTreeMap, HashMap, HashSet, VecDeque},
    io::{Read, Write},
    path::{Component, Path, PathBuf},
    process::{Child, ChildStderr, ChildStdin, ChildStdout, Command, Stdio},
    sync::{
        Arc,
        atomic::{AtomicBool, Ordering},
        mpsc::{self, Receiver, SyncSender, TryRecvError, TrySendError},
    },
    thread,
    time::{Duration, Instant},
};

use serde_json::{Value, json};
use unicode_segmentation::UnicodeSegmentation;

use crate::{
    editor::Context,
    plugin::{
        CompletionEdit, CompletionItem, Diagnostic, DiagnosticSeverity, DocumentEvent,
        DocumentEventKind, DocumentSnapshot, PluginUpdate, SemanticToken, SemanticTokenKind,
        SemanticTokenModifier, TextPosition, UpdateEffect,
    },
    snippet,
};

const MAX_BODY: usize = 4 * 1024 * 1024;
const MAX_HEADER: usize = 16 * 1024;
const QUEUE_SIZE: usize = 128;
const MAX_SERVERS: usize = 16;
const MAX_VERSION: u64 = i32::MAX as u64;
const TICK: Duration = Duration::from_millis(5);

#[derive(Clone, Debug)]
pub struct ServerConfig {
    pub language_id: String,
    pub command: Vec<String>,
    pub extensions: Vec<String>,
    pub initialization_options: Value,
    pub settings: Value,
}

#[derive(Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
struct ServerKey {
    config: usize,
    root: PathBuf,
}

struct CachedDocument {
    snapshot: Arc<DocumentSnapshot>,
    servers: Vec<ServerKey>,
    diagnostics: HashMap<usize, Contribution>,
    semantic_tokens: Option<Arc<[SemanticToken]>>,
}

struct Contribution {
    items: Vec<Diagnostic>,
    bytes: usize,
}

enum Output {
    Update(PluginUpdate),
    Diagnostics {
        snapshot: Arc<DocumentSnapshot>,
        items: Vec<Diagnostic>,
    },
    SemanticTokens {
        snapshot: Arc<DocumentSnapshot>,
        tokens: Option<Arc<[SemanticToken]>>,
    },
}

struct Worker {
    input: SyncSender<Input>,
    output: Receiver<Output>,
    failure: Receiver<String>,
    cancelled: Arc<AtomicBool>,
    retired: bool,
    thread: Option<thread::JoinHandle<()>>,
}

enum Input {
    Document {
        snapshot: Arc<DocumentSnapshot>,
        event: DocumentEventKind,
        served: bool,
        primary: bool,
    },
    Position(PositionKind, Source),
    Diagnostics(usize),
}

#[derive(Clone, Copy)]
enum PositionKind {
    Definition,
    Hover,
    Completion,
}

impl PositionKind {
    fn name(self) -> &'static str {
        match self {
            Self::Definition => "definition",
            Self::Hover => "hover",
            Self::Completion => "completion",
        }
    }
}

struct Source {
    buffer_id: usize,
    revision: u64,
    row: usize,
    column: usize,
    request_id: u64,
}

/// Callbacks only enqueue snapshots. Each lazy server owns its process and I/O on
/// one worker; neither spawning, disk reads, pipe I/O nor reaping runs in poll.
pub struct Lsp {
    configs: Vec<Arc<ServerConfig>>,
    cwd: PathBuf,
    documents: HashMap<usize, CachedDocument>,
    servers: BTreeMap<ServerKey, Worker>,
    updates: Vec<PluginUpdate>,
    changed_diagnostics: HashSet<usize>,
    changed_semantics: HashSet<usize>,
}

impl Lsp {
    pub fn new(configs: Vec<ServerConfig>) -> Self {
        Self {
            configs: configs.into_iter().map(Arc::new).collect(),
            cwd: std::env::current_dir().unwrap_or_else(|_| PathBuf::from("/")),
            documents: HashMap::new(),
            servers: BTreeMap::new(),
            updates: Vec::new(),
            changed_diagnostics: HashSet::new(),
            changed_semantics: HashSet::new(),
        }
    }

    pub fn is_enabled(&self) -> bool {
        !self.configs.is_empty()
    }

    pub fn notify_document(&mut self, event: &DocumentEvent) -> Result<(), String> {
        if !self.is_enabled() {
            return Ok(());
        }
        let snapshot = self
            .documents
            .get(&event.document.id)
            .filter(|cached| {
                matches!(event.event, DocumentEventKind::Close)
                    || (cached.snapshot.revision == event.document.revision
                        && cached.snapshot.path == event.document.path
                        && cached.snapshot.root == event.document.root)
            })
            .map(|cached| Arc::clone(&cached.snapshot))
            .unwrap_or_else(|| Arc::new(event.document.clone()));
        let mut startup_error = None;
        if matches!(event.event, DocumentEventKind::Close) {
            self.documents.remove(&snapshot.id);
            self.changed_diagnostics.remove(&snapshot.id);
            self.changed_semantics.remove(&snapshot.id);
        } else if self.documents.get(&snapshot.id).is_none_or(|cached| {
            cached.snapshot.path != snapshot.path || cached.snapshot.root != snapshot.root
        }) {
            let keys = snapshot
                .path
                .as_ref()
                .and_then(|path| {
                    let extension = Path::new(path).extension()?.to_str()?;
                    let matches = |config: &ServerConfig| {
                        config
                            .extensions
                            .iter()
                            .any(|candidate| candidate == extension)
                    };
                    let language = &self
                        .configs
                        .iter()
                        .find(|config| matches(config))?
                        .language_id;
                    let root = absolute(Path::new(&snapshot.root), &self.cwd);
                    Some(
                        self.configs
                            .iter()
                            .enumerate()
                            .filter(|(_, config)| {
                                config.language_id == *language && matches(config)
                            })
                            .map(|(config, _)| ServerKey {
                                config,
                                root: root.clone(),
                            })
                            .collect::<Vec<_>>(),
                    )
                })
                .unwrap_or_default();
            self.documents.insert(
                snapshot.id,
                CachedDocument {
                    snapshot: Arc::clone(&snapshot),
                    servers: keys,
                    diagnostics: HashMap::new(),
                    semantic_tokens: None,
                },
            );
            self.changed_diagnostics.insert(snapshot.id);
            self.changed_semantics.insert(snapshot.id);
            for index in 0..self.documents[&snapshot.id].servers.len() {
                let key = self.documents[&snapshot.id].servers[index].clone();
                if let Err(error) = self.start(&key) {
                    startup_error.get_or_insert(error);
                }
            }
        } else {
            let cached = self.documents.get_mut(&snapshot.id).unwrap();
            if !Arc::ptr_eq(&cached.snapshot, &snapshot) {
                cached.snapshot = Arc::clone(&snapshot);
                cached.diagnostics.clear();
                cached.semantic_tokens = None;
                self.changed_semantics.insert(snapshot.id);
                self.changed_diagnostics.insert(snapshot.id);
            }
        }
        let mut failures = Vec::new();
        for (server_key, worker) in &self.servers {
            if worker.retired {
                continue;
            }
            if worker
                .input
                .try_send(Input::Document {
                    snapshot: Arc::clone(&snapshot),
                    event: event.event,
                    served: self
                        .documents
                        .get(&snapshot.id)
                        .is_some_and(|doc| doc.servers.contains(server_key)),
                    primary: self
                        .documents
                        .get(&snapshot.id)
                        .is_some_and(|doc| doc.servers.first() == Some(server_key)),
                })
                .is_err()
            {
                failures.push(server_key.clone());
            }
        }
        for key in failures {
            self.retire(
                &key,
                "LSP server stopped: document/command queue is full or unavailable".into(),
            );
        }
        startup_error.map_or(Ok(()), Err)
    }

    pub fn definition(&mut self, context: &Context, request_id: u64) -> Result<(), String> {
        self.position(context, request_id, PositionKind::Definition)
    }

    pub fn hover(&mut self, context: &Context, request_id: u64) -> Result<(), String> {
        self.position(context, request_id, PositionKind::Hover)
    }

    pub fn completion(&mut self, context: &Context, request_id: u64) -> Result<(), String> {
        self.position(context, request_id, PositionKind::Completion)
    }

    fn position(
        &mut self,
        context: &Context,
        request_id: u64,
        kind: PositionKind,
    ) -> Result<(), String> {
        let source = Source {
            buffer_id: context.buffer_id,
            revision: context.revision,
            row: context
                .row
                .checked_sub(1)
                .ok_or_else(|| format!("LSP {} row is invalid", kind.name()))?,
            column: context
                .column
                .checked_sub(1)
                .ok_or_else(|| format!("LSP {} column is invalid", kind.name()))?,
            request_id,
        };
        self.command(
            context.buffer_id,
            Input::Position(kind, source),
            Some(request_id),
        )
    }

    pub fn diagnostics(&mut self, buffer_id: usize) -> Result<(), String> {
        self.command(buffer_id, Input::Diagnostics(buffer_id), None)
    }

    pub fn poll(&mut self) -> Vec<PluginUpdate> {
        let failures: Vec<_> = self
            .servers
            .iter()
            .filter_map(|(key, worker)| {
                if worker.retired {
                    return None;
                }
                match worker.failure.try_recv() {
                    Ok(reason) => Some((key.clone(), reason)),
                    Err(TryRecvError::Disconnected) => Some((
                        key.clone(),
                        "LSP server stopped: worker exited unexpectedly".into(),
                    )),
                    Err(TryRecvError::Empty) => None,
                }
            })
            .collect();
        for (key, reason) in failures {
            self.retire(&key, reason);
        }
        // Ordered storage keeps attribution deterministic without per-poll sorting.
        for (key, worker) in &mut self.servers {
            if worker.retired {
                continue;
            }
            let first_update = self.updates.len();
            for _ in 0..QUEUE_SIZE {
                let Ok(output) = worker.output.try_recv() else {
                    break;
                };
                match output {
                    Output::Update(update) => self.updates.push(update),
                    Output::Diagnostics { snapshot, items } => {
                        let Some(doc) = self.documents.get_mut(&snapshot.id).filter(|doc| {
                            doc.servers.contains(key) && Arc::ptr_eq(&doc.snapshot, &snapshot)
                        }) else {
                            continue;
                        };
                        let bytes = items.iter().map(|item| item.message.len()).sum::<usize>();
                        let siblings = doc
                            .diagnostics
                            .iter()
                            .filter(|(config, _)| **config != key.config)
                            .map(|(_, contribution)| contribution.bytes)
                            .sum::<usize>();
                        if bytes > MAX_BODY.saturating_sub(siblings) {
                            self.updates.truncate(first_update);
                            Self::retire_worker(
                                key,
                                worker,
                                &mut self.documents,
                                &mut self.changed_diagnostics,
                                &mut self.changed_semantics,
                                &mut self.updates,
                                "LSP server stopped: combined diagnostic messages exceed 4 MiB"
                                    .into(),
                            );
                            break;
                        }
                        if items.is_empty() {
                            doc.diagnostics.remove(&key.config);
                        } else {
                            doc.diagnostics
                                .insert(key.config, Contribution { items, bytes });
                        }
                        self.changed_diagnostics.insert(snapshot.id);
                    }
                    Output::SemanticTokens { snapshot, tokens } => {
                        let Some(doc) = self.documents.get_mut(&snapshot.id).filter(|doc| {
                            doc.servers.first() == Some(key)
                                && Arc::ptr_eq(&doc.snapshot, &snapshot)
                        }) else {
                            continue;
                        };
                        doc.semantic_tokens = tokens;
                        self.changed_semantics.insert(snapshot.id);
                    }
                }
            }
        }
        let mut effects: Vec<_> = self
            .changed_diagnostics
            .drain()
            .filter_map(|id| {
                let doc = self.documents.get(&id)?;
                let count = doc
                    .diagnostics
                    .values()
                    .map(|value| value.items.len())
                    .sum();
                let mut items = Vec::with_capacity(count);
                for key in &doc.servers {
                    if let Some(contribution) = doc.diagnostics.get(&key.config) {
                        items.extend(contribution.items.iter().cloned());
                    }
                }
                Some(UpdateEffect::Diagnostics {
                    buffer_id: id,
                    revision: doc.snapshot.revision,
                    items,
                })
            })
            .collect();
        effects.extend(self.changed_semantics.drain().filter_map(|id| {
            let doc = self.documents.get(&id)?;
            Some(UpdateEffect::SemanticTokens {
                buffer_id: id,
                revision: doc.snapshot.revision,
                tokens: doc.semantic_tokens.clone(),
            })
        }));
        if !effects.is_empty() {
            self.updates.push(PluginUpdate {
                request_id: None,
                effects,
            });
        }
        std::mem::take(&mut self.updates)
    }

    fn start(&mut self, key: &ServerKey) -> Result<(), String> {
        if self.servers.contains_key(key) {
            return Ok(());
        }
        if self.servers.len() >= MAX_SERVERS {
            return Err("LSP server/project limit of 16 reached".into());
        }
        let (input, receiver) = mpsc::sync_channel(QUEUE_SIZE);
        let (sender, output) = mpsc::sync_channel(QUEUE_SIZE);
        let (failed, failure) = mpsc::sync_channel(1);
        let cancelled = Arc::new(AtomicBool::new(false));
        let stop = Arc::clone(&cancelled);
        let config = Arc::clone(&self.configs[key.config]);
        let root = key.root.clone();
        let initial: Vec<_> = self
            .documents
            .values()
            .map(|doc| {
                (
                    Arc::clone(&doc.snapshot),
                    doc.servers.contains(key),
                    doc.servers.first() == Some(key),
                )
            })
            .collect();
        let started = thread::Builder::new()
            .name(format!("lsp-{}", config.language_id))
            .spawn(move || {
                let mut runtime =
                    match Runtime::new(Arc::clone(&config), root, receiver, sender, stop) {
                        Ok(runtime) => runtime,
                        Err(reason) => {
                            let _ = failed.try_send(format!(
                                "LSP: {} server failed to start: {}",
                                config.language_id,
                                clean(&reason)
                            ));
                            return;
                        }
                    };
                let result = initial
                    .into_iter()
                    .try_for_each(|(snapshot, served, primary)| {
                        runtime.document(snapshot, DocumentEventKind::Open, served, primary)
                    })
                    .and_then(|()| runtime.run());
                if let Err(reason) = result {
                    let _ = failed.try_send(format!("LSP server stopped: {}", clean(&reason)));
                }
            });
        let (thread, startup_error) = match started {
            Ok(thread) => (Some(thread), None),
            Err(error) => (None, Some(error)),
        };
        self.servers.insert(
            key.clone(),
            Worker {
                input,
                output,
                failure,
                cancelled,
                retired: false,
                thread,
            },
        );
        if let Some(error) = startup_error {
            self.retire(key, format!("LSP: server failed to start: {error}"));
        }
        Ok(())
    }

    fn command(
        &mut self,
        buffer_id: usize,
        input: Input,
        request_id: Option<u64>,
    ) -> Result<(), String> {
        let key = self
            .documents
            .get(&buffer_id)
            .and_then(|doc| doc.servers.first().cloned());
        if let Some(key) = key
            && let Some(worker) = self.servers.get(&key)
            && !worker.retired
        {
            if worker.input.try_send(input).is_ok() {
                return Ok(());
            }
            self.retire(
                &key,
                "LSP server stopped: document/command queue is full or unavailable".into(),
            );
        }
        if self.updates.len() >= QUEUE_SIZE {
            return Err("LSP update queue is full".into());
        }
        self.updates
            .push(message("no running server for this document", request_id));
        Ok(())
    }

    fn retire(&mut self, key: &ServerKey, reason: String) {
        let Some(worker) = self.servers.get_mut(key) else {
            return;
        };
        Self::retire_worker(
            key,
            worker,
            &mut self.documents,
            &mut self.changed_diagnostics,
            &mut self.changed_semantics,
            &mut self.updates,
            reason,
        );
    }

    fn retire_worker(
        key: &ServerKey,
        worker: &mut Worker,
        documents: &mut HashMap<usize, CachedDocument>,
        changed: &mut HashSet<usize>,
        changed_semantics: &mut HashSet<usize>,
        updates: &mut Vec<PluginUpdate>,
        reason: String,
    ) {
        if worker.retired {
            return;
        }
        worker.retired = true;
        worker.cancelled.store(true, Ordering::Release);
        for (id, doc) in documents {
            if doc.servers.first() == Some(key) {
                doc.semantic_tokens = None;
                changed_semantics.insert(*id);
            }
            if doc.servers.contains(key) {
                doc.diagnostics.remove(&key.config);
                changed.insert(*id);
            }
        }
        // Retirement cleanup remains independent of the command-message queue.
        updates.push(PluginUpdate {
            request_id: None,
            effects: vec![UpdateEffect::Message { text: reason }],
        });
    }
}

impl Drop for Lsp {
    fn drop(&mut self) {
        for worker in self.servers.values() {
            worker.cancelled.store(true, Ordering::Release);
        }
        // Application exit must not race detached workers' process cleanup.
        for worker in self.servers.values_mut() {
            if let Some(thread) = worker.thread.take() {
                let _ = thread.join();
            }
        }
    }
}

struct Document {
    snapshot: Arc<DocumentSnapshot>,
    uri: String,
    version: u64,
    opened: bool,
    saved: bool,
    primary: bool,
    semantic_dirty: bool,
}

#[derive(Default)]
struct SyncOptions {
    open_close: bool,
    change: u64,
    save: bool,
    include_text: bool,
}

enum Request {
    Initialize,
    Definition(Source),
    Hover(Source),
    Completion(Source, Arc<DocumentSnapshot>),
    SemanticTokens(Arc<DocumentSnapshot>),
    Shutdown,
}

struct Runtime {
    config: Arc<ServerConfig>,
    root: PathBuf,
    input: Receiver<Input>,
    output: SyncSender<Output>,
    cancelled: Arc<AtomicBool>,
    transport: Transport,
    snapshots: HashMap<usize, Arc<DocumentSnapshot>>,
    documents: HashMap<usize, Document>,
    requests: HashMap<u64, Request>,
    pending: VecDeque<(PositionKind, Source)>,
    semantic_inflight: HashSet<usize>,
    next_id: u64,
    ready: bool,
    definition: bool,
    hover: bool,
    completion: bool,
    semantic: Option<SemanticOptions>,
    sync: SyncOptions,
}

impl Runtime {
    fn new(
        config: Arc<ServerConfig>,
        root: PathBuf,
        input: Receiver<Input>,
        output: SyncSender<Output>,
        cancelled: Arc<AtomicBool>,
    ) -> Result<Self, String> {
        let transport = Transport::spawn(&config.command, &root)?;
        let mut runtime = Self {
            config,
            root,
            input,
            output,
            cancelled,
            transport,
            snapshots: HashMap::new(),
            documents: HashMap::new(),
            requests: HashMap::new(),
            pending: VecDeque::new(),
            semantic_inflight: HashSet::new(),
            next_id: 1,
            ready: false,
            definition: false,
            hover: false,
            completion: false,
            semantic: None,
            sync: SyncOptions::default(),
        };
        runtime.request(
            "initialize",
            Some(json!({
                "processId": std::process::id(),
                "clientInfo": {"name": "vamp"},
                "rootUri": file_uri(&runtime.root)?,
                "workspaceFolders": runtime.workspace_folders()?,
                "initializationOptions": runtime.config.initialization_options,
                "capabilities": {
                    "general": {"positionEncodings": ["utf-16"]},
                    "workspace": {"configuration": true, "workspaceFolders": true,
                        "semanticTokens": {"refreshSupport": true}},
                    "textDocument": {
                        "synchronization": {"dynamicRegistration": false, "willSave": false,
                            "willSaveWaitUntil": false, "didSave": true},
                        "definition": {"dynamicRegistration": false, "linkSupport": true},
                        "hover": {"dynamicRegistration": false,
                            "contentFormat": ["plaintext", "markdown"]},
                        "completion": {
                            "dynamicRegistration": false, "contextSupport": true,
                            "insertTextMode": 1,
                            "completionItem": {
                                "snippetSupport": true, "commitCharactersSupport": false,
                                "insertReplaceSupport": true,
                                "insertTextModeSupport": {"valueSet": [1]},
                                "resolveSupport": {"properties": []}
                            },
                            "completionList": {
                                "itemDefaults": ["editRange", "insertTextFormat", "insertTextMode"]
                            }
                        },
                        "publishDiagnostics": {"relatedInformation": false, "versionSupport": true},
                        "semanticTokens": {
                            "dynamicRegistration": false,
                            "requests": {"range": true, "full": true},
                            "tokenTypes": SEMANTIC_TYPES.iter().map(|(name, _)| *name).collect::<Vec<_>>(),
                            "tokenModifiers": SEMANTIC_MODIFIERS.iter().map(|(name, _)| *name).collect::<Vec<_>>(),
                            "formats": ["relative"],
                            "overlappingTokenSupport": false,
                            "multilineTokenSupport": false,
                            "serverCancelSupport": false,
                            "augmentsSyntaxTokens": true
                        }
                    }
                }
            })),
            Request::Initialize,
        )?;
        Ok(runtime)
    }

    fn run(&mut self) -> Result<(), String> {
        while !self.cancelled.load(Ordering::Acquire) {
            for _ in 0..QUEUE_SIZE {
                match self.input.try_recv() {
                    Ok(Input::Document {
                        snapshot,
                        event,
                        served,
                        primary,
                    }) => self.document(snapshot, event, served, primary)?,
                    Ok(Input::Position(kind, source)) => self.position(kind, source)?,
                    Ok(Input::Diagnostics(id)) => {
                        let text = if !self.documents.contains_key(&id) {
                            "no running server for this document".to_owned()
                        } else if !self.ready {
                            "server initializing".to_owned()
                        } else {
                            format!(
                                "diagnostics update automatically from {}",
                                self.config.language_id
                            )
                        };
                        self.emit(message(&text, None))?;
                    }
                    Err(TryRecvError::Empty) => break,
                    Err(TryRecvError::Disconnected) => {
                        self.cancelled.store(true, Ordering::Release);
                        break;
                    }
                }
            }
            self.transport.flush()?;
            self.transport.drain_stderr()?;
            self.transport.read()?;
            for _ in 0..QUEUE_SIZE {
                let Some(frame) = self.transport.frame()? else {
                    break;
                };
                self.incoming(frame)?;
            }
            if self.ready && self.semantic.is_some() && self.requests.len() < QUEUE_SIZE {
                let pending: Vec<_> = self
                    .documents
                    .iter()
                    .filter(|(id, doc)| {
                        doc.primary && doc.semantic_dirty && !self.semantic_inflight.contains(*id)
                    })
                    .map(|(id, _)| *id)
                    .collect();
                for id in pending {
                    self.semantic_request(id)?;
                }
            }
            self.transport.check_alive()?;
            thread::sleep(TICK);
        }
        self.shutdown();
        Ok(())
    }

    fn emit(&self, update: PluginUpdate) -> Result<(), String> {
        self.send(Output::Update(update))
    }

    fn emit_diagnostics(
        &self,
        snapshot: &Arc<DocumentSnapshot>,
        items: Vec<Diagnostic>,
    ) -> Result<(), String> {
        self.send(Output::Diagnostics {
            snapshot: Arc::clone(snapshot),
            items,
        })
    }

    fn emit_semantics(
        &self,
        snapshot: &Arc<DocumentSnapshot>,
        tokens: Option<Arc<[SemanticToken]>>,
    ) -> Result<(), String> {
        self.send(Output::SemanticTokens {
            snapshot: Arc::clone(snapshot),
            tokens,
        })
    }

    fn send(&self, output: Output) -> Result<(), String> {
        self.output.try_send(output).map_err(|error| match error {
            TrySendError::Full(_) => "LSP incoming update queue is full".into(),
            TrySendError::Disconnected(_) => "LSP client disconnected".into(),
        })
    }

    fn document(
        &mut self,
        snapshot: Arc<DocumentSnapshot>,
        event: DocumentEventKind,
        served: bool,
        primary: bool,
    ) -> Result<(), String> {
        let id = snapshot.id;
        let old = self.documents.remove(&id);
        if matches!(event, DocumentEventKind::Close) {
            self.snapshots.remove(&id);
            if let Some(old) = old {
                self.close(&old)?;
            }
            return Ok(());
        }
        self.snapshots.insert(id, Arc::clone(&snapshot));
        if !served {
            if let Some(old) = old {
                self.close(&old)?;
                self.emit_diagnostics(&snapshot, Vec::new())?;
            }
            return Ok(());
        }
        let path = snapshot.path.as_ref().ok_or("LSP document has no path")?;
        let uri = file_uri(&absolute(Path::new(path), &self.root))?;
        let same_uri = old.as_ref().is_some_and(|old| old.uri == uri);
        if let Some(old) = &old
            && !same_uri
        {
            self.close(old)?;
        }
        let changed = old
            .as_ref()
            .is_some_and(|old| old.snapshot.revision != snapshot.revision);
        let version = if same_uri {
            old.as_ref().unwrap().version + u64::from(changed)
        } else {
            1
        };
        if version > MAX_VERSION {
            return Err("LSP document version exhausted".into());
        }
        let mut doc = Document {
            snapshot: Arc::clone(&snapshot),
            uri,
            version,
            opened: same_uri && old.as_ref().is_some_and(|old| old.opened),
            saved: matches!(event, DocumentEventKind::Save),
            primary,
            semantic_dirty: primary
                && old.as_ref().is_none_or(|old| {
                    old.semantic_dirty || !old.primary || !Arc::ptr_eq(&old.snapshot, &snapshot)
                }),
        };
        if changed {
            self.emit_diagnostics(&doc.snapshot, Vec::new())?;
        }
        if self.ready {
            if !doc.opened {
                self.open(&mut doc)?;
            } else if changed {
                self.change(old.as_ref().unwrap(), &doc)?;
            }
            if doc.saved {
                self.save(&mut doc)?;
            }
        }
        self.documents.insert(id, doc);
        self.semantic_request(id)?;
        Ok(())
    }

    fn open(&mut self, doc: &mut Document) -> Result<(), String> {
        doc.opened = true;
        if self.sync.open_close {
            self.notify(
                "textDocument/didOpen",
                Some(json!({"textDocument": {
                    "uri": doc.uri, "languageId": self.config.language_id,
                    "version": doc.version, "text": doc.snapshot.text
                }})),
            )?;
        }
        Ok(())
    }

    fn change(&mut self, old: &Document, doc: &Document) -> Result<(), String> {
        if self.sync.change == 0 {
            return Ok(());
        }
        let mut change = json!({"text": doc.snapshot.text});
        if self.sync.change == 2 {
            let text = normalized(&old.snapshot.text);
            let mut lines = text.split('\n');
            let mut row = 0;
            let mut last = lines.next().unwrap_or("");
            for line in lines {
                row += 1;
                last = line;
            }
            change["range"] = json!({"start": {"line": 0, "character": 0},
                "end": {"line": row, "character": last.encode_utf16().count()}});
        }
        self.notify(
            "textDocument/didChange",
            Some(json!({
                "textDocument": {"uri": doc.uri, "version": doc.version}, "contentChanges": [change]
            })),
        )
    }

    fn save(&mut self, doc: &mut Document) -> Result<(), String> {
        if self.sync.save {
            let mut params = json!({"textDocument": {"uri": doc.uri}});
            if self.sync.include_text {
                params["text"] = json!(doc.snapshot.text);
            }
            self.notify("textDocument/didSave", Some(params))?;
        }
        doc.saved = false;
        Ok(())
    }

    fn close(&mut self, doc: &Document) -> Result<(), String> {
        if self.ready && doc.opened && self.sync.open_close {
            self.notify(
                "textDocument/didClose",
                Some(json!({"textDocument": {"uri": doc.uri}})),
            )?;
        }
        Ok(())
    }

    fn position(&mut self, kind: PositionKind, source: Source) -> Result<(), String> {
        let Some(doc) = self.documents.get(&source.buffer_id) else {
            return Ok(());
        };
        if doc.snapshot.revision != source.revision {
            return Ok(());
        }
        if !self.ready {
            if self.pending.len() == QUEUE_SIZE {
                return Err("LSP pending command queue is full".into());
            }
            self.pending.push_back((kind, source));
            return Ok(());
        }
        let (supported, method, unsupported) = match kind {
            PositionKind::Definition => (
                self.definition,
                "textDocument/definition",
                "server does not support go-to-definition",
            ),
            PositionKind::Hover => (
                self.hover,
                "textDocument/hover",
                "server does not support hover",
            ),
            PositionKind::Completion => (
                self.completion,
                "textDocument/completion",
                "server does not support completion",
            ),
        };
        if !supported {
            return self.emit(message(unsupported, Some(source.request_id)));
        }
        let position = outbound_position(&doc.snapshot.text, source.row, source.column)?;
        let mut params = json!({
            "textDocument": {"uri": doc.uri}, "position": position
        });
        if matches!(kind, PositionKind::Completion) {
            params["context"] = json!({"triggerKind": 1});
        }
        let request = match kind {
            PositionKind::Definition => Request::Definition(source),
            PositionKind::Hover => Request::Hover(source),
            PositionKind::Completion => Request::Completion(source, Arc::clone(&doc.snapshot)),
        };
        self.request(method, Some(params), request)
    }

    fn semantic_request(&mut self, id: usize) -> Result<(), String> {
        let Some(options) = &self.semantic else {
            return Ok(());
        };
        let Some(doc) = self.documents.get(&id).filter(|doc| {
            self.ready && doc.primary && doc.semantic_dirty && !self.semantic_inflight.contains(&id)
        }) else {
            return Ok(());
        };
        if self.requests.len() == QUEUE_SIZE {
            return Ok(());
        }
        let snapshot = Arc::clone(&doc.snapshot);
        let mut params = json!({"textDocument": {"uri": doc.uri}});
        let method = if options.full {
            "textDocument/semanticTokens/full"
        } else {
            let text = normalized(&snapshot.text);
            let (row, line) = text.split('\n').enumerate().last().unwrap();
            params["range"] = json!({"start": {"line": 0, "character": 0},
                "end": {"line": row, "character": line.encode_utf16().count()}});
            "textDocument/semanticTokens/range"
        };
        self.request(method, Some(params), Request::SemanticTokens(snapshot))?;
        self.semantic_inflight.insert(id);
        self.documents.get_mut(&id).unwrap().semantic_dirty = false;
        Ok(())
    }

    fn completion_result(
        &self,
        source: Source,
        snapshot: Arc<DocumentSnapshot>,
        result: Result<&Value, String>,
    ) -> Result<(), String> {
        if !self
            .documents
            .get(&source.buffer_id)
            .is_some_and(|doc| doc.primary && Arc::ptr_eq(&doc.snapshot, &snapshot))
        {
            return Ok(());
        }
        match result.and_then(|value| {
            decode_completion(&snapshot.text, value, &source, snapshot.path.as_deref())
        }) {
            Ok(items) => self.emit(PluginUpdate {
                request_id: Some(source.request_id),
                effects: vec![UpdateEffect::Completion {
                    buffer_id: source.buffer_id,
                    revision: source.revision,
                    items,
                }],
            }),
            Err(reason) => self.emit(message(
                &format!("completion failed: {reason}"),
                Some(source.request_id),
            )),
        }
    }

    fn semantic_result(
        &mut self,
        snapshot: Arc<DocumentSnapshot>,
        result: Result<&Value, String>,
    ) -> Result<(), String> {
        self.semantic_inflight.remove(&snapshot.id);
        let fresh = self.documents.get(&snapshot.id).is_some_and(|doc| {
            doc.primary && !doc.semantic_dirty && Arc::ptr_eq(&doc.snapshot, &snapshot)
        });
        // Stale responses are discarded before inspecting their token payload.
        if fresh {
            let decoded = result.and_then(|result| {
                decode_semantic_tokens(&snapshot.text, result, self.semantic.as_ref().unwrap())
            });
            match decoded {
                Ok(tokens) => self.emit_semantics(&snapshot, tokens)?,
                Err(reason) => {
                    self.emit_semantics(&snapshot, None)?;
                    self.emit(message(&format!("semantic tokens failed: {reason}"), None))?;
                }
            }
        }
        self.semantic_request(snapshot.id)
    }

    fn request(
        &mut self,
        method: &str,
        params: Option<Value>,
        context: Request,
    ) -> Result<(), String> {
        if self.next_id > MAX_VERSION {
            return Err("LSP request IDs exhausted".into());
        }
        if self.requests.len() == QUEUE_SIZE {
            return Err("LSP pending request queue is full".into());
        }
        let id = self.next_id;
        self.next_id += 1;
        let mut frame = json!({"jsonrpc": "2.0", "id": id, "method": method});
        if let Some(params) = params {
            frame["params"] = params;
        }
        self.transport.send(frame)?;
        self.requests.insert(id, context);
        Ok(())
    }

    fn notify(&mut self, method: &str, params: Option<Value>) -> Result<(), String> {
        let mut frame = json!({"jsonrpc": "2.0", "method": method});
        if let Some(params) = params {
            frame["params"] = params;
        }
        self.transport.send(frame)
    }

    fn incoming(&mut self, frame: Value) -> Result<(), String> {
        let object = frame
            .as_object()
            .filter(|object| object.get("jsonrpc") == Some(&json!("2.0")))
            .ok_or("invalid LSP JSON-RPC frame")?;
        if let Some(method) = object.get("method") {
            let method = method.as_str().ok_or("invalid LSP method")?;
            if let Some(id) = object.get("id") {
                return self.server_request(
                    id,
                    method,
                    object.get("params").unwrap_or(&Value::Null),
                );
            }
            match method {
                "textDocument/publishDiagnostics" => self.publish_diagnostics(&frame["params"]),
                "window/showMessage" => {
                    let text = frame["params"]["message"]
                        .as_str()
                        .ok_or("invalid LSP showMessage")?;
                    self.emit(message(text, None))
                }
                _ => Ok(()),
            }
        } else {
            let id = frame["id"].as_u64().ok_or("unexpected LSP response")?;
            let context = self.requests.remove(&id).ok_or("unexpected LSP response")?;
            if object.contains_key("result") == object.contains_key("error") {
                return match context {
                    Request::Hover(source) => {
                        self.hover_result(source, Err("unexpected LSP response".into()))
                    }
                    Request::Completion(source, snapshot) => self.completion_result(
                        source,
                        snapshot,
                        Err("unexpected LSP response".into()),
                    ),
                    Request::SemanticTokens(snapshot) => {
                        self.semantic_result(snapshot, Err("unexpected LSP response".into()))
                    }
                    _ => Err("unexpected LSP response".into()),
                };
            }
            if let Some(error) = object.get("error") {
                let text = error["message"]
                    .as_str()
                    .filter(|_| error["code"].as_i64().is_some());
                let Some(text) = text else {
                    return match context {
                        Request::Hover(source) => {
                            self.hover_result(source, Err("invalid LSP error response".into()))
                        }
                        Request::Completion(source, snapshot) => self.completion_result(
                            source,
                            snapshot,
                            Err("invalid LSP error response".into()),
                        ),
                        Request::SemanticTokens(snapshot) => {
                            self.semantic_result(snapshot, Err("invalid LSP error response".into()))
                        }
                        _ => Err("invalid LSP error response".into()),
                    };
                };
                match context {
                    Request::Initialize => Err(format!("initialization failed: {text}")),
                    Request::Definition(source) => self.emit(message(
                        &format!("definition failed: {text}"),
                        Some(source.request_id),
                    )),
                    Request::Hover(source) => self.hover_result(source, Err(text.to_owned())),
                    Request::Completion(source, snapshot) => {
                        self.completion_result(source, snapshot, Err(text.to_owned()))
                    }
                    Request::SemanticTokens(snapshot) => {
                        self.semantic_result(snapshot, Err(text.to_owned()))
                    }
                    Request::Shutdown => Ok(()),
                }
            } else {
                match context {
                    Request::Initialize => self.initialized(&frame["result"]),
                    Request::Definition(source) => self.definition_result(source, &frame["result"]),
                    Request::Hover(source) => self.hover_result(source, Ok(&frame["result"])),
                    Request::Completion(source, snapshot) => {
                        self.completion_result(source, snapshot, Ok(&frame["result"]))
                    }
                    Request::SemanticTokens(snapshot) => {
                        self.semantic_result(snapshot, Ok(&frame["result"]))
                    }
                    Request::Shutdown => self.notify("exit", None),
                }
            }
        }
    }

    fn initialized(&mut self, result: &Value) -> Result<(), String> {
        let capabilities = result["capabilities"]
            .as_object()
            .ok_or("invalid LSP initialize result")?;
        if capabilities
            .get("positionEncoding")
            .is_some_and(|encoding| encoding != "utf-16")
        {
            return Err("server selected unsupported position encoding".into());
        }
        self.sync = sync_options(capabilities.get("textDocumentSync").unwrap_or(&json!(0)))?;
        self.definition = capabilities
            .get("definitionProvider")
            .is_some_and(|value| value == true || value.is_object());
        self.hover = capabilities
            .get("hoverProvider")
            .is_some_and(|value| value == true || value.is_object());
        self.completion = capabilities
            .get("completionProvider")
            .is_some_and(Value::is_object);
        match semantic_options(capabilities.get("semanticTokensProvider")) {
            Ok(options) => self.semantic = options,
            Err(reason) => {
                self.emit(message(&format!("semantic tokens failed: {reason}"), None))?
            }
        }
        self.notify("initialized", Some(json!({})))?;
        self.notify(
            "workspace/didChangeConfiguration",
            Some(json!({"settings": self.config.settings})),
        )?;
        self.ready = true;
        let documents = std::mem::take(&mut self.documents);
        for (id, mut doc) in documents {
            self.open(&mut doc)?;
            if doc.saved {
                self.save(&mut doc)?;
            }
            self.documents.insert(id, doc);
            self.semantic_request(id)?;
        }
        while let Some((kind, source)) = self.pending.pop_front() {
            self.position(kind, source)?;
        }
        Ok(())
    }

    fn workspace_folders(&self) -> Result<Value, String> {
        Ok(
            json!([{"uri": file_uri(&self.root)?, "name": self.root.file_name().and_then(|name| name.to_str()).unwrap_or("/")}]),
        )
    }

    fn server_request(&mut self, id: &Value, method: &str, params: &Value) -> Result<(), String> {
        if !(id.is_string() || id.as_i64().is_some() || id.as_u64().is_some()) {
            return Err("invalid LSP server request ID".into());
        }
        let mut response = json!({"jsonrpc": "2.0", "id": id});
        response["result"] = match method {
            "workspace/configuration" => {
                let items = params["items"]
                    .as_array()
                    .ok_or("invalid LSP configuration request")?;
                let mut values = Vec::with_capacity(items.len());
                for item in items {
                    if !item.is_object() {
                        return Err("invalid LSP configuration item".into());
                    }
                    let value = match item.get("section") {
                        None | Some(Value::Null) => &self.config.settings,
                        Some(Value::String(section)) if section.is_empty() => &self.config.settings,
                        Some(Value::String(section)) => {
                            self.config.settings.get(section).unwrap_or_else(|| {
                                section
                                    .split('.')
                                    .try_fold(&self.config.settings, |value, part| value.get(part))
                                    .unwrap_or(&Value::Null)
                            })
                        }
                        _ => return Err("invalid LSP configuration section".into()),
                    };
                    values.push(value.clone());
                }
                Value::Array(values)
            }
            "workspace/workspaceFolders" => self.workspace_folders()?,
            "window/showMessageRequest" => Value::Null,
            "workspace/semanticTokens/refresh" => {
                self.transport
                    .send(json!({"jsonrpc": "2.0", "id": id, "result": null}))?;
                if self.semantic.is_some() {
                    let ids: Vec<_> = self
                        .documents
                        .iter()
                        .filter(|(_, doc)| doc.primary)
                        .map(|(id, _)| *id)
                        .collect();
                    for id in ids {
                        let doc = self.documents.get_mut(&id).unwrap();
                        doc.semantic_dirty = true;
                        self.semantic_request(id)?;
                    }
                }
                return Ok(());
            }
            _ => {
                response.as_object_mut().unwrap().remove("result");
                response["error"] =
                    json!({"code": -32601, "message": "Method not supported by vamp"});
                return self.transport.send(response);
            }
        };
        self.transport.send(response)
    }

    fn publish_diagnostics(&self, params: &Value) -> Result<(), String> {
        let uri = params["uri"]
            .as_str()
            .ok_or("invalid LSP publishDiagnostics")?;
        let items = params["diagnostics"]
            .as_array()
            .ok_or("invalid LSP publishDiagnostics")?;
        let Some(doc) = self.documents.values().find(|doc| doc.uri == uri) else {
            return Ok(());
        };
        if let Some(version) = params.get("version").filter(|value| !value.is_null()) {
            let version = version
                .as_i64()
                .filter(|value| i32::try_from(*value).is_ok())
                .ok_or("invalid LSP diagnostic version")?;
            if version < doc.version as i64 {
                return Ok(());
            }
            if version > doc.version as i64 {
                return Err("LSP diagnostics refer to an unknown future version".into());
            }
        }
        // Without a version only the revision at arrival is knowable.
        let text = normalized(&doc.snapshot.text);
        let mut diagnostics = Vec::with_capacity(items.len());
        for item in items {
            let (start, end) = range(&text, &item["range"])?;
            let severity = match item.get("severity").unwrap_or(&json!(1)).as_u64() {
                Some(1) => DiagnosticSeverity::Error,
                Some(2) => DiagnosticSeverity::Warning,
                Some(3) => DiagnosticSeverity::Information,
                Some(4) => DiagnosticSeverity::Hint,
                _ => return Err("invalid LSP diagnostic severity".into()),
            };
            diagnostics.push(Diagnostic {
                start,
                end,
                severity,
                message: clean(
                    item["message"]
                        .as_str()
                        .ok_or("invalid LSP diagnostic message")?,
                ),
            });
        }
        self.emit_diagnostics(&doc.snapshot, diagnostics)
    }

    fn definition_result(&self, source: Source, result: &Value) -> Result<(), String> {
        if self
            .documents
            .get(&source.buffer_id)
            .is_none_or(|doc| doc.snapshot.revision != source.revision)
        {
            return Ok(());
        }
        let result = self.definition_effects(&source, result);
        match result {
            Ok(effects) => self.emit(PluginUpdate {
                request_id: Some(source.request_id),
                effects,
            }),
            Err(reason) => self.emit(message(
                &format!("definition failed: {reason}"),
                Some(source.request_id),
            )),
        }
    }

    fn hover_result(&self, source: Source, result: Result<&Value, String>) -> Result<(), String> {
        let Some(doc) = self
            .documents
            .get(&source.buffer_id)
            .filter(|doc| doc.snapshot.revision == source.revision)
        else {
            return Ok(());
        };
        match result.and_then(|result| hover_text(result, &doc.snapshot.text)) {
            Ok(Some(text)) => self.emit(PluginUpdate {
                request_id: Some(source.request_id),
                effects: vec![UpdateEffect::Hover {
                    buffer_id: source.buffer_id,
                    revision: source.revision,
                    text,
                }],
            }),
            Ok(None) => self.emit(message("no hover information", Some(source.request_id))),
            Err(reason) => self.emit(message(
                &format!("hover failed: {reason}"),
                Some(source.request_id),
            )),
        }
    }

    fn definition_effects(
        &self,
        source: &Source,
        result: &Value,
    ) -> Result<Vec<UpdateEffect>, String> {
        let (location, count) = match result {
            Value::Null => return Ok(message("no definition found", None).effects),
            Value::Array(locations) if locations.is_empty() => {
                return Ok(message("no definition found", None).effects);
            }
            Value::Array(locations) => (&locations[0], locations.len()),
            location => (location, 1),
        };
        if !location.is_object() {
            return Err("invalid LSP definition location".into());
        }
        let link = location.get("targetUri").is_some();
        let path = uri_path(
            location[if link { "targetUri" } else { "uri" }]
                .as_str()
                .ok_or("invalid definition URI")?,
        )?;
        let identity = std::fs::canonicalize(&path).unwrap_or_else(|_| path.clone());
        let cached = self.snapshots.values().find(|doc| {
            doc.path.as_ref().is_some_and(|candidate| {
                let candidate = absolute(Path::new(candidate), &self.root);
                std::fs::canonicalize(&candidate).unwrap_or(candidate) == identity
            })
        });
        let disk;
        let text = if let Some(doc) = cached {
            &doc.text
        } else {
            if !std::fs::metadata(&path)
                .map_err(|error| error.to_string())?
                .is_file()
            {
                return Err("definition target must be a regular file".into());
            }
            let mut bytes = Vec::new();
            std::fs::File::open(&path)
                .map_err(|error| error.to_string())?
                .take((MAX_BODY + 1) as u64)
                .read_to_end(&mut bytes)
                .map_err(|error| error.to_string())?;
            if bytes.len() > MAX_BODY {
                return Err("definition target exceeds 4 MiB".into());
            }
            disk = String::from_utf8(bytes).map_err(|_| "definition target is not UTF-8")?;
            &disk
        };
        let text = normalized(text);
        if link {
            range(&text, &location["targetRange"])?;
        }
        let (start, _) = range(
            &text,
            &location[if link {
                "targetSelectionRange"
            } else {
                "range"
            }],
        )?;
        let mut effects = vec![UpdateEffect::Navigate {
            buffer_id: source.buffer_id,
            revision: source.revision,
            path,
            row: start.row,
            column: start.column,
        }];
        if count > 1 {
            effects.push(UpdateEffect::Message {
                text: format!("LSP: selected first of {count} definitions"),
            });
        }
        Ok(effects)
    }

    fn shutdown(&mut self) {
        if !self.ready {
            return;
        }
        if self.request("shutdown", None, Request::Shutdown).is_err() {
            return;
        }
        let deadline = Instant::now() + Duration::from_millis(200);
        let mut exiting = false;
        while Instant::now() < deadline {
            if self.transport.flush().is_err() || self.transport.read().is_err() {
                break;
            }
            // A flushed exit notification is not proof the peer has consumed it.
            if exiting
                && self.transport.flushed()
                && self.transport.child.try_wait().ok().flatten().is_some()
            {
                break;
            }
            match self.transport.frame() {
                Ok(Some(frame)) => {
                    let shutdown = frame["id"].as_u64().is_some_and(|id| {
                        matches!(self.requests.get(&id), Some(Request::Shutdown))
                    });
                    if shutdown {
                        self.notify("exit", None).ok();
                        exiting = true;
                    }
                }
                Ok(None) => thread::sleep(TICK),
                Err(_) => break,
            }
            let _ = self.transport.drain_stderr();
        }
    }
}

const SEMANTIC_TYPES: &[(&str, SemanticTokenKind)] = &[
    ("namespace", SemanticTokenKind::Namespace),
    ("type", SemanticTokenKind::Type),
    ("class", SemanticTokenKind::Class),
    ("enum", SemanticTokenKind::Enum),
    ("interface", SemanticTokenKind::Interface),
    ("struct", SemanticTokenKind::Struct),
    ("typeParameter", SemanticTokenKind::TypeParameter),
    ("parameter", SemanticTokenKind::Parameter),
    ("variable", SemanticTokenKind::Variable),
    ("property", SemanticTokenKind::Property),
    ("enumMember", SemanticTokenKind::EnumMember),
    ("event", SemanticTokenKind::Event),
    ("function", SemanticTokenKind::Function),
    ("method", SemanticTokenKind::Method),
    ("macro", SemanticTokenKind::Macro),
    ("keyword", SemanticTokenKind::Keyword),
    ("modifier", SemanticTokenKind::Modifier),
    ("comment", SemanticTokenKind::Comment),
    ("string", SemanticTokenKind::String),
    ("number", SemanticTokenKind::Number),
    ("regexp", SemanticTokenKind::Regexp),
    ("operator", SemanticTokenKind::Operator),
    ("decorator", SemanticTokenKind::Decorator),
];

const SEMANTIC_MODIFIERS: &[(&str, SemanticTokenModifier)] = &[
    ("declaration", SemanticTokenModifier::Declaration),
    ("definition", SemanticTokenModifier::Definition),
    ("readonly", SemanticTokenModifier::Readonly),
    ("static", SemanticTokenModifier::Static),
    ("deprecated", SemanticTokenModifier::Deprecated),
    ("abstract", SemanticTokenModifier::Abstract),
    ("async", SemanticTokenModifier::Async),
    ("modification", SemanticTokenModifier::Modification),
    ("documentation", SemanticTokenModifier::Documentation),
    ("defaultLibrary", SemanticTokenModifier::DefaultLibrary),
];

struct SemanticOptions {
    full: bool,
    types: Vec<Option<SemanticTokenKind>>,
    modifiers: [u16; 31],
    modifier_mask: u32,
}

fn semantic_options(value: Option<&Value>) -> Result<Option<SemanticOptions>, String> {
    let Some(value) = value.filter(|value| !value.is_null() && *value != false) else {
        return Ok(None);
    };
    let object = value
        .as_object()
        .ok_or("invalid semantic tokens capability")?;
    let supported = |name| -> Result<bool, String> {
        match object.get(name) {
            None | Some(Value::Bool(false)) => Ok(false),
            Some(Value::Bool(true) | Value::Object(_)) => Ok(true),
            _ => Err("invalid semantic tokens request capability".into()),
        }
    };
    let full = supported("full")?;
    if !full && !supported("range")? {
        return Ok(None);
    }
    let types = value["legend"]["tokenTypes"]
        .as_array()
        .filter(|items| items.len() <= 65536)
        .ok_or("invalid semantic token type legend")?;
    let names = value["legend"]["tokenModifiers"]
        .as_array()
        .ok_or("invalid semantic token modifier legend")?;
    let types = types
        .iter()
        .map(|value| {
            let name = value.as_str().ok_or("invalid semantic token type legend")?;
            Ok(SEMANTIC_TYPES
                .iter()
                .find(|(candidate, _)| *candidate == name)
                .map(|(_, kind)| *kind))
        })
        .collect::<Result<Vec<_>, String>>()?;
    let mut modifiers = [0; 31];
    for (index, value) in names.iter().enumerate() {
        let name = value
            .as_str()
            .ok_or("invalid semantic token modifier legend")?;
        if let Some(slot) = modifiers.get_mut(index) {
            *slot = SEMANTIC_MODIFIERS
                .iter()
                .find(|(candidate, _)| *candidate == name)
                .map_or(0, |(_, modifier)| *modifier as u16);
        }
    }
    Ok(Some(SemanticOptions {
        full,
        types,
        modifiers,
        modifier_mask: (1u32 << names.len().min(31)) - 1,
    }))
}

// Both cursors only advance: strict codepoint boundaries and outward grapheme
// rounding cost O(document + tokens), including many tokens in one grapheme.
struct SemanticLine<'a> {
    chars: std::str::Chars<'a>,
    graphemes: unicode_segmentation::Graphemes<'a>,
    units: usize,
    bytes: usize,
    column: usize,
    grapheme_start: usize,
    grapheme_end: usize,
}

impl<'a> SemanticLine<'a> {
    fn new(line: &'a str) -> Self {
        let mut graphemes = line.graphemes(true);
        let grapheme_end = graphemes.next().map_or(0, str::len);
        Self {
            chars: line.chars(),
            graphemes,
            units: 0,
            bytes: 0,
            column: 0,
            grapheme_start: 0,
            grapheme_end,
        }
    }

    fn column(&mut self, target: usize, end: bool) -> Result<usize, String> {
        while self.units < target {
            let Some(character) = self.chars.next() else {
                if end {
                    break; // LSP: an overlong end truncates at EOL, never wraps.
                }
                return Err("semantic token start is beyond its line".into());
            };
            let next = self.units + character.len_utf16();
            if next > target {
                return Err("semantic token splits a UTF-16 surrogate pair".into());
            }
            self.units = next;
            self.bytes += character.len_utf8();
        }
        while self.grapheme_end > self.grapheme_start && self.bytes >= self.grapheme_end {
            self.grapheme_start = self.grapheme_end;
            self.column += 1;
            self.grapheme_end += self.graphemes.next().map_or(0, str::len);
        }
        Ok(self.column + usize::from(end && self.bytes > self.grapheme_start))
    }
}

fn decode_semantic_tokens(
    text: &str,
    result: &Value,
    options: &SemanticOptions,
) -> Result<Option<Arc<[SemanticToken]>>, String> {
    if result.is_null() {
        return Ok(None);
    }
    if result
        .get("resultId")
        .is_some_and(|value| !value.is_string())
    {
        return Err("invalid semantic token result ID".into());
    }
    let data = result["data"]
        .as_array()
        .ok_or("invalid semantic token data")?;
    if !data.len().is_multiple_of(5) {
        return Err("incomplete semantic token data".into());
    }
    let count = data.len() / 5;
    if count > MAX_BODY / std::mem::size_of::<SemanticToken>() {
        return Err("decoded semantic tokens exceed 4 MiB".into());
    }
    if count == 0 {
        return Ok(None);
    }
    let text = normalized(text);
    let mut lines = text.split('\n');
    let mut line = SemanticLine::new(lines.next().unwrap());
    let mut row = 0usize;
    let mut start = 0usize;
    let mut previous_end = 0usize;
    let mut tokens = Vec::with_capacity(count);
    for chunk in data.chunks_exact(5) {
        let integer = |index: usize| -> Result<usize, String> {
            chunk[index]
                .as_u64()
                .filter(|value| *value <= MAX_VERSION)
                .map(|value| value as usize)
                .ok_or_else(|| "invalid semantic token integer".into())
        };
        let delta_row = integer(0)?;
        let delta_start = integer(1)?;
        let length = integer(2)?;
        let kind = integer(3)?;
        let mask = integer(4)? as u32;
        let next_row = row
            .checked_add(delta_row)
            .filter(|value| *value <= MAX_VERSION as usize)
            .ok_or("semantic token row overflow")?;
        if delta_row != 0 {
            for _ in row..next_row {
                line =
                    SemanticLine::new(lines.next().ok_or("semantic token row is out of bounds")?);
            }
            row = next_row;
            start = delta_start;
            previous_end = 0;
        } else {
            start = start
                .checked_add(delta_start)
                .filter(|value| *value <= MAX_VERSION as usize)
                .ok_or("semantic token column overflow")?;
        }
        let end = start
            .checked_add(length)
            .ok_or("semantic token length overflow")?;
        let token_kind = options
            .types
            .get(kind)
            .filter(|_| kind < 65536)
            .ok_or("semantic token type is outside its legend")?;
        if mask & !options.modifier_mask != 0 {
            return Err("semantic token modifier is outside its legend".into());
        }
        if start < previous_end {
            return Err("overlapping semantic tokens are unsupported".into());
        }
        let start_column = line.column(start, false)?;
        let end_column = line.column(end, true)?;
        previous_end = end.min(line.units);
        if length == 0 || start_column >= end_column {
            continue;
        }
        if let Some(kind) = token_kind {
            let mut modifiers = 0;
            let mut remaining = mask;
            while remaining != 0 {
                modifiers |= options.modifiers[remaining.trailing_zeros() as usize];
                remaining &= remaining - 1;
            }
            tokens.push(SemanticToken {
                start: TextPosition {
                    row,
                    column: start_column,
                },
                end: TextPosition {
                    row,
                    column: end_column,
                },
                kind: *kind,
                modifiers,
            });
        }
    }
    Ok((!tokens.is_empty()).then(|| tokens.into()))
}

fn sync_options(value: &Value) -> Result<SyncOptions, String> {
    if let Some(kind) = value.as_u64().filter(|kind| *kind <= 2) {
        return Ok(SyncOptions {
            open_close: kind != 0,
            change: kind,
            ..SyncOptions::default()
        });
    }
    let object = value
        .as_object()
        .ok_or("invalid LSP textDocumentSync capability")?;
    let open_close = object
        .get("openClose")
        .unwrap_or(&Value::Bool(false))
        .as_bool()
        .ok_or("invalid LSP synchronization options")?;
    let change = object
        .get("change")
        .unwrap_or(&json!(0))
        .as_u64()
        .filter(|kind| *kind <= 2)
        .ok_or("invalid LSP synchronization options")?;
    let (save, include_text) = match object.get("save") {
        None | Some(Value::Bool(false)) => (false, false),
        Some(Value::Bool(true)) => (true, false),
        Some(Value::Object(save)) => (
            true,
            save.get("includeText")
                .unwrap_or(&Value::Bool(false))
                .as_bool()
                .ok_or("invalid LSP synchronization options")?,
        ),
        _ => return Err("invalid LSP synchronization options".into()),
    };
    Ok(SyncOptions {
        open_close,
        change,
        save,
        include_text,
    })
}

struct Transport {
    child: Child,
    #[cfg(windows)]
    job: WindowsJob,
    #[cfg(unix)]
    stdin: ChildStdin,
    #[cfg(unix)]
    stdout: ChildStdout,
    #[cfg(unix)]
    stderr: ChildStderr,
    #[cfg(not(unix))]
    portable: PortablePipes,
    incoming: Vec<u8>,
    consumed: usize,
    writes: VecDeque<Vec<u8>>,
    #[cfg(unix)]
    written: usize,
}

impl Transport {
    fn spawn(argv: &[String], root: &Path) -> Result<Self, String> {
        let program = argv
            .first()
            .filter(|program| !program.is_empty())
            .ok_or("LSP command argv is empty")?;
        let mut command = Command::new(program);
        command
            .args(&argv[1..])
            .current_dir(root)
            .stdin(Stdio::piped())
            .stdout(Stdio::piped())
            .stderr(Stdio::piped());
        #[cfg(unix)]
        {
            use std::os::unix::process::CommandExt;
            command.process_group(0);
        }
        let mut child = command.spawn().map_err(|error| error.to_string())?;
        #[cfg(windows)]
        let job = match WindowsJob::new(&child) {
            Ok(job) => job,
            Err(error) => {
                let _ = child.kill();
                let _ = child.wait();
                return Err(error);
            }
        };
        let stdin = child.stdin.take().expect("piped LSP stdin");
        let stdout = child.stdout.take().expect("piped LSP stdout");
        let stderr = child.stderr.take().expect("piped LSP stderr");
        let transport = Self {
            child,
            #[cfg(windows)]
            job,
            #[cfg(unix)]
            stdin,
            #[cfg(unix)]
            stdout,
            #[cfg(unix)]
            stderr,
            #[cfg(not(unix))]
            portable: PortablePipes::new(stdin, stdout, stderr),
            incoming: Vec::new(),
            consumed: 0,
            writes: VecDeque::new(),
            #[cfg(unix)]
            written: 0,
        };
        #[cfg(unix)]
        {
            nonblocking(&transport.stdin)?;
            nonblocking(&transport.stdout)?;
            nonblocking(&transport.stderr)?;
        }
        Ok(transport)
    }

    fn send(&mut self, frame: Value) -> Result<(), String> {
        if self.writes.len() == QUEUE_SIZE {
            return Err("LSP outgoing queue is full".into());
        }
        let body = serde_json::to_vec(&frame).map_err(|error| error.to_string())?;
        if body.len() > MAX_BODY {
            return Err("LSP outgoing frame exceeds 4 MiB".into());
        }
        let mut bytes = format!("Content-Length: {}\r\n\r\n", body.len()).into_bytes();
        bytes.extend_from_slice(&body);
        self.writes.push_back(bytes);
        Ok(())
    }

    #[cfg(unix)]
    fn flush(&mut self) -> Result<(), String> {
        for _ in 0..QUEUE_SIZE {
            let Some(bytes) = self.writes.front() else {
                break;
            };
            let result = self.stdin.write(&bytes[self.written..]);
            match result {
                Ok(0) => return Err("LSP server closed stdin".into()),
                Ok(count) => {
                    self.written += count;
                    if self.written == bytes.len() {
                        self.writes.pop_front();
                        self.written = 0;
                    }
                }
                Err(error) if error.kind() == std::io::ErrorKind::WouldBlock => break,
                Err(error) if error.kind() == std::io::ErrorKind::Interrupted => continue,
                Err(error) => return Err(error.to_string()),
            }
        }
        Ok(())
    }

    #[cfg(not(unix))]
    fn flush(&mut self) -> Result<(), String> {
        for _ in 0..QUEUE_SIZE {
            let Some(bytes) = self.writes.pop_front() else {
                break;
            };
            self.portable.pending.fetch_add(1, Ordering::AcqRel);
            match self
                .portable
                .writes
                .as_ref()
                .ok_or("LSP writer unavailable")?
                .try_send(bytes)
            {
                Ok(()) => {}
                Err(TrySendError::Full(bytes)) => {
                    self.portable.pending.fetch_sub(1, Ordering::Release);
                    self.writes.push_front(bytes);
                    break;
                }
                Err(TrySendError::Disconnected(_)) => {
                    self.portable.pending.fetch_sub(1, Ordering::Release);
                    return Err("LSP server closed stdin".into());
                }
            }
        }
        Ok(())
    }

    fn flushed(&self) -> bool {
        #[cfg(unix)]
        {
            self.writes.is_empty()
        }
        #[cfg(not(unix))]
        {
            self.writes.is_empty() && self.portable.pending.load(Ordering::Acquire) == 0
        }
    }

    fn read(&mut self) -> Result<(), String> {
        if self.consumed != 0 {
            self.incoming.drain(..self.consumed);
            self.consumed = 0;
        }
        let mut bytes = [0; 8192];
        for _ in 0..8 {
            if self.incoming.len() >= MAX_BODY + MAX_HEADER {
                break;
            }
            let remaining = (MAX_BODY + MAX_HEADER - self.incoming.len()).min(bytes.len());
            #[cfg(unix)]
            let result = self.stdout.read(&mut bytes[..remaining]);
            #[cfg(not(unix))]
            let result = self.portable.read(&mut bytes[..remaining]);
            match result {
                Ok(0) => return Err("LSP server exited or truncated a frame".into()),
                Ok(count) => self.incoming.extend_from_slice(&bytes[..count]),
                Err(error) if error.kind() == std::io::ErrorKind::WouldBlock => break,
                Err(error) if error.kind() == std::io::ErrorKind::Interrupted => continue,
                Err(error) => return Err(error.to_string()),
            }
        }
        Ok(())
    }

    fn frame(&mut self) -> Result<Option<Value>, String> {
        let bytes = &self.incoming[self.consumed..];
        let Some(end) = bytes.windows(4).position(|bytes| bytes == b"\r\n\r\n") else {
            if bytes.len() > MAX_HEADER {
                return Err("LSP headers exceed 16 KiB".into());
            }
            return Ok(None);
        };
        if end + 4 > MAX_HEADER {
            return Err("LSP headers exceed 16 KiB".into());
        }
        let header = &bytes[..end];
        if !header.is_ascii() {
            return Err("LSP headers must be ASCII".into());
        }
        let header = std::str::from_utf8(header).map_err(|_| "invalid LSP header")?;
        let mut length = None;
        for line in header.split("\r\n") {
            let (name, value) = line.split_once(':').ok_or("invalid LSP header")?;
            if name.is_empty() || line.contains(['\r', '\n']) {
                return Err("invalid LSP header newline".into());
            }
            let value = value.trim();
            if name.eq_ignore_ascii_case("content-length") {
                if length.is_some()
                    || value.is_empty()
                    || !value.bytes().all(|byte| byte.is_ascii_digit())
                {
                    return Err("invalid LSP Content-Length".into());
                }
                length = Some(
                    value
                        .parse::<usize>()
                        .map_err(|_| "invalid LSP Content-Length")?,
                );
            } else if name.eq_ignore_ascii_case("content-type") {
                for parameter in value.split(';').skip(1) {
                    if let Some((name, encoding)) = parameter.trim().split_once('=')
                        && name.trim().eq_ignore_ascii_case("charset")
                        && !["utf8", "utf-8"]
                            .iter()
                            .any(|supported| encoding.trim().eq_ignore_ascii_case(supported))
                    {
                        return Err("unsupported LSP content encoding".into());
                    }
                }
            }
        }
        let length = length
            .filter(|length| (1..=MAX_BODY).contains(length))
            .ok_or("invalid LSP body length")?;
        let body_start = end + 4;
        if bytes.len() < body_start + length {
            return Ok(None);
        }
        let frame = serde_json::from_slice(&bytes[body_start..body_start + length])
            .map_err(|error| format!("invalid LSP JSON: {error}"))?;
        self.consumed += body_start + length;
        Ok(Some(frame))
    }

    #[cfg(unix)]
    fn drain_stderr(&mut self) -> Result<(), String> {
        let mut bytes = [0; 8192];
        for _ in 0..8 {
            match self.stderr.read(&mut bytes) {
                Ok(0) => break,
                Ok(_) => {}
                Err(error) if error.kind() == std::io::ErrorKind::WouldBlock => break,
                Err(error) if error.kind() == std::io::ErrorKind::Interrupted => continue,
                Err(error) => return Err(error.to_string()),
            }
        }
        Ok(())
    }

    #[cfg(not(unix))]
    fn drain_stderr(&mut self) -> Result<(), String> {
        match self.portable.failures.try_recv() {
            Ok(reason) => Err(reason),
            Err(_) => Ok(()),
        }
    }

    fn check_alive(&mut self) -> Result<(), String> {
        if let Some(status) = self.child.try_wait().map_err(|error| error.to_string())? {
            return Err(format!("LSP server exited: {status}"));
        }
        Ok(())
    }
}

impl Drop for Transport {
    fn drop(&mut self) {
        // This runs only on the already bounded server worker, never on input or
        // poll. Kill the entire group even if the direct child has already exited.
        #[cfg(unix)]
        unsafe {
            kill(-(self.child.id() as i32), 9);
        }
        #[cfg(windows)]
        self.job.terminate();
        let _ = self.child.kill();
        let _ = self.child.wait();
        #[cfg(not(unix))]
        self.portable.stop();
    }
}

// Anonymous Windows pipes cannot be made nonblocking through std. Three bounded
// pipe workers keep the same framed transport off the editor and server workers.
#[cfg(not(unix))]
struct PortablePipes {
    writes: Option<SyncSender<Vec<u8>>>,
    reads: Option<Receiver<Vec<u8>>>,
    failures: Receiver<String>,
    chunk: Vec<u8>,
    consumed: usize,
    pending: Arc<std::sync::atomic::AtomicUsize>,
    workers: Vec<thread::JoinHandle<()>>,
}

#[cfg(not(unix))]
impl PortablePipes {
    fn new(mut stdin: ChildStdin, mut stdout: ChildStdout, mut stderr: ChildStderr) -> Self {
        let (writes, outgoing) = mpsc::sync_channel::<Vec<u8>>(QUEUE_SIZE);
        let (incoming, reads) = mpsc::sync_channel(QUEUE_SIZE);
        let (failed, failures) = mpsc::sync_channel(1);
        let pending = Arc::new(std::sync::atomic::AtomicUsize::new(0));
        let written = Arc::clone(&pending);
        let write_failed = failed.clone();
        let writer = thread::spawn(move || {
            while let Ok(bytes) = outgoing.recv() {
                let result = stdin.write_all(&bytes).and_then(|()| stdin.flush());
                written.fetch_sub(1, Ordering::Release);
                if let Err(error) = result {
                    let _ = write_failed.try_send(error.to_string());
                    break;
                }
            }
        });
        let reader = thread::spawn(move || {
            loop {
                let mut bytes = vec![0; 8192];
                match stdout.read(&mut bytes) {
                    Ok(0) => {
                        let _ = failed.try_send("LSP server exited or truncated a frame".into());
                        break;
                    }
                    Ok(count) => {
                        bytes.truncate(count);
                        if incoming.send(bytes).is_err() {
                            break;
                        }
                    }
                    Err(error) if error.kind() == std::io::ErrorKind::Interrupted => continue,
                    Err(error) => {
                        let _ = failed.try_send(error.to_string());
                        break;
                    }
                }
            }
        });
        let drain = thread::spawn(move || {
            let mut bytes = [0; 8192];
            loop {
                match stderr.read(&mut bytes) {
                    Ok(0) => break,
                    Ok(_) => {}
                    Err(error) if error.kind() == std::io::ErrorKind::Interrupted => continue,
                    Err(_) => break,
                }
            }
        });
        Self {
            writes: Some(writes),
            reads: Some(reads),
            failures,
            chunk: Vec::new(),
            consumed: 0,
            pending,
            workers: vec![writer, reader, drain],
        }
    }

    fn read(&mut self, buffer: &mut [u8]) -> std::io::Result<usize> {
        if self.consumed == self.chunk.len() {
            self.chunk = match self
                .reads
                .as_ref()
                .ok_or(std::io::ErrorKind::BrokenPipe)?
                .try_recv()
            {
                Ok(bytes) => bytes,
                Err(TryRecvError::Empty) => return Err(std::io::ErrorKind::WouldBlock.into()),
                Err(TryRecvError::Disconnected) => return Ok(0),
            };
            self.consumed = 0;
        }
        let count = buffer.len().min(self.chunk.len() - self.consumed);
        buffer[..count].copy_from_slice(&self.chunk[self.consumed..self.consumed + count]);
        self.consumed += count;
        Ok(count)
    }

    fn stop(&mut self) {
        self.writes.take();
        self.reads.take();
        for worker in self.workers.drain(..) {
            let _ = worker.join();
        }
    }
}

#[cfg(windows)]
struct WindowsJob(*mut std::ffi::c_void);

#[cfg(windows)]
impl WindowsJob {
    fn new(child: &Child) -> Result<Self, String> {
        use std::os::windows::io::AsRawHandle;
        let handle = unsafe { CreateJobObjectW(std::ptr::null(), std::ptr::null()) };
        if handle.is_null() {
            return Err(std::io::Error::last_os_error().to_string());
        }
        let job = Self(handle);
        let mut limits: JobLimits = unsafe { std::mem::zeroed() };
        limits.basic.flags = 0x2000; // JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE.
        if unsafe {
            SetInformationJobObject(
                handle,
                9,
                &limits as *const _ as *const std::ffi::c_void,
                std::mem::size_of::<JobLimits>() as u32,
            ) == 0
                || AssignProcessToJobObject(handle, child.as_raw_handle()) == 0
        } {
            return Err(std::io::Error::last_os_error().to_string());
        }
        Ok(job)
    }

    fn terminate(&self) {
        unsafe {
            TerminateJobObject(self.0, 1);
        }
    }
}

#[cfg(windows)]
impl Drop for WindowsJob {
    fn drop(&mut self) {
        unsafe {
            CloseHandle(self.0);
        }
    }
}

#[cfg(windows)]
#[repr(C)]
struct BasicJobLimits {
    process_time: i64,
    job_time: i64,
    flags: u32,
    min_working_set: usize,
    max_working_set: usize,
    active_processes: u32,
    affinity: usize,
    priority: u32,
    scheduling: u32,
}

#[cfg(windows)]
#[repr(C)]
struct JobLimits {
    basic: BasicJobLimits,
    io: [u64; 6],
    process_memory: usize,
    job_memory: usize,
    peak_process_memory: usize,
    peak_job_memory: usize,
}

#[cfg(windows)]
#[link(name = "kernel32")]
unsafe extern "system" {
    fn CreateJobObjectW(
        attributes: *const std::ffi::c_void,
        name: *const u16,
    ) -> *mut std::ffi::c_void;
    fn SetInformationJobObject(
        job: *mut std::ffi::c_void,
        class: u32,
        information: *const std::ffi::c_void,
        length: u32,
    ) -> i32;
    fn AssignProcessToJobObject(job: *mut std::ffi::c_void, process: *mut std::ffi::c_void) -> i32;
    fn TerminateJobObject(job: *mut std::ffi::c_void, code: u32) -> i32;
    fn CloseHandle(handle: *mut std::ffi::c_void) -> i32;
}

#[cfg(unix)]
fn nonblocking(pipe: &impl std::os::fd::AsRawFd) -> Result<(), String> {
    #[cfg(any(target_os = "linux", target_os = "android"))]
    const O_NONBLOCK: i32 = 0x800;
    #[cfg(not(any(target_os = "linux", target_os = "android")))]
    const O_NONBLOCK: i32 = 4;
    let fd = pipe.as_raw_fd();
    unsafe {
        let flags = fcntl(fd, 3);
        if flags == -1 || fcntl(fd, 4, flags | O_NONBLOCK) == -1 {
            return Err(std::io::Error::last_os_error().to_string());
        }
    }
    Ok(())
}

#[cfg(unix)]
unsafe extern "C" {
    fn fcntl(fd: i32, command: i32, ...) -> i32;
    fn kill(pid: i32, signal: i32) -> i32;
}

fn message(text: &str, request_id: Option<u64>) -> PluginUpdate {
    PluginUpdate {
        request_id,
        effects: vec![UpdateEffect::Message {
            text: format!("LSP: {}", clean(text)),
        }],
    }
}

fn hover_text(result: &Value, source: &str) -> Result<Option<String>, String> {
    if result.is_null() {
        return Ok(None);
    }
    let object = result.as_object().ok_or("invalid LSP hover result")?;
    if let Some(value) = object.get("range") {
        range(&normalized(source), value)?;
    }
    let contents = object.get("contents").ok_or("missing LSP hover contents")?;
    let mut text = String::new();
    match contents {
        Value::Array(items) => {
            for item in items {
                append_hover(&mut text, marked_string(item)?)?;
            }
        }
        Value::Object(object) if object.contains_key("kind") => {
            match object.get("kind").and_then(Value::as_str) {
                Some("plaintext" | "markdown") => {}
                _ => return Err("invalid LSP hover markup kind".into()),
            }
            append_hover(
                &mut text,
                object
                    .get("value")
                    .and_then(Value::as_str)
                    .ok_or("invalid LSP hover markup value")?,
            )?;
        }
        value => append_hover(&mut text, marked_string(value)?)?,
    }
    Ok((!text.trim().is_empty()).then_some(text))
}

fn marked_string(value: &Value) -> Result<&str, String> {
    match value {
        Value::String(text) => Ok(text),
        Value::Object(object) if object.get("language").and_then(Value::as_str).is_some() => object
            .get("value")
            .and_then(Value::as_str)
            .ok_or_else(|| "invalid LSP marked string value".into()),
        _ => Err("invalid LSP marked string".into()),
    }
}

fn append_hover(text: &mut String, value: &str) -> Result<(), String> {
    if value.is_empty() {
        return Ok(());
    }
    if !text.is_empty() {
        if text.len() + 2 > MAX_BODY {
            return Err("LSP hover contents exceed 4 MiB".into());
        }
        text.push_str("\n\n");
    }
    let mut characters = value.chars().peekable();
    while let Some(character) = characters.next() {
        let character = match character {
            '\r' => {
                if characters.peek() == Some(&'\n') {
                    characters.next();
                }
                '\n'
            }
            '\n' | '\t' => character,
            character if character.is_control() => ' ',
            character => character,
        };
        let length = if character == '\t' {
            4
        } else {
            character.len_utf8()
        };
        if text.len() + length > MAX_BODY {
            return Err("LSP hover contents exceed 4 MiB".into());
        }
        if character == '\t' {
            text.push_str("    ");
        } else {
            text.push(character);
        }
    }
    Ok(())
}

fn clean(text: &str) -> String {
    text.chars()
        .map(|character| {
            if character.is_control() {
                ' '
            } else {
                character
            }
        })
        .collect()
}

fn normalized(text: &str) -> std::borrow::Cow<'_, str> {
    if text.contains('\r') {
        std::borrow::Cow::Owned(text.replace("\r\n", "\n").replace('\r', "\n"))
    } else {
        std::borrow::Cow::Borrowed(text)
    }
}

fn completion_string<'a>(value: &'a Value, field: &str) -> Result<Option<&'a str>, String> {
    value
        .get(field)
        .map(|value| {
            value
                .as_str()
                .ok_or_else(|| format!("invalid completion {field}"))
        })
        .transpose()
}

fn completion_text(text: &str, multiline: bool) -> Result<(), String> {
    if text
        .chars()
        .any(|ch| ch.is_control() && !(multiline && matches!(ch, '\n' | '\r' | '\t')))
    {
        return Err("unsupported control character in completion".into());
    }
    Ok(())
}

fn completion_position(lines: &[&str], value: &Value) -> Result<TextPosition, String> {
    let (row, units) = coordinates(value)?;
    let line = lines
        .get(row)
        .ok_or("completion line is outside the document")?;
    let mut offset = 0;
    let mut column = 0;
    for cluster in line.graphemes(true) {
        if units == offset {
            return Ok(TextPosition { row, column });
        }
        offset += cluster.encode_utf16().count();
        if units < offset {
            return Err("completion position splits a grapheme".into());
        }
        column += 1;
    }
    if units != offset {
        return Err("completion position is outside the line".into());
    }
    Ok(TextPosition { row, column })
}

fn completion_range(lines: &[&str], value: &Value) -> Result<(TextPosition, TextPosition), String> {
    let start = completion_position(lines, &value["start"])?;
    let end = completion_position(lines, &value["end"])?;
    if (start.row, start.column) > (end.row, end.column) {
        return Err("reversed completion range".into());
    }
    Ok((start, end))
}

fn completion_primary_range(
    lines: &[&str],
    value: &Value,
    source: &Source,
) -> Result<(TextPosition, TextPosition), String> {
    let (start, end) = completion_range(lines, value)?;
    if start.row != source.row
        || end.row != source.row
        || start.column > source.column
        || end.column < source.column
    {
        return Err(
            "completion primary range must be single-line and contain the request position".into(),
        );
    }
    Ok((start, end))
}

fn completion_replacement(
    lines: &[&str],
    value: &Value,
    source: &Source,
) -> Result<(TextPosition, TextPosition), String> {
    if value.get("insert").is_some() || value.get("replace").is_some() {
        if value.get("range").is_some() || value.get("start").is_some() {
            return Err("ambiguous completion edit range".into());
        }
        let (start, end) = completion_primary_range(lines, &value["replace"], source)?;
        let (insert_start, insert_end) = completion_primary_range(lines, &value["insert"], source)?;
        if insert_start != start || insert_end.column > end.column {
            return Err("completion insert range is not a prefix of replace range".into());
        }
        Ok((start, end))
    } else {
        completion_primary_range(lines, value.get("range").unwrap_or(value), source)
    }
}

fn decode_completion(
    text: &str,
    result: &Value,
    source: &Source,
    path: Option<&str>,
) -> Result<Vec<CompletionItem>, String> {
    if result.is_null() {
        return Ok(Vec::new());
    }
    let (items, defaults) = if let Some(items) = result.as_array() {
        (items, None)
    } else {
        if !result["isIncomplete"].is_boolean() {
            return Err("invalid completion list isIncomplete".into());
        }
        let defaults = result.get("itemDefaults");
        if defaults.is_some_and(|value| !value.is_object()) {
            return Err("invalid completion itemDefaults".into());
        }
        if let Some(defaults) = defaults {
            for key in defaults.as_object().unwrap().keys() {
                if !matches!(
                    key.as_str(),
                    "editRange" | "insertTextFormat" | "insertTextMode"
                ) {
                    return Err("unsupported completion item default".into());
                }
            }
        }
        (
            result["items"]
                .as_array()
                .ok_or("invalid completion items")?,
            defaults,
        )
    };
    let text = normalized(text);
    let lines: Vec<_> = text.split('\n').collect();
    let line = lines
        .get(source.row)
        .ok_or("completion request line is outside the document")?;
    let mut prefix_start = 0;
    let mut prefix_byte = 0;
    let mut caret_byte = line.len();
    let mut columns = 0;
    for (column, (byte, cluster)) in line.grapheme_indices(true).enumerate() {
        if column == source.column {
            caret_byte = byte;
            break;
        }
        if !cluster
            .chars()
            .next()
            .is_some_and(|ch| ch.is_alphanumeric() || ch == '_')
        {
            prefix_start = column + 1;
            prefix_byte = byte + cluster.len();
        }
        columns = column + 1;
    }
    if source.column > columns {
        return Err("completion request column is outside the line".into());
    }
    let prefix = &line[prefix_byte..caret_byte];
    // The prefix is the only guessed range; explicit server edits remain authoritative.
    if let Some(defaults) = defaults {
        if let Some(range) = defaults.get("editRange") {
            completion_replacement(&lines, range, source)?;
        }
        for name in ["insertTextFormat", "insertTextMode"] {
            if defaults
                .get(name)
                .is_some_and(|value| !matches!(value.as_u64(), Some(1 | 2)))
            {
                return Err(format!("invalid completion default {name}"));
            }
        }
    }
    let word_end = caret_byte
        + line[caret_byte..]
            .graphemes(true)
            .take_while(|cluster| {
                cluster
                    .chars()
                    .next()
                    .is_some_and(|ch| ch.is_alphanumeric() || ch == '_')
            })
            .map(str::len)
            .sum::<usize>();
    let mut decoded = Vec::with_capacity(items.len());
    let mut bytes = 0usize;
    for item in items {
        if !item.is_object() {
            return Err("invalid completion item".into());
        }
        let label = completion_string(item, "label")?.ok_or("missing completion label")?;
        completion_text(label, false)?;
        let detail = completion_string(item, "detail")?;
        let sort = completion_string(item, "sortText")?.unwrap_or(label);
        let filter = completion_string(item, "filterText")?;
        let insert = completion_string(item, "insertText")?;
        let edit_text = completion_string(item, "textEditText")?;
        if let Some(detail) = detail {
            completion_text(detail, true)?;
        }
        completion_text(sort, false)?;
        if let Some(filter) = filter {
            completion_text(filter, false)?;
        }
        let property = |name: &str| {
            item.get(name)
                .or_else(|| defaults.and_then(|value| value.get(name)))
        };
        let format = property("insertTextFormat")
            .map(|value| {
                value
                    .as_u64()
                    .filter(|value| matches!(value, 1 | 2))
                    .ok_or("invalid completion insertTextFormat")
            })
            .transpose()?
            .unwrap_or(1);
        let mode = property("insertTextMode")
            .map(|value| {
                value
                    .as_u64()
                    .filter(|value| matches!(value, 1 | 2))
                    .ok_or("invalid completion insertTextMode")
            })
            .transpose()?
            .unwrap_or(1);
        // No indentation adjustment, server commands or lazy edit resolution.
        if mode == 2 || item.get("command").is_some() {
            continue;
        }
        let explicit = item.get("textEdit");
        let default_range = defaults.and_then(|value| value.get("editRange"));
        let (start, end, new_text) = if let Some(edit) = explicit {
            if !edit.is_object() {
                return Err("invalid completion textEdit".into());
            }
            let (start, end) = completion_replacement(&lines, edit, source)?;
            let new_text =
                completion_string(edit, "newText")?.ok_or("missing completion newText")?;
            (start, end, new_text)
        } else if let Some(range) = default_range {
            let (start, end) = completion_replacement(&lines, range, source)?;
            (start, end, edit_text.or(insert).unwrap_or(label))
        } else {
            (
                TextPosition {
                    row: source.row,
                    column: prefix_start,
                },
                TextPosition {
                    row: source.row,
                    column: source.column,
                },
                insert.unwrap_or(label),
            )
        };
        let filter_prefix = if explicit.is_some() || default_range.is_some() {
            let byte = line
                .grapheme_indices(true)
                .nth(start.column)
                .map_or(line.len(), |(byte, _)| byte);
            &line[byte..caret_byte]
        } else {
            prefix
        };
        completion_text(new_text, true)?;
        let (new_text, snippet) = if format == 2 {
            match snippet::expand(new_text, |name| {
                completion_variable(name, line, &line[prefix_byte..word_end], source.row, path)
            }) {
                Ok((text, snippet)) => (text, Some(snippet)),
                Err(snippet::Error::Unsupported) => continue,
                Err(snippet::Error::Invalid) => return Err("invalid completion snippet".into()),
                Err(snippet::Error::Limit) => {
                    return Err("completion snippet exceeds expansion limits".into());
                }
            }
        } else {
            (normalized(new_text).into_owned(), None)
        };
        completion_text(&new_text, true)?;
        let mut edits = vec![CompletionEdit {
            start,
            end,
            text: new_text,
        }];
        if let Some(additional) = item.get("additionalTextEdits") {
            for edit in additional
                .as_array()
                .ok_or("invalid completion additionalTextEdits")?
            {
                let (start, end) = completion_range(&lines, &edit["range"])?;
                let new_text =
                    completion_string(edit, "newText")?.ok_or("missing additional edit newText")?;
                completion_text(new_text, true)?;
                edits.push(CompletionEdit {
                    start,
                    end,
                    text: normalized(new_text).into_owned(),
                });
            }
        }
        edits.sort_unstable_by_key(|edit| {
            (
                (edit.start.row, edit.start.column),
                (edit.end.row, edit.end.column),
            )
        });
        if edits.windows(2).any(|pair| {
            (pair[0].end.row, pair[0].end.column) > (pair[1].start.row, pair[1].start.column)
                || pair[0].start == pair[1].start
        }) {
            return Err("overlapping completion edits".into());
        }
        let size = label.len()
            + detail.map_or(0, str::len)
            + edits.iter().map(|edit| edit.text.len()).sum::<usize>();
        bytes = bytes
            .checked_add(size)
            .filter(|bytes| *bytes <= MAX_BODY)
            .ok_or("completion text exceeds 4 MiB")?;
        if (filter.is_some() || (explicit.is_none() && default_range.is_none()))
            && !filter.unwrap_or(label).starts_with(filter_prefix)
        {
            continue;
        }
        let primary_edit = edits
            .iter()
            .position(|edit| edit.start == start && edit.end == end)
            .unwrap();
        decoded.push((
            sort,
            CompletionItem {
                label: label.to_owned(),
                detail: detail.map(str::to_owned),
                primary_edit,
                edits,
                snippet,
            },
        ));
    }
    decoded.sort_by(|a, b| a.0.cmp(b.0));
    Ok(decoded.into_iter().map(|(_, item)| item).collect())
}

fn completion_variable(
    name: &str,
    line: &str,
    word: &str,
    row: usize,
    path: Option<&str>,
) -> Option<String> {
    let value = match name {
        "TM_SELECTED_TEXT" => "",
        "TM_CURRENT_LINE" => line,
        "TM_CURRENT_WORD" => word,
        "TM_LINE_INDEX" => return Some(row.to_string()),
        "TM_LINE_NUMBER" => return Some((row + 1).to_string()),
        "TM_FILEPATH" => path.unwrap_or(""),
        "TM_FILENAME" => path
            .and_then(|path| Path::new(path).file_name())
            .and_then(|name| name.to_str())
            .unwrap_or(""),
        "TM_FILENAME_BASE" => path
            .and_then(|path| Path::new(path).file_stem())
            .and_then(|name| name.to_str())
            .unwrap_or(""),
        "TM_DIRECTORY" => path
            .and_then(|path| Path::new(path).parent())
            .and_then(|name| name.to_str())
            .unwrap_or(""),
        _ => return None,
    };
    // Context values have the same per-expansion bound as template text.
    (value.len() <= 256 * 1024).then(|| value.to_owned())
}

fn coordinates(value: &Value) -> Result<(usize, usize), String> {
    let line = value["line"]
        .as_u64()
        .filter(|number| *number <= MAX_VERSION)
        .ok_or("invalid LSP line")?;
    let character = value["character"]
        .as_u64()
        .filter(|number| *number <= MAX_VERSION)
        .ok_or("invalid LSP character")?;
    Ok((line as usize, character as usize))
}

fn position(text: &str, value: &Value, round_up: bool) -> Result<TextPosition, String> {
    let (row, units) = coordinates(value)?;
    let line = text
        .split('\n')
        .nth(row)
        .ok_or("LSP line is outside the document")?;
    let mut offset = 0;
    let mut column = 0;
    for cluster in line.graphemes(true) {
        if units == offset {
            return Ok(TextPosition { row, column });
        }
        let width = cluster.encode_utf16().count();
        if units < offset + width {
            return Ok(TextPosition {
                row,
                column: column + usize::from(round_up),
            });
        }
        offset += width;
        column += 1;
    }
    Ok(TextPosition { row, column })
}

fn range(text: &str, value: &Value) -> Result<(TextPosition, TextPosition), String> {
    if coordinates(&value["start"])? > coordinates(&value["end"])? {
        return Err("reversed LSP range".into());
    }
    Ok((
        position(text, &value["start"], false)?,
        position(text, &value["end"], true)?,
    ))
}

fn outbound_position(text: &str, row: usize, column: usize) -> Result<Value, String> {
    let text = normalized(text);
    let line = text
        .split('\n')
        .nth(row)
        .ok_or("LSP source row is outside the document")?;
    let mut units = 0;
    let mut clusters = line.graphemes(true);
    for _ in 0..column {
        units += clusters
            .next()
            .ok_or("LSP source column is outside the line")?
            .encode_utf16()
            .count();
    }
    Ok(json!({"line": row, "character": units}))
}

fn absolute(path: &Path, cwd: &Path) -> PathBuf {
    let joined;
    let path = if path.is_absolute() {
        path
    } else {
        joined = cwd.join(path);
        &joined
    };
    let mut result = PathBuf::new();
    for component in path.components() {
        match component {
            Component::CurDir => {}
            Component::ParentDir => {
                result.pop();
            }
            component => result.push(component.as_os_str()),
        }
    }
    result
}

fn file_uri(path: &Path) -> Result<String, String> {
    use std::fmt::Write as _;
    let path = path.to_str().ok_or("LSP path is not UTF-8")?;
    #[cfg(windows)]
    let windows_path = path.replace('\\', "/");
    #[cfg(windows)]
    let path = windows_path.as_str();
    let mut uri = String::from("file://");
    #[cfg(windows)]
    if path.as_bytes().get(1) == Some(&b':') {
        uri.push('/');
    }
    for byte in path.bytes() {
        if byte.is_ascii_alphanumeric() || b"-._~/".contains(&byte) {
            uri.push(byte as char);
        } else {
            let _ = write!(uri, "%{byte:02X}");
        }
    }
    Ok(uri)
}

fn uri_path(uri: &str) -> Result<PathBuf, String> {
    let unsupported = || format!("unsupported local file URI: {}", clean(uri));
    let mut path = uri.strip_prefix("file:").ok_or_else(unsupported)?;
    if let Some(rest) = path.strip_prefix("//") {
        let (host, _) = rest.split_once('/').ok_or_else(unsupported)?;
        if !host.is_empty() && host != "localhost" {
            return Err(unsupported());
        }
        path = &rest[host.len()..];
    }
    if !path.starts_with('/') || path.contains(['?', '#']) {
        return Err(unsupported());
    }
    let mut decoded = Vec::with_capacity(path.len());
    let bytes = path.as_bytes();
    let mut index = 0;
    while index < bytes.len() {
        if bytes[index] == b'%' {
            let pair = bytes.get(index + 1..index + 3).ok_or_else(unsupported)?;
            let hex = std::str::from_utf8(pair).map_err(|_| unsupported())?;
            decoded.push(u8::from_str_radix(hex, 16).map_err(|_| unsupported())?);
            index += 3;
        } else {
            decoded.push(bytes[index]);
            index += 1;
        }
    }
    let path = String::from_utf8(decoded).map_err(|_| "invalid UTF-8 file URI")?;
    if path.chars().any(char::is_control) {
        return Err("invalid control character in file URI".into());
    }
    #[cfg(windows)]
    let path = if path.as_bytes().get(2) == Some(&b':') && path.starts_with('/') {
        path[1..].to_owned()
    } else {
        path
    };
    Ok(absolute(Path::new(&path), Path::new("/")))
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn semantic_decoder_validates_raw_coordinates_before_grapheme_rounding() {
        let options = semantic_options(Some(&json!({
            "full": true,
            "legend": {"tokenTypes": ["custom", "function"],
                "tokenModifiers": ["custom", "readonly", "declaration", "async"]}
        })))
        .unwrap()
        .unwrap();
        let tokens = decode_semantic_tokens(
            "A😀e\u{301}Z\r\n\r\nx",
            &json!({"data": [
                0, 1, 2, 1, 2,
                0, 2, 1, 1, 12,
                0, 1, 1, 1, 0,
                0, 1, 999, 0, 0,
                1, 0, 1, 1, 0,
                1, 0, 0, 1, 0,
                0, 0, 100, 1, 0
            ]}),
            &options,
        )
        .unwrap()
        .unwrap();
        assert_eq!(tokens.len(), 4);
        assert_eq!(tokens[0].start, TextPosition { row: 0, column: 1 });
        assert_eq!(tokens[0].end, TextPosition { row: 0, column: 2 });
        assert_eq!(tokens[0].modifiers, SemanticTokenModifier::Readonly as u16);
        assert_eq!(tokens[1].start, TextPosition { row: 0, column: 2 });
        assert_eq!(tokens[1].end, TextPosition { row: 0, column: 3 });
        assert_eq!(
            tokens[1].modifiers,
            SemanticTokenModifier::Declaration as u16 | SemanticTokenModifier::Async as u16
        );
        assert_eq!(tokens[2].start, tokens[1].start);
        assert_eq!(tokens[3].end, TextPosition { row: 2, column: 1 });
        for data in [
            json!([0, 0, 1, 1]),
            json!([-1, 0, 1, 1, 0]),
            json!([0, 0.5, 1, 1, 0]),
            json!([0, 0, 1, 1, 2147483648u64]),
            json!([0, 0, 1, 65536, 0]),
            json!([0, 0, 1, 2, 0]),
            json!([0, 0, 1, 1, 16]),
            json!([0, 2, 1, 0, 0]),
            json!([0, 1, 1, 1, 0]),
            json!([0, 7, 1, 1, 0]),
            json!([2, 0, 1, 1, 0]),
            json!([0, 1, 2, 1, 0, 0, 1, 1, 1, 0]),
        ] {
            assert!(
                decode_semantic_tokens("A😀e\u{301}Z\n", &json!({"data": data}), &options).is_err(),
                "{data}"
            );
        }
        let count = MAX_BODY / std::mem::size_of::<SemanticToken>() + 1;
        assert!(
            decode_semantic_tokens("x", &json!({"data": vec![0; count * 5]}), &options).is_err()
        );
    }

    #[test]
    fn unicode_positions_and_local_uri_boundaries() {
        let text = "A😀e\u{301}Z\n";
        let (start, end) = range(
            text,
            &json!({"start": {"line": 0, "character": 2}, "end": {"line": 0, "character": 4}}),
        )
        .unwrap();
        assert_eq!(start, TextPosition { row: 0, column: 1 });
        assert_eq!(end, TextPosition { row: 0, column: 3 });
        assert_eq!(
            outbound_position(text, 0, 3).unwrap(),
            json!({"line": 0, "character": 5})
        );
        assert_eq!(
            position(text, &json!({"line": 1, "character": 999}), false).unwrap(),
            TextPosition { row: 1, column: 0 }
        );
        assert!(
            range(
                text,
                &json!({"start": {"line": 0, "character": 3}, "end": {"line": 0, "character": 2}})
            )
            .is_err()
        );
        let path = Path::new("/tmp/source space λ😀.rs");
        assert_eq!(uri_path(&file_uri(path).unwrap()).unwrap(), path);
        assert_eq!(
            uri_path("file://localhost/tmp/a%23b").unwrap(),
            Path::new("/tmp/a#b")
        );
        for uri in [
            "https://example.invalid/a",
            "file://remote/tmp/a",
            "file:///tmp/%",
            "file:///tmp/%00",
            "file:///tmp/%FF",
            "file:///tmp/a?query",
        ] {
            assert!(uri_path(uri).is_err(), "{uri}");
        }
    }

    #[test]
    fn hover_contents_keep_lines_and_reject_malformed_values() {
        let source = "A😀e\u{301}Z\r\n";
        for kind in ["plaintext", "markdown"] {
            assert_eq!(
                hover_text(
                    &json!({
                        "contents": {"kind": kind, "value": "😀e\u{301}\r\n\tcode\rnext\u{1b}\u{85}"},
                        "range": {"start": {"line": 0, "character": 2},
                            "end": {"line": 0, "character": 4}}
                    }),
                    source,
                )
                .unwrap(),
                Some("😀e\u{301}\n    code\nnext  ".into()),
            );
        }
        for contents in [
            json!("line\ncode"),
            json!({"language": "rust", "value": "line\ncode"}),
        ] {
            assert_eq!(
                hover_text(&json!({"contents": contents}), source).unwrap(),
                Some("line\ncode".into()),
            );
        }
        assert_eq!(
            hover_text(
                &json!({"contents": ["first", {"language": "rust", "value": "fn main() {\n}"}]}),
                source,
            )
            .unwrap(),
            Some("first\n\nfn main() {\n}".into()),
        );
        for value in [
            Value::Null,
            json!({"contents": ""}),
            json!({"contents": []}),
        ] {
            assert_eq!(hover_text(&value, source).unwrap(), None);
        }
        for value in [
            json!({}),
            json!({"contents": {"kind": "html", "value": "x"}}),
            json!({"contents": {"language": "rust", "value": false}}),
            json!({"contents": [{"kind": "markdown", "value": "x"}]}),
            json!({"contents": "x", "range": {"start": {"line": 1, "character": 0},
                "end": {"line": 0, "character": 0}}}),
            json!({"contents": "\t".repeat(MAX_BODY / 4 + 1)}),
        ] {
            assert!(hover_text(&value, source).is_err());
        }
    }

    #[test]
    fn completion_text_limit_counts_the_atomic_batch_not_sort_metadata() {
        let source = Source {
            buffer_id: 1,
            revision: 0,
            row: 0,
            column: 0,
            request_id: 1,
        };
        let at_limit = json!([{"label": "x", "insertText": "x".repeat(MAX_BODY - 1)}]);
        assert_eq!(
            decode_completion("", &at_limit, &source, None).unwrap()[0].edits[0]
                .text
                .len(),
            MAX_BODY - 1
        );
        let over_limit = json!([
            {"label": "x", "insertText": "x".repeat(MAX_BODY - 1)},
            {"label": "y"}
        ]);
        assert!(
            decode_completion("", &over_limit, &source, None)
                .unwrap_err()
                .contains("4 MiB")
        );
    }
}