vamp/src/diff.rs

use crate::editor::{BackspaceKind, Input};
use crate::files::{FilePicker, FilePresentation, FileView, fuzzy_match};
use std::{
    borrow::Cow,
    collections::{BTreeMap, BTreeSet},
    path::{Path, PathBuf},
    sync::Arc,
    time::{Duration, Instant},
};
use unicode_segmentation::UnicodeSegmentation;
use unicode_width::UnicodeWidthStr;

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum RepoKind {
    Jujutsu,
    Git,
}
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum Category {
    Revisions,
    Bookmarks,
    Commits,
    Branches,
    Tags,
    Open,
    Closed,
    All,
}
impl Category {
    pub fn label(self) -> &'static str {
        match self {
            Self::Revisions => "Revisions",
            Self::Bookmarks => "Bookmarks",
            Self::Commits => "Commits",
            Self::Branches => "Branches",
            Self::Tags => "Tags",
            Self::Open => "Open",
            Self::Closed => "Closed",
            Self::All => "All",
        }
    }
}
impl RepoKind {
    pub fn categories(self) -> &'static [Category] {
        match self {
            Self::Jujutsu => &[Category::Revisions, Category::Bookmarks],
            Self::Git => &[Category::Commits, Category::Branches, Category::Tags],
        }
    }
}
#[derive(Clone, Copy, Debug, Default, Eq, PartialEq)]
pub enum ComparisonMode {
    #[default]
    BranchChanges,
    Snapshots,
}
impl ComparisonMode {
    pub fn label(self) -> &'static str {
        match self {
            Self::BranchChanges => "Branch changes",
            Self::Snapshots => "Snapshots",
        }
    }
}
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum RevisionTarget {
    Working,
    Commit(String),
    Bookmark {
        name: String,
        remote: Option<String>,
    },
    Remote {
        remote: String,
        reference: String,
    },
    RemoteCommit {
        remote: String,
        reference: String,
        id: String,
    },
    PullRequest {
        remote: String,
        number: u64,
    },
    ListRemote(String),
    FetchRemote(String),
}
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Revision {
    pub target: RevisionTarget,
    pub label: String,
    pub description: String,
    pub category: Category,
}
#[derive(Debug)]
pub struct Catalog {
    pub kind: RepoKind,
    pub entries: Vec<Revision>,
    pub notice: Option<String>,
}
#[derive(Debug)]
pub struct PullRequestPage {
    pub kind: RepoKind,
    pub remote: String,
    pub entries: Vec<(Revision, String)>,
    pub has_more: bool,
}

#[derive(Debug, Default)]
struct PullRequestCategory {
    complete: bool,
    error: Option<String>,
}
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum FileStatus {
    Added,
    Removed,
    Modified,
    Renamed,
}
#[derive(Debug)]
pub struct Hunk {
    pub before_start: usize,
    pub before_len: usize,
    pub after_start: usize,
    pub after_len: usize,
}
#[derive(Debug)]
pub struct ChangedFile {
    /// Immutable file-side identities, independent of unrelated revision changes.
    pub review_revisions: Option<[String; 2]>,
    pub path: PathBuf,
    pub old_path: Option<PathBuf>,
    pub status: FileStatus,
    /// False means text, EOF flags, hunks and notices have not been prepared.
    pub loaded: bool,
    pub before: Vec<String>,
    pub after: Vec<String>,
    pub before_eof: bool,
    pub after_eof: bool,
    pub hunks: Vec<Hunk>,
    pub notice: Option<String>,
}
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct BlameRevision {
    pub store: PathBuf,
    pub commit: String,
    pub(crate) keepalive: Option<Arc<crate::repository::Temporary>>,
}

#[derive(Debug)]
pub struct Comparison {
    /// Immutable comparison-context IDs; captured review text also freezes live edits.
    pub revisions: [String; 2],
    /// Git object stores and immutable attribution baselines for Before/After.
    pub blame: [Option<BlameRevision>; 2],
    pub before: String,
    pub after: String,
    pub files: Vec<ChangedFile>,
    pub root: PathBuf,
    /// Eligible regular-file After paths; loaded binary details are excluded.
    pub editable_paths: BTreeSet<PathBuf>,
}
#[derive(Debug)]
pub enum Operation {
    Discover,
    PullRequests(String),
    ResolvePullRequest(Revision),
    ListRemote(String),
    FetchRemote(String),
    Compare {
        before: Revision,
        after: Revision,
        mode: ComparisonMode,
        buffers: Box<[(PathBuf, String)]>,
    },
    RefreshCheckout {
        comparison_id: u64,
        follow_working: bool,
        buffers: Box<[(PathBuf, String)]>,
    },
    LoadFile {
        comparison_id: u64,
        file_index: usize,
    },
}
#[derive(Debug)]
pub enum Outcome {
    Catalog(Catalog),
    PullRequestPage {
        closed: bool,
        result: Result<PullRequestPage, String>,
    },
    ResolvedPullRequest {
        before: Revision,
        after: Revision,
    },
    Comparison(Comparison),
    CheckoutRefresh(Option<Comparison>),
    BookmarkRefresh(Comparison),
    File {
        comparison_id: u64,
        file_index: usize,
        file: ChangedFile,
    },
}
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum Phase {
    Before,
    After,
    View,
}
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum Focus {
    Files,
    Before,
    After,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq, serde::Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum ViewMode {
    Split,
    Unified,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub(crate) enum PaneLayout {
    Split,
    Before,
    After,
    Empty,
}
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum RowKind {
    Equal,
    Added,
    Removed,
    Changed,
}
#[derive(Debug)]
pub struct AlignedRow {
    pub before: Option<usize>,
    pub after: Option<usize>,
    pub kind: RowKind,
    /// Changed grapheme ranges, paired as [before, after]; empty on unpaired rows.
    pub changes: Vec<[std::ops::Range<usize>; 2]>,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum DisplayRow {
    Line(usize),
    Fold { start: usize, end: usize },
}

/// Borrowed painting and native-coordinate projection of a loaded comparison.
#[derive(Clone, Copy, Debug)]
pub(crate) struct ColumnView<'a> {
    diff: &'a DiffState,
    file: &'a ChangedFile,
    pub info: ColumnInfo<'a>,
}

#[derive(Clone, Copy, Debug)]
pub(crate) struct ColumnInfo<'a> {
    pub focus: Focus,
    pub unified: bool,
    pub display_len: usize,
    pub source_extents: [usize; 2],
    pub stored_lengths: [usize; 2],
    pub empty: ColumnEmpty<'a>,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub(crate) enum ColumnEmpty<'a> {
    EmptyFile,
    NoChanges,
    Notice(&'a str),
}

#[derive(Debug)]
pub(crate) enum ColumnRow<'a> {
    Fold { hidden: usize },
    Gap,
    Text(ColumnLine<'a>),
}

#[derive(Debug)]
pub(crate) struct ColumnLine<'a> {
    pub text: Option<&'a str>,
    pub side: Focus,
    pub source_row: usize,
    pub numbers: [Option<usize>; 2],
    pub changed: bool,
    pub emphasize: bool,
    pub newline: bool,
    changes: Cow<'a, [[std::ops::Range<usize>; 2]]>,
}

impl<'a> ColumnLine<'a> {
    pub fn changes(&self) -> impl Iterator<Item = &std::ops::Range<usize>> + '_ {
        let side = usize::from(self.side == Focus::After);
        let emphasize = self.emphasize;
        self.changes
            .iter()
            .filter(move |_| emphasize)
            .map(move |change| &change[side])
    }
}

impl<'a> ColumnView<'a> {
    fn new(diff: &'a DiffState, file: &'a ChangedFile, focus: Focus) -> Self {
        let stored_lengths = [file.before.len(), file.after.len()];
        let last = diff.rows.last();
        let source_extents = [
            stored_lengths[0].max(last.and_then(|row| row.before).map_or(0, |row| row + 1)),
            stored_lengths[1].max(last.and_then(|row| row.after).map_or(0, |row| row + 1)),
        ];
        Self {
            diff,
            file,
            info: ColumnInfo {
                focus,
                unified: diff.view_mode == ViewMode::Unified,
                display_len: diff.display_rows.len(),
                source_extents,
                stored_lengths,
                empty: match file.notice.as_deref() {
                    Some(notice) => ColumnEmpty::Notice(notice),
                    None if diff.rows.is_empty() => ColumnEmpty::EmptyFile,
                    None => ColumnEmpty::NoChanges,
                },
            },
        }
    }

    pub fn row(&self, display: usize) -> Option<ColumnRow<'a>> {
        let index = match *self.diff.display_rows.get(display)? {
            DisplayRow::Fold { start, end } => {
                return Some(ColumnRow::Fold {
                    hidden: end - start,
                });
            }
            DisplayRow::Line(index) => index,
        };
        let row = self.diff.rows.get(index)?;
        let side = if self.info.unified {
            self.diff.display_focus(display).unwrap_or(self.info.focus)
        } else {
            self.info.focus
        };
        let Some(source_row) = self.diff.display_source_row(side, display) else {
            return Some(ColumnRow::Gap);
        };
        let before = side == Focus::Before;
        let side_index = usize::from(!before);
        let (source, eof) = if before {
            (&self.file.before, self.file.before_eof)
        } else {
            (&self.file.after, self.file.after_eof)
        };
        let text = source
            .get(source_row)
            .map(String::as_str)
            .or_else(|| (self.diff.terminal_rows[side_index] == Some(source_row)).then_some(""));
        Some(ColumnRow::Text(ColumnLine {
            text,
            side,
            source_row,
            numbers: [
                self.diff.display_source_row(Focus::Before, display),
                self.diff.display_source_row(Focus::After, display),
            ],
            changed: row.kind == RowKind::Changed
                || (before && row.kind == RowKind::Removed)
                || (!before && row.kind == RowKind::Added),
            emphasize: !matches!(self.file.status, FileStatus::Added | FileStatus::Removed),
            newline: source_row + 1 < source.len() || eof,
            changes: if self.diff.lazy_changes && row.kind == RowKind::Changed {
                match (row.before, row.after) {
                    (Some(_), Some(_)) if self.file.hunks.is_empty() => {
                        // A stitched History base is changed by another column.
                        Cow::Owned(vec![[0..usize::MAX, 0..usize::MAX]])
                    }
                    (Some(before), Some(after)) => Cow::Owned(character_changes(
                        &self.file.before[before],
                        &self.file.after[after],
                    )),
                    _ => Cow::Borrowed(&row.changes),
                }
            } else {
                Cow::Borrowed(&row.changes)
            },
        }))
    }

    /// Caret mapping stays on this view's native side, even in unified display.
    pub fn native_row(&self, display: usize) -> Option<usize> {
        self.diff.display_source_row(self.info.focus, display)
    }

    /// Finds the first visible native match; never reveals folded source rows.
    pub fn display_row(&self, native: usize) -> Option<usize> {
        self.diff.source_display_row(self.info.focus, native)
    }

    /// Review anchors may address either coordinate of unified equal context,
    /// independently of the focused native caret mapping.
    pub fn review_row(&self, side: Focus, display: usize) -> Option<usize> {
        self.diff.display_source_row(side, display).or_else(|| {
            let DisplayRow::Line(index) = *self.diff.display_rows.get(display)? else {
                return None;
            };
            let row = self.diff.rows.get(index)?;
            if !self.info.unified || row.kind != RowKind::Equal {
                return None;
            }
            match side {
                Focus::Before => row.before,
                Focus::After => row.after,
                Focus::Files => None,
            }
        })
    }
}

/// Immutable left/base/right conflict History, aligned at shared base rows.
pub(crate) struct HistoryComparison {
    columns: [DiffState; 3],
}

/// A read-only projection for the existing diff painter.
pub(crate) struct HistoryColumn<'a> {
    pub diff: &'a DiffState,
    pub file: &'a ChangedFile,
    pub focus: Focus,
    pub lines: &'a [String],
}

impl<'a> HistoryColumn<'a> {
    pub fn view(&self) -> ColumnView<'a> {
        ColumnView::new(self.diff, self.file, self.focus)
    }

    pub fn source_row(&self, display: usize) -> Option<usize> {
        self.diff.display_source_row(self.focus, display)
    }
}

impl HistoryComparison {
    pub fn new(sources: [&str; 3]) -> Self {
        let [left, base, right] =
            sources.map(|text| text.split('\n').map(str::to_owned).collect::<Vec<_>>());
        let mut columns: [DiffState; 3] = std::array::from_fn(|_| DiffState::new());
        columns[0].set_resolver_text(&base, &left);
        columns[1].set_resolver_text(&base, &base);
        columns[2].set_resolver_text(&base, &right);
        let mut left = std::mem::take(&mut columns[0].rows).into_iter().peekable();
        let mut right = std::mem::take(&mut columns[2].rows).into_iter().peekable();
        columns[1].rows.clear();
        // Stitch the existing pairwise alignments at their shared base rows.
        while left.peek().is_some() || right.peek().is_some() {
            let insertion = left.peek().is_some_and(|row| row.before.is_none())
                || right.peek().is_some_and(|row| row.before.is_none());
            let take_left = left
                .peek()
                .is_some_and(|row| !insertion || row.before.is_none());
            let take_right = right
                .peek()
                .is_some_and(|row| !insertion || row.before.is_none());
            let empty = || AlignedRow {
                before: None,
                after: None,
                kind: RowKind::Equal,
                changes: Vec::new(),
            };
            let left_row = if take_left {
                left.next().unwrap()
            } else {
                empty()
            };
            let right_row = if take_right {
                right.next().unwrap()
            } else {
                empty()
            };
            let base = left_row.before.or(right_row.before);
            let changed = left_row.kind != RowKind::Equal || right_row.kind != RowKind::Equal;
            let changes = left_row
                .changes
                .iter()
                .chain(&right_row.changes)
                .map(|change| [change[0].clone(), change[0].clone()])
                .collect();
            columns[1].rows.push(AlignedRow {
                before: base,
                after: base,
                kind: if changed {
                    RowKind::Changed
                } else {
                    RowKind::Equal
                },
                changes,
            });
            columns[0].rows.push(left_row);
            columns[2].rows.push(right_row);
        }
        for column in &mut columns {
            column.display_rows = (0..column.rows.len()).map(DisplayRow::Line).collect();
            column.rebuild_source_ranges();
        }
        Self { columns }
    }

    pub fn row_count(&self) -> usize {
        self.columns[0].rows.len()
    }

    pub fn column(&self, index: usize) -> HistoryColumn<'_> {
        let diff = &self.columns[index];
        let file = &diff.comparison.as_ref().unwrap().files[0];
        let focus = if index == 1 {
            Focus::Before
        } else {
            Focus::After
        };
        let lines = if focus == Focus::Before {
            &file.before
        } else {
            &file.after
        };
        HistoryColumn {
            diff,
            file,
            focus,
            lines,
        }
    }
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum RequestKind {
    Catalog,
    Comparison,
    CheckoutRefresh {
        comparison_id: u64,
    },
    File {
        comparison_id: u64,
        file_index: usize,
    },
}

#[derive(Debug)]
pub struct DiffState {
    pub catalog: Option<Catalog>,
    pub pull_request_remote: Option<String>,
    pull_request_categories: [PullRequestCategory; 2],
    pull_request_order: BTreeMap<u64, (String, usize)>,
    pub(crate) pull_request_retries: [u64; 2],
    pub phase: Phase,
    pub word_wrap: bool,
    pub view_mode: ViewMode,
    pub comparison_mode: ComparisonMode,
    pub split_axis: crate::panes::SplitAxis,
    pub writable_side: Option<Focus>,
    pub side_labels: [&'static str; 2],
    pub before: Option<Revision>,
    pub after: Option<Revision>,
    pub category: usize,
    pub query: String,
    pub selected: usize,
    pub loading: bool,
    pub error: Option<String>,
    pub comparison: Option<Comparison>,
    pub local_review: crate::review::ReviewState,
    pub comparison_id: Option<u64>,
    pub reviewed_files: BTreeSet<usize>,
    pub(crate) snapshot_roots: [Option<PathBuf>; 2],
    pub files: Option<FilePicker>,
    pub files_visible: bool,
    pub file: usize,
    external_file_selection: bool,
    pub(crate) requested_file: Option<usize>,
    pub focus: Focus,
    pub top: usize,
    pub left_before: usize,
    pub left_after: usize,
    pub rows: Vec<AlignedRow>,
    pub display_rows: Vec<DisplayRow>,
    display_sides: Vec<Option<Focus>>,
    source_ranges: [Vec<(std::ops::Range<usize>, usize)>; 2],
    lazy_changes: bool,
    /// Native empty terminal rows explicitly supplied by resolver text.
    terminal_rows: [Option<usize>; 2],
    excerpt_ranges: Vec<(usize, usize)>,
    pub live_buffer: Option<usize>,
    pub live_revision: Option<u64>,
    pub follow_caret: bool,
    live_files: BTreeSet<usize>,
    /// Files edited since their frozen comparison was loaded; undo does not restore review marks.
    modified_after: BTreeSet<usize>,
    frozen_after: BTreeMap<usize, (Vec<String>, bool)>,
    context_lines: usize,
    expand_lines: usize,
    pending: Option<Operation>,
    request_kind: Option<RequestKind>,
    visible: Vec<usize>,
    search_fields: Vec<[String; 3]>,
    search_matches: Vec<([usize; 7], usize)>,
    max_before: usize,
    max_after: usize,
    after_widths: Vec<usize>,
    pub(crate) refresh_view: Option<(usize, usize, usize)>,
}

impl DiffState {
    pub(crate) fn new() -> Self {
        Self {
            catalog: None,
            pull_request_remote: None,
            pull_request_categories: Default::default(),
            pull_request_order: BTreeMap::new(),
            pull_request_retries: [0; 2],
            phase: Phase::Before,
            word_wrap: false,
            view_mode: ViewMode::Split,
            comparison_mode: ComparisonMode::default(),
            split_axis: crate::panes::SplitAxis::Vertical,
            writable_side: Some(Focus::After),
            side_labels: ["Before", "After"],
            before: None,
            after: None,
            category: 0,
            query: String::new(),
            selected: 0,
            loading: true,
            error: None,
            comparison: None,
            local_review: Default::default(),
            comparison_id: None,
            reviewed_files: BTreeSet::new(),
            files: None,
            snapshot_roots: [None, None],
            files_visible: true,
            file: 0,
            external_file_selection: false,
            requested_file: None,
            focus: Focus::Files,
            top: 0,
            left_before: 0,
            left_after: 0,
            rows: Vec::new(),
            display_rows: Vec::new(),
            display_sides: Vec::new(),
            source_ranges: Default::default(),
            lazy_changes: false,
            terminal_rows: [None; 2],
            excerpt_ranges: Vec::new(),
            live_buffer: None,
            live_revision: None,
            follow_caret: false,
            live_files: BTreeSet::new(),
            modified_after: BTreeSet::new(),
            frozen_after: BTreeMap::new(),
            context_lines: 10,
            expand_lines: 15,
            pending: Some(Operation::Discover),
            request_kind: Some(RequestKind::Catalog),
            visible: Vec::new(),
            search_fields: Vec::new(),
            search_matches: Vec::new(),
            max_before: 0,
            max_after: 0,
            after_widths: Vec::new(),
            refresh_view: None,
        }
    }
    pub(crate) fn set_resolver_paths(&mut self, root: PathBuf, paths: Vec<PathBuf>, file: usize) {
        let comparison = self.comparison.get_or_insert_with(|| Comparison {
            revisions: Default::default(),
            blame: Default::default(),
            before: "Resolution".into(),
            after: "Other version".into(),
            files: Vec::new(),
            root: root.clone(),
            editable_paths: BTreeSet::new(),
        });
        comparison.root = root.clone();
        comparison.files.resize_with(paths.len(), || ChangedFile {
            review_revisions: None,
            path: PathBuf::new(),
            old_path: None,
            status: FileStatus::Modified,
            loaded: true,
            before: Vec::new(),
            after: Vec::new(),
            before_eof: false,
            after_eof: false,
            hunks: Vec::new(),
            notice: None,
        });
        for (entry, path) in comparison.files.iter_mut().zip(&paths) {
            entry.path.clone_from(path);
        }
        self.file = file;
        self.external_file_selection = true;
        self.requested_file = None;
        if self.files.is_none() {
            self.files = Some(FilePicker::from_paths(root, paths.into_iter().enumerate()));
        } else if let Some(files) = &mut self.files {
            files.replace_paths(paths.into_iter().enumerate());
        }
        if let Some(files) = &mut self.files {
            files.reveal_path(&comparison.files[file].path);
        }
    }

    pub(crate) fn set_resolver_text(&mut self, draft: &[String], alternative: &[String]) {
        fn content(lines: &[String]) -> (&[String], bool) {
            let eof = lines.len() > 1 && lines.last().is_some_and(String::is_empty);
            let content = if eof {
                &lines[..lines.len() - 1]
            } else {
                lines
            };
            (
                if !eof && content.len() == 1 && content[0].is_empty() {
                    &[]
                } else {
                    content
                },
                eof,
            )
        }
        let (before, before_eof) = content(draft);
        let (after, after_eof) = content(alternative);
        let before_terminal = (draft.len() > before.len()).then_some(before.len());
        let after_terminal = (alternative.len() > after.len()).then_some(after.len());
        if self.phase == Phase::View
            && self.terminal_rows == [before_terminal, after_terminal]
            && self.comparison.as_ref().is_some_and(|comparison| {
                let file = &comparison.files[self.file];
                file.before == before
                    && file.after == after
                    && file.before_eof == before_eof
                    && file.after_eof == after_eof
            })
        {
            return;
        }
        let viewport = (self.top, self.left_before, self.left_after);
        if self.comparison.is_none() {
            self.set_resolver_paths(PathBuf::new(), vec![PathBuf::from("resolution.txt")], 0);
        }
        self.phase = Phase::View;
        self.loading = false;
        self.pending = None;
        self.request_kind = None;
        self.side_labels = ["Resolution", "Other version"];
        self.comparison_id = Some(self.comparison_id.unwrap_or(0).wrapping_add(1));
        self.live_files.clear();
        let file = &mut self.comparison.as_mut().unwrap().files[self.file];
        file.before = before.to_vec();
        file.after = after.to_vec();
        file.before_eof = before_eof;
        file.after_eof = after_eof;
        file.hunks = line_hunks(before, before_eof, after, after_eof);
        self.align_file();
        self.terminal_rows = [before_terminal, after_terminal];
        if before_terminal.is_some() || after_terminal.is_some() {
            self.rows.push(AlignedRow {
                before: before_terminal,
                after: after_terminal,
                kind: RowKind::Equal,
                changes: Vec::new(),
            });
            self.reset_excerpts();
        }
        if self.comparison.as_ref().unwrap().files[self.file]
            .hunks
            .is_empty()
        {
            self.excerpt_ranges = vec![(0, self.rows.len())];
            self.rebuild_display_rows();
        }
        self.top = viewport.0.min(self.display_rows.len().saturating_sub(1));
        self.left_before = viewport.1.min(self.max_before);
        self.left_after = viewport.2.min(self.max_after);
    }

    pub(crate) fn pull_requests(remote: String) -> Self {
        Self {
            pull_request_remote: Some(remote.clone()),
            pending: Some(Operation::PullRequests(remote)),
            ..Self::new()
        }
    }

    pub fn categories(&self) -> &'static [Category] {
        if self.pull_request_remote.is_some() {
            &[Category::Open, Category::Closed, Category::All]
        } else {
            self.catalog
                .as_ref()
                .map_or(&[], |catalog| catalog.kind.categories())
        }
    }

    pub fn entries(&self) -> impl ExactSizeIterator<Item = &Revision> {
        self.visible
            .iter()
            .map(|&index| &self.catalog.as_ref().expect("visible catalog").entries[index])
    }

    pub(crate) fn take_request(&mut self) -> Option<Operation> {
        if !self.request_ready() {
            return None;
        }
        self.pending.take()
    }

    pub(crate) fn request_queued(&self) -> bool {
        self.pending.is_some()
    }

    pub(crate) fn request_ready(&self) -> bool {
        self.pending.as_ref().is_some_and(|operation| {
            !self.refresh_pending() || matches!(operation, Operation::RefreshCheckout { .. })
        })
    }

    pub(crate) fn refresh_pending(&self) -> bool {
        matches!(self.request_kind, Some(RequestKind::CheckoutRefresh { .. }))
    }

    pub(crate) fn queue_checkout_refresh(&mut self, follow_working: bool) {
        if self.phase != Phase::View
            || self.loading
            || self.request_kind.is_some()
            || self.pending.is_some()
            || self.comparison.is_none()
            || self.after.as_ref().is_none_or(|after| {
                after.target == RevisionTarget::Working
                    && !follow_working
                    && !self
                        .before
                        .iter()
                        .chain(self.after.iter())
                        .any(|revision| matches!(revision.target, RevisionTarget::Bookmark { .. }))
            })
        {
            return;
        }
        if let Some(comparison_id) = self.comparison_id {
            self.request_kind = Some(RequestKind::CheckoutRefresh { comparison_id });
            self.pending = Some(Operation::RefreshCheckout {
                comparison_id,
                follow_working,
                buffers: Box::new([]),
            });
        }
    }

    fn queue(&mut self, operation: Operation) {
        if self.refresh_pending()
            && !matches!(self.pending, Some(Operation::RefreshCheckout { .. }))
            && matches!(operation, Operation::LoadFile { .. })
        {
            // Finish the refresh before issuing a load against its frozen comparison ID.
            self.pending = Some(operation);
            self.loading = true;
            self.error = None;
            return;
        }
        self.request_kind = Some(match operation {
            Operation::Compare { .. } | Operation::ResolvePullRequest(_) => RequestKind::Comparison,
            Operation::LoadFile {
                comparison_id,
                file_index,
            } => RequestKind::File {
                comparison_id,
                file_index,
            },
            _ => RequestKind::Catalog,
        });
        self.pending = Some(operation);
        self.loading = true;
        self.error = None;
    }

    pub(crate) fn apply_result(&mut self, id: u64, result: Result<Outcome, String>) -> bool {
        if let Ok(Outcome::PullRequestPage { closed, result }) = result {
            if self.pull_request_remote.is_none()
                || self.request_kind != Some(RequestKind::Catalog)
                || self.pending.is_some()
            {
                return false;
            }
            self.error = None;
            let status = &mut self.pull_request_categories[usize::from(closed)];
            match result {
                Ok(page) => {
                    status.complete = !page.has_more;
                    status.error = None;
                    let selected = self
                        .entries()
                        .nth(self.selected)
                        .map(|entry| entry.target.clone());
                    let catalog = self.catalog.get_or_insert_with(|| Catalog {
                        kind: page.kind,
                        entries: Vec::new(),
                        notice: Some(format!("Pull requests from {}", page.remote)),
                    });
                    for (revision, created_at) in page.entries {
                        let RevisionTarget::PullRequest { number, .. } = &revision.target else {
                            continue;
                        };
                        if let Some((date, index)) = self.pull_request_order.get_mut(number) {
                            *date = created_at;
                            catalog.entries[*index] = revision;
                        } else {
                            self.pull_request_order
                                .insert(*number, (created_at, catalog.entries.len()));
                            catalog.entries.push(revision);
                        }
                    }
                    let number = |revision: &Revision| match revision.target {
                        RevisionTarget::PullRequest { number, .. } => number,
                        _ => unreachable!("pull request catalog"),
                    };
                    catalog.entries.sort_unstable_by(|a, b| {
                        self.pull_request_order[&number(b)]
                            .0
                            .cmp(&self.pull_request_order[&number(a)].0)
                            .then_with(|| number(b).cmp(&number(a)))
                    });
                    for (index, revision) in catalog.entries.iter().enumerate() {
                        self.pull_request_order
                            .get_mut(&number(revision))
                            .expect("known pull request")
                            .1 = index;
                    }
                    self.prepare_search();
                    self.refilter();
                    if let Some(target) = selected {
                        self.select_target(&target);
                    }
                }
                Err(error) => status.error = Some(error),
            }
            self.loading = self
                .pull_request_categories
                .iter()
                .any(|status| !status.complete && status.error.is_none());
            return true;
        }
        if self.request_kind.is_none() {
            return false;
        }
        if let Some(RequestKind::CheckoutRefresh { comparison_id }) = self.request_kind {
            if self.comparison_id != Some(comparison_id) {
                return false;
            }
            self.request_kind = None;
            let deferred = self.pending.take();
            let bookmark_refresh = matches!(&result, Ok(Outcome::BookmarkRefresh(_)));
            match result {
                Ok(Outcome::CheckoutRefresh(None)) => {
                    let recovered = self.error.take().is_some();
                    if let Some(operation) = deferred {
                        self.queue(operation);
                    }
                    return recovered;
                }
                Ok(Outcome::CheckoutRefresh(Some(mut comparison)))
                | Ok(Outcome::BookmarkRefresh(mut comparison)) => {
                    let pinned = !bookmark_refresh
                        && self
                            .after
                            .as_ref()
                            .is_some_and(|after| after.target != RevisionTarget::Working);
                    let view = (self.top, self.left_before, self.left_after);
                    if !pinned {
                        self.reviewed_files.clear();
                    }
                    let selected = self
                        .comparison
                        .as_ref()
                        .and_then(|comparison| comparison.files.get(self.file))
                        .map(|file| &file.path);
                    self.file = selected
                        .and_then(|path| {
                            comparison.files.iter().position(|file| &file.path == path)
                        })
                        .unwrap_or(0);
                    if pinned {
                        self.restore_frozen_after();
                        if let Some(old) = self.comparison.take() {
                            // Checkout-only refreshes retain the frozen catalog order.
                            for (file, replacement) in
                                old.files.into_iter().zip(&mut comparison.files)
                            {
                                if file.loaded {
                                    *replacement = file;
                                }
                            }
                        }
                    }
                    if let Some(files) = &mut self.files {
                        files.replace_paths(
                            comparison
                                .files
                                .iter()
                                .enumerate()
                                .map(|(index, file)| (index, file.path.clone())),
                        );
                    }
                    self.comparison = Some(comparison);
                    self.comparison_id = Some(id);
                    self.snapshot_roots = [None, None];
                    self.frozen_after.clear();
                    self.live_files.clear();
                    self.modified_after.clear();
                    self.live_buffer = None;
                    self.live_revision = None;
                    self.refresh_view = None;
                    self.follow_caret = false;
                    self.prepare_selected_file();
                    if pinned || bookmark_refresh {
                        if self.selected_file_loaded() {
                            self.top = view.0.min(self.display_rows.len().saturating_sub(1));
                            self.left_before = view.1.min(self.max_before);
                            self.left_after = view.2.min(self.max_after);
                        } else {
                            self.refresh_view = Some(view);
                        }
                    }
                    self.normalize_focus();
                }
                Err(error) => {
                    self.revoke_pinned_after();
                    if let Some(operation) = deferred {
                        self.queue(operation);
                    }
                    self.error = Some(error);
                }
                Ok(_) => {
                    self.revoke_pinned_after();
                    self.error = Some("Unexpected repository response".into());
                }
            }
            return true;
        }
        if matches!(
            result,
            Ok(Outcome::CheckoutRefresh(_) | Outcome::BookmarkRefresh(_))
        ) {
            return false;
        }
        if let Ok(Outcome::File {
            comparison_id,
            file_index,
            ..
        }) = &result
            && (self.comparison_id != Some(*comparison_id)
                || self.request_kind
                    != Some(RequestKind::File {
                        comparison_id: *comparison_id,
                        file_index: *file_index,
                    }))
        {
            return false;
        }
        let expected = self.request_kind.take();
        self.loading = false;
        self.pending = None;
        match result {
            Ok(Outcome::ResolvedPullRequest { before, after })
                if expected == Some(RequestKind::Comparison) =>
            {
                self.before = Some(before);
                self.after = Some(after);
                self.queue_comparison();
            }
            Ok(Outcome::Catalog(catalog)) if expected == Some(RequestKind::Catalog) => {
                let selected = self
                    .entries()
                    .nth(self.selected)
                    .map(|entry| entry.target.clone());
                self.catalog = Some(catalog);
                self.prepare_search();
                self.category = self.category.min(self.categories().len() - 1);
                self.refilter();
                if let Some(target) = selected {
                    self.select_target(&target);
                }
                self.error = None;
            }
            Ok(Outcome::Comparison(comparison)) if expected == Some(RequestKind::Comparison) => {
                self.live_files.clear();
                self.modified_after.clear();
                self.frozen_after.clear();
                self.refresh_view = None;
                self.reviewed_files.clear();
                self.live_buffer = None;
                self.live_revision = None;
                self.follow_caret = false;
                let mut files = FilePicker::from_paths(
                    comparison.root.clone(),
                    comparison
                        .files
                        .iter()
                        .enumerate()
                        .map(|(index, file)| (index, file.path.clone())),
                );
                if let Some(file) = comparison.files.first() {
                    files.reveal_path(&file.path);
                }
                self.files = Some(files);
                self.comparison = Some(comparison);
                self.comparison_id = Some(id);
                self.snapshot_roots = [None, None];
                self.phase = Phase::View;
                self.file = 0;
                self.focus = if self.files_visible {
                    Focus::Files
                } else {
                    self.first_text_focus()
                };
                self.prepare_selected_file();
            }
            Ok(Outcome::File {
                comparison_id,
                file_index,
                file,
            }) if expected
                == Some(RequestKind::File {
                    comparison_id,
                    file_index,
                }) =>
            {
                if let Some(comparison) = &mut self.comparison
                    && let Some(selected) = comparison.files.get_mut(file_index)
                    && selected.path == file.path
                    && file.loaded
                {
                    if file
                        .notice
                        .as_deref()
                        .is_some_and(|notice| notice.contains("Binary or non-UTF-8"))
                    {
                        comparison.editable_paths.remove(&file.path);
                    }
                    *selected = file;
                    if self.file == file_index {
                        self.align_file();
                        if let Some(view) = self.refresh_view.take() {
                            self.top = view.0.min(self.display_rows.len().saturating_sub(1));
                            self.left_before = view.1.min(self.max_before);
                            self.left_after = view.2.min(self.max_after);
                        }
                        self.normalize_focus();
                    }
                    self.error = None;
                } else {
                    self.error = Some("Unexpected changed-file response".into());
                }
            }
            Ok(_) => self.error = Some("Unexpected repository response".into()),
            Err(error) => self.error = Some(error),
        }
        true
    }
    fn prepare_search(&mut self) {
        self.search_fields = self
            .catalog
            .as_ref()
            .into_iter()
            .flat_map(|catalog| &catalog.entries)
            .map(|entry| {
                let id = match &entry.target {
                    RevisionTarget::Commit(id) => id.as_str(),
                    _ => "",
                };
                [
                    entry.label.to_lowercase(),
                    id.to_lowercase(),
                    entry.description.to_lowercase(),
                ]
            })
            .collect();
    }

    pub(crate) fn pull_request_partial(&self) -> bool {
        self.pull_request_remote.is_some()
            && self
                .pull_request_categories
                .iter()
                .enumerate()
                .any(|(index, status)| {
                    (self.category == 2 || self.category == index) && !status.complete
                })
    }

    pub(crate) fn pull_request_notice(&self) -> Option<String> {
        self.pull_request_remote.as_ref()?;
        let statuses: Vec<_> = self
            .pull_request_categories
            .iter()
            .enumerate()
            .filter(|(index, _)| self.category == 2 || self.category == *index)
            .map(|(index, status)| {
                let category = [Category::Open, Category::Closed][index];
                let count = self.catalog.as_ref().map_or(0, |catalog| {
                    catalog
                        .entries
                        .iter()
                        .filter(|entry| entry.category == category)
                        .count()
                });
                let state = status.error.as_deref().unwrap_or(if status.complete {
                    "complete"
                } else {
                    "loading…"
                });
                format!("{}: {count} loaded · {state}", category.label())
            })
            .collect();
        Some(format!(
            "{}{}",
            statuses.join(" · "),
            if self.pull_request_partial() {
                " · partial search"
            } else {
                ""
            }
        ))
    }

    fn retry_pull_requests(&mut self) {
        for (index, status) in self.pull_request_categories.iter_mut().enumerate() {
            if (self.category == 2 || self.category == index) && status.error.take().is_some() {
                self.pull_request_retries[index] += 1;
                self.loading = true;
            }
        }
    }

    fn refilter(&mut self) {
        self.visible.clear();
        self.search_matches.clear();
        if let Some(catalog) = &self.catalog {
            let category = self.categories()[self.category];
            let query = self.query.to_lowercase();
            let tokens: Vec<_> = query.split_whitespace().collect();
            // Repository revisions pin Working outside its category and query.
            if self.pull_request_remote.is_none() {
                self.visible.extend(
                    catalog
                        .entries
                        .iter()
                        .enumerate()
                        .filter(|(_, entry)| entry.target == RevisionTarget::Working)
                        .map(|(index, _)| index),
                );
            }
            for (index, entry) in catalog.entries.iter().enumerate() {
                if entry.target == RevisionTarget::Working
                    || (entry.category != category
                        && !(category == Category::All
                            && matches!(entry.category, Category::Open | Category::Closed)))
                {
                    continue;
                }
                if tokens.is_empty() {
                    self.visible.push(index);
                    continue;
                }
                let score = tokens.iter().try_fold([0usize; 7], |mut score, token| {
                    let best = self.search_fields[index]
                        .iter()
                        .enumerate()
                        .filter_map(|(field_index, field)| {
                            fuzzy_match(field, token).map(|[kind, boundaries, gaps, start, len]| {
                                [
                                    kind,
                                    usize::from(field_index == 2),
                                    boundaries,
                                    gaps,
                                    start,
                                    len,
                                ]
                            })
                        })
                        .min()?;
                    // A scattered token never outranks an all-contiguous match.
                    score[0] = score[0].max(best[0]);
                    for (total, value) in score[1..].iter_mut().zip(best) {
                        *total += value;
                    }
                    Some(score)
                });
                if let Some(score) = score {
                    self.search_matches.push((score, index));
                }
            }
            self.search_matches.sort_unstable();
            self.visible
                .extend(self.search_matches.iter().map(|(_, index)| *index));
        }
        self.selected = 0;
    }

    fn select_target(&mut self, target: &RevisionTarget) {
        let selected = self.entries().position(|entry| &entry.target == target);
        if let Some(index) = selected {
            self.selected = index;
        }
    }

    pub(crate) fn navigate(&mut self, forward: bool) {
        if self.phase == Phase::View {
            if self.focus == Focus::Files {
                if let Some(files) = &mut self.files {
                    files.navigate(forward);
                }
            } else {
                self.top = if forward {
                    self.top
                        .saturating_add(1)
                        .min(self.display_rows.len().saturating_sub(1))
                } else {
                    self.top.saturating_sub(1)
                };
            }
        } else {
            let count = self.visible.len();
            self.selected = if count == 0 {
                0
            } else if forward {
                (self.selected + 1) % count
            } else if self.selected == 0 {
                count - 1
            } else {
                self.selected - 1
            };
        }
    }

    /// Handles selector filtering and comparison navigation; the editor owns selector closing.
    pub(crate) fn handle(&mut self, input: Input) {
        if input == Input::Escape {
            if self.phase == Phase::View
                && self.focus == Focus::Files
                && let Some(files) = &mut self.files
            {
                if files.searching && files.view == FileView::Tree {
                    files.finish_search();
                } else {
                    files.focused = false;
                    self.files_visible = files.presentation == FilePresentation::Docked;
                    self.focus = self.first_text_focus();
                }
            }
            return;
        }
        if self.phase == Phase::View {
            self.handle_view(input);
            return;
        }
        match input {
            Input::Char('\t') | Input::BackTab => {
                let count = self.categories().len();
                if count > 0 {
                    self.category = if input == Input::BackTab {
                        (self.category + count - 1) % count
                    } else {
                        (self.category + 1) % count
                    };
                    self.refilter();
                }
            }
            Input::Up | Input::Ctrl('k') => self.navigate(false),
            Input::Down | Input::Ctrl('j') => self.navigate(true),
            Input::Home => self.selected = 0,
            Input::End => self.selected = self.visible.len().saturating_sub(1),
            Input::Enter
                if !self.loading
                    || (self.pull_request_remote.is_some()
                        && self.request_kind == Some(RequestKind::Catalog)) =>
            {
                self.choose()
            }
            Input::Ctrl('r') if self.pull_request_remote.is_some() => self.retry_pull_requests(),
            input if BackspaceKind::from_input(&input).is_some() => {
                if BackspaceKind::from_input(&input)
                    .expect("guard checked backspace input")
                    .truncate(&mut self.query)
                {
                    self.refilter();
                }
            }
            Input::Ctrl('u') => {
                self.query.clear();
                self.refilter();
            }
            Input::Char(ch) if !ch.is_control() => {
                self.query.push(ch);
                self.refilter();
            }
            _ => {}
        }
    }

    fn choose(&mut self) {
        let revision = self.entries().nth(self.selected).cloned();
        let Some(revision) = revision else {
            self.retry_pull_requests();
            // Listing can be retried without closing the modal.
            if self.catalog.is_none()
                && (self.pull_request_remote.is_none() || self.error.is_some())
            {
                self.queue(match &self.pull_request_remote {
                    Some(remote) => Operation::PullRequests(remote.clone()),
                    None => Operation::Discover,
                });
            }
            return;
        };
        if matches!(revision.target, RevisionTarget::PullRequest { .. }) {
            self.before = None;
            self.after = None;
            self.comparison_mode = ComparisonMode::BranchChanges;
            self.phase = Phase::After;
            self.queue(Operation::ResolvePullRequest(revision));
            return;
        }
        if revision.target == RevisionTarget::Working {
            self.comparison_mode = ComparisonMode::Snapshots;
        }
        if let RevisionTarget::FetchRemote(remote) = &revision.target {
            self.queue(Operation::FetchRemote(remote.clone()));
        } else if let RevisionTarget::ListRemote(remote) = &revision.target {
            self.queue(Operation::ListRemote(remote.clone()));
        } else if self.phase == Phase::Before {
            self.before = Some(revision);
            self.after = None;
            self.phase = Phase::After;
            self.query.clear();
            self.refilter();
            self.error = None;
        } else {
            self.after = Some(revision);
            self.queue_comparison();
        }
    }

    pub(crate) fn working_selected(&self) -> bool {
        [&self.before, &self.after]
            .into_iter()
            .flatten()
            .any(|revision| revision.target == RevisionTarget::Working)
    }

    pub(crate) fn toggle_comparison_mode(&mut self) -> Result<(), &'static str> {
        if self.pull_request_remote.is_some() && self.after.is_none() {
            return Err("Select a pull request before changing comparison mode");
        }
        if self.working_selected() {
            return Err("Working checkout requires Snapshots");
        }
        self.comparison_mode = match self.comparison_mode {
            ComparisonMode::BranchChanges => ComparisonMode::Snapshots,
            ComparisonMode::Snapshots => ComparisonMode::BranchChanges,
        };
        self.queue_comparison();
        Ok(())
    }

    fn queue_comparison(&mut self) {
        if let (Some(before), Some(after)) = (&self.before, &self.after) {
            let operation = Operation::Compare {
                before: before.clone(),
                after: after.clone(),
                mode: self.comparison_mode,
                buffers: Box::new([]),
            };
            self.phase = Phase::After;
            self.comparison = None;
            self.comparison_id = None;
            self.reviewed_files.clear();
            self.queue(operation);
        }
    }

    fn handle_view(&mut self, input: Input) {
        if self.focus == Focus::Files {
            let files = self.files.as_mut().expect("comparison file picker");
            match input {
                Input::Char('\t') | Input::BackTab => {}
                Input::Ctrl('t') => {
                    files.toggle_view();
                    return;
                }
                Input::Home => {
                    files.selected = 0;
                    return;
                }
                Input::End => {
                    let last = files.entries().len().saturating_sub(1);
                    files.selected = last;
                    return;
                }
                Input::Up | Input::Ctrl('k') => {
                    files.navigate(false);
                    return;
                }
                Input::Down | Input::Ctrl('j') => {
                    files.navigate(true);
                    return;
                }
                Input::Left | Input::Ctrl('h') => {
                    files.left();
                    return;
                }
                Input::Right | Input::Ctrl('l') => {
                    files.right();
                    return;
                }
                Input::Enter => {
                    if let Some(path) = files.accept()
                        && self.select_file(&path)
                    {
                        self.focus = self
                            .writable_side
                            .unwrap_or_else(|| self.first_text_focus());
                        self.normalize_focus();
                        let files = self.files.as_mut().expect("comparison picker");
                        files.focused = false;
                        self.files_visible = files.presentation == FilePresentation::Docked;
                    }
                    return;
                }
                _ => {
                    files.handle_filter_input(&input);
                    return;
                }
            }
        }
        match input {
            Input::Char('\t') => self.cycle_focus(true),
            Input::BackTab => self.cycle_focus(false),
            Input::Up | Input::Ctrl('k') => self.navigate(false),
            Input::Down | Input::Ctrl('j') => self.navigate(true),
            Input::Left | Input::Ctrl('h') => match self.focus {
                Focus::Before => self.left_before = self.left_before.saturating_sub(1),
                Focus::After => self.left_after = self.left_after.saturating_sub(1),
                Focus::Files => {}
            },
            Input::Right | Input::Ctrl('l') => match self.focus {
                Focus::Before => {
                    self.left_before = self.left_before.saturating_add(1).min(self.max_before)
                }
                Focus::After => {
                    self.left_after = self.left_after.saturating_add(1).min(self.max_after)
                }
                Focus::Files => {}
            },
            Input::Home | Input::End => {
                self.top = if input == Input::Home {
                    0
                } else {
                    self.display_rows.len().saturating_sub(1)
                };
            }
            Input::Char('[') => self.expand_excerpt(false),
            Input::Char(']') => self.expand_excerpt(true),
            _ => {}
        }
        if let Some(files) = &mut self.files {
            files.focused = self.files_visible && self.focus == Focus::Files;
        }
    }

    pub(crate) fn pane_layout(&self) -> PaneLayout {
        let Some(file) = self
            .comparison
            .as_ref()
            .and_then(|comparison| comparison.files.get(self.file))
        else {
            return PaneLayout::Empty;
        };
        if !file.loaded {
            return PaneLayout::Empty;
        }
        match file.status {
            FileStatus::Added => PaneLayout::After,
            FileStatus::Removed => PaneLayout::Before,
            FileStatus::Renamed
                if file.before == file.after && file.before_eof == file.after_eof =>
            {
                PaneLayout::Empty
            }
            FileStatus::Modified | FileStatus::Renamed => PaneLayout::Split,
        }
    }

    pub(crate) fn bound_horizontal(&self, focus: Focus, left: usize, width: usize) -> usize {
        let columns = match focus {
            Focus::Before => self.max_before,
            Focus::After => self.max_after,
            Focus::Files => return 0,
        };
        let columns = if self.pane_layout() == PaneLayout::Split {
            self.max_before.max(self.max_after)
        } else {
            columns
        };
        left.min(columns.saturating_add(1).saturating_sub(width.max(1)))
    }

    pub(crate) fn scroll_horizontal(&mut self, focus: Focus, right: bool, width: usize) {
        let left = match focus {
            Focus::Before => self.left_before,
            Focus::After => self.left_after,
            Focus::Files => return,
        };
        let left = if right {
            left.saturating_add(3)
        } else {
            left.saturating_sub(3)
        };
        let left = self.bound_horizontal(focus, left, width);
        if self.pane_layout() == PaneLayout::Split {
            self.left_before = left;
            self.left_after = left;
        } else {
            match focus {
                Focus::Before => self.left_before = left,
                Focus::After => self.left_after = left,
                Focus::Files => unreachable!(),
            }
        }
        self.follow_caret = false;
    }

    pub(crate) fn first_text_focus(&self) -> Focus {
        if self.pane_layout() == PaneLayout::After {
            Focus::After
        } else {
            Focus::Before
        }
    }

    pub(crate) fn normalize_focus(&mut self) {
        self.focus = match (self.focus, self.pane_layout()) {
            (Focus::Files, _) if self.files_visible => Focus::Files,
            (Focus::Before | Focus::After, PaneLayout::Split) => self.focus,
            (Focus::Before | Focus::After, PaneLayout::Empty)
                if self
                    .comparison
                    .as_ref()
                    .is_some_and(|comparison| !comparison.files.is_empty())
                    && !self.selected_file_loaded() =>
            {
                self.focus
            }
            _ => self.first_text_focus(),
        };
        if let Some(files) = &mut self.files {
            files.focused = self.files_visible && self.focus == Focus::Files;
        }
    }

    fn cycle_focus(&mut self, forward: bool) {
        let first = self.first_text_focus();
        let last = if self.pane_layout() == PaneLayout::Split {
            Focus::After
        } else {
            first
        };
        self.focus = match self.focus {
            Focus::Files => {
                if forward {
                    first
                } else {
                    last
                }
            }
            focus if forward && focus != last => last,
            focus if !forward && focus != first => first,
            _ if self.files_visible => Focus::Files,
            _ => {
                if forward {
                    first
                } else {
                    last
                }
            }
        };
    }

    fn revoke_pinned_after(&mut self) {
        if self
            .after
            .as_ref()
            .is_none_or(|after| after.target == RevisionTarget::Working)
        {
            return;
        }
        if let Some(comparison) = &mut self.comparison {
            comparison.editable_paths.clear();
        }
        self.restore_frozen_after();
        self.live_files.clear();
        self.live_buffer = None;
        self.live_revision = None;
        self.follow_caret = false;
        self.rebuild_file(false);
        self.refresh_files();
    }

    fn restore_frozen_after(&mut self) {
        if let Some(comparison) = &mut self.comparison {
            for (index, (after, eof)) in std::mem::take(&mut self.frozen_after) {
                if let Some(file) = comparison.files.get_mut(index) {
                    file.after = after;
                    file.after_eof = eof;
                    file.hunks =
                        line_hunks(&file.before, file.before_eof, &file.after, file.after_eof);
                    update_eof_notice(file);
                }
            }
        }
    }
    pub fn side_label(&self, focus: Focus) -> &'static str {
        self.side_labels[usize::from(focus == Focus::After)]
    }

    pub fn side_editable(&self, focus: Focus) -> bool {
        self.writable_side == Some(focus)
            && match focus {
                Focus::Before => self.selected_file_loaded(),
                Focus::After => self.after_editable(),
                Focus::Files => false,
            }
    }

    pub fn after_editable(&self) -> bool {
        matches!(self.pane_layout(), PaneLayout::Split | PaneLayout::After)
            && self.live_after_available()
    }

    pub(crate) fn live_after_available(&self) -> bool {
        self.phase == Phase::View
            && self.selected_file_loaded()
            && self.comparison.as_ref().is_some_and(|comparison| {
                comparison.files.get(self.file).is_some_and(|file| {
                    comparison.editable_paths.contains(&file.path)
                        && file.status != FileStatus::Removed
                        && file
                            .notice
                            .as_deref()
                            .is_none_or(|notice| !notice.contains("Binary or non-UTF-8"))
                })
            })
    }

    pub(crate) fn toggle_reviewed(&mut self) -> Result<bool, &'static str> {
        let comparison = self
            .comparison
            .as_ref()
            .filter(|_| self.phase == Phase::View)
            .ok_or("No comparison is open")?;
        let index = if self.focus == Focus::Files {
            let entry = self
                .files
                .as_ref()
                .filter(|_| self.files_visible)
                .and_then(|files| files.entries().nth(files.selected))
                .ok_or("No changed file is selected")?;
            if entry.is_dir {
                return Err("Select a changed file, not a directory");
            }
            entry
                .catalog_index
                .filter(|&index| {
                    comparison
                        .files
                        .get(index)
                        .is_some_and(|file| file.path == entry.path)
                })
                .ok_or("No changed file is selected")?
        } else {
            self.file
        };
        let file = comparison
            .files
            .get(index)
            .ok_or("No changed file is selected")?;
        let reviewed = !self.reviewed_files.remove(&index);
        if reviewed {
            self.reviewed_files.insert(index);
        }
        // Resolver comparisons have no immutable revision identity or local review.
        if comparison
            .revisions
            .iter()
            .all(|revision| !revision.is_empty())
        {
            if self.local_review.document.root.as_os_str().is_empty() {
                self.local_review.document = crate::review::Review::new(comparison.root.clone());
            }
            let records = &mut self.local_review.document.reviewed_files;
            records.retain(|record| {
                record.revisions
                    != *file
                        .review_revisions
                        .as_ref()
                        .unwrap_or(&comparison.revisions)
                    || record.path != file.path
            });
            if reviewed {
                records.push(crate::review::ReviewedFile {
                    revisions: file
                        .review_revisions
                        .as_ref()
                        .unwrap_or(&comparison.revisions)
                        .clone(),
                    path: file.path.clone(),
                    old_path: file.old_path.clone(),
                    after: self
                        .live_files
                        .contains(&index)
                        .then(|| file.after.join("\n")),
                });
            }
            self.local_review.dirty = true;
        }
        Ok(reviewed)
    }

    pub(crate) fn restore_reviewed_files(&mut self) {
        self.reviewed_files.clear();
        let Some(comparison) = self.comparison.as_ref() else {
            return;
        };
        if self.local_review.document.root != comparison.root {
            return;
        }
        let records: BTreeMap<_, _> = self
            .local_review
            .document
            .reviewed_files
            .iter()
            .map(|record| ((&record.revisions, &record.path), record))
            .collect();
        for (index, file) in comparison.files.iter().enumerate() {
            let Some(record) = records
                .get(&(
                    file.review_revisions
                        .as_ref()
                        .unwrap_or(&comparison.revisions),
                    &file.path,
                ))
                .filter(|record| record.old_path == file.old_path)
            else {
                continue;
            };
            let matches = match &record.after {
                None => !self.modified_after.contains(&index),
                Some(text) => {
                    file.loaded
                        && (text
                            .split('\n')
                            .eq(file.after.iter().map(String::as_str).chain(
                                ((!self.live_files.contains(&index) && file.after_eof)
                                    || file.after.is_empty())
                                .then_some(""),
                            )))
                }
            };
            if matches {
                self.reviewed_files.insert(index);
            }
        }
    }

    pub(crate) fn select_file(&mut self, path: &Path) -> bool {
        let Some(index) = self.comparison.as_ref().and_then(|comparison| {
            let relative = path.strip_prefix(&comparison.root).unwrap_or(path);
            comparison
                .files
                .iter()
                .position(|file| file.path == relative)
        }) else {
            return false;
        };
        self.select_file_index(index)
    }

    pub(crate) fn select_file_index(&mut self, index: usize) -> bool {
        let Some(file) = self
            .comparison
            .as_ref()
            .and_then(|comparison| comparison.files.get(index))
        else {
            return false;
        };
        if let Some(files) = &mut self.files {
            files.reveal_path(&file.path);
        }
        if self.external_file_selection {
            self.requested_file = Some(index);
            return true;
        }
        self.file = index;
        self.refresh_view = None;
        self.normalize_focus();
        self.live_buffer = None;
        self.live_revision = None;
        self.follow_caret = false;
        self.prepare_selected_file();
        true
    }

    pub fn selected_file_loaded(&self) -> bool {
        self.comparison
            .as_ref()
            .and_then(|comparison| comparison.files.get(self.file))
            .is_some_and(|file| file.loaded)
    }

    pub(crate) fn column_view(&self, focus: Focus) -> Option<ColumnView<'_>> {
        if focus == Focus::Files {
            return None;
        }
        let file = self
            .comparison
            .as_ref()?
            .files
            .get(self.file)
            .filter(|file| file.loaded)?;
        Some(ColumnView::new(self, file, focus))
    }

    /// Source rows without the editor's virtual terminal row, preserving real empty lines.
    pub(crate) fn after_source_lines<'a>(&self, file: &'a ChangedFile) -> &'a [String] {
        if self.live_files.contains(&self.file) {
            live_content(&file.after, file.after_eof)
        } else {
            &file.after
        }
    }

    fn prepare_selected_file(&mut self) {
        if self.refresh_pending() {
            self.align_file();
            if !self.selected_file_loaded() {
                self.load_file(self.file);
            } else if matches!(self.pending, Some(Operation::LoadFile { .. })) {
                self.pending = None;
                self.loading = false;
            }
            return;
        }
        self.pending = None;
        self.request_kind = None;
        self.loading = false;
        self.error = None;
        if self.selected_file_loaded() {
            self.align_file();
        } else {
            self.align_file();
            if let Some(comparison_id) = self.comparison_id
                && self
                    .comparison
                    .as_ref()
                    .is_some_and(|comparison| !comparison.files.is_empty())
            {
                self.queue(Operation::LoadFile {
                    comparison_id,
                    file_index: self.file,
                });
            }
        }
    }

    pub(crate) fn load_file(&mut self, file_index: usize) {
        if let Some(comparison_id) = self.comparison_id {
            self.queue(Operation::LoadFile {
                comparison_id,
                file_index,
            });
        }
    }

    pub(crate) fn source_range_changed(
        &self,
        file_index: usize,
        focus: Focus,
        first: usize,
        last: usize,
    ) -> bool {
        if file_index == self.file {
            return self.rows.iter().any(|aligned| {
                aligned.kind != RowKind::Equal
                    && match focus {
                        Focus::Before => aligned
                            .before
                            .is_some_and(|row| first <= row && row <= last),
                        Focus::After => {
                            aligned.after.is_some_and(|row| first <= row && row <= last)
                        }
                        Focus::Files => false,
                    }
            });
        }
        self.comparison
            .as_ref()
            .and_then(|comparison| comparison.files.get(file_index))
            .is_some_and(|file| {
                file.hunks.iter().any(|hunk| {
                    let (start, len) = match focus {
                        Focus::Before => (hunk.before_start, hunk.before_len),
                        Focus::After => (hunk.after_start, hunk.after_len),
                        Focus::Files => return false,
                    };
                    len > 0 && start <= last && first < start.saturating_add(len)
                })
            })
    }

    pub(crate) fn reveal_source_row(&mut self, focus: Focus, row: usize) {
        if self.source_display_row(focus, row).is_none()
            && let Some(index) = self.rows.iter().position(|aligned| match focus {
                Focus::Before => aligned.before == Some(row),
                Focus::After => aligned.after == Some(row),
                Focus::Files => false,
            })
        {
            self.excerpt_ranges.push((index, index + 1));
            self.excerpt_ranges.sort_unstable();
            self.rebuild_display_rows();
        }
        if let Some(display) = self.source_display_row(focus, row) {
            self.top = display;
            self.follow_caret = true;
        }
    }

    pub(crate) fn source_display_row(&self, focus: Focus, source_row: usize) -> Option<usize> {
        let ranges = self.source_ranges.get(usize::from(focus == Focus::After))?;
        if focus == Focus::Files {
            return None;
        }
        let index = ranges.partition_point(|(range, _)| range.end <= source_row);
        let (range, display) = ranges.get(index)?;
        range
            .contains(&source_row)
            .then(|| display + source_row - range.start)
    }

    pub(crate) fn display_focus(&self, display: usize) -> Option<Focus> {
        self.display_sides.get(display).copied().flatten()
    }

    pub(crate) fn display_source_row(&self, focus: Focus, display: usize) -> Option<usize> {
        if self
            .display_focus(display)
            .is_some_and(|side| side != focus)
        {
            return None;
        }
        match *self.display_rows.get(display)? {
            DisplayRow::Line(index) => self.source_row(focus, index),
            DisplayRow::Fold { .. } => None,
        }
    }

    pub(crate) fn set_view_mode(&mut self, view_mode: ViewMode) {
        if self.view_mode == view_mode {
            return;
        }
        let anchor = self.display_rows.get(self.top).copied();
        let side = self.display_focus(self.top).or_else(|| {
            [self.focus, Focus::After, Focus::Before]
                .into_iter()
                .find(|&focus| self.display_source_row(focus, self.top).is_some())
        });
        self.view_mode = view_mode;
        self.rebuild_display_rows();
        self.restore_top(anchor, side);
    }

    pub fn after_display_row(&self, source_row: usize) -> Option<usize> {
        self.source_display_row(Focus::After, source_row)
    }

    fn source_row(&self, focus: Focus, index: usize) -> Option<usize> {
        match focus {
            Focus::Before => self.rows[index].before,
            Focus::After => self.rows[index].after,
            Focus::Files => None,
        }
    }

    pub(crate) fn source_rows_in(
        &self,
        focus: Focus,
        start: usize,
        end: usize,
    ) -> impl DoubleEndedIterator<Item = usize> + '_ {
        let ranges = &self.source_ranges[usize::from(focus == Focus::After)];
        let first = ranges.partition_point(|(range, _)| range.end <= start);
        let last = ranges
            .partition_point(|(range, _)| range.start < end)
            .max(first);
        ranges[first..last].iter().flat_map(move |(range, _)| {
            let start = range.start.max(start);
            start..if focus == Focus::Files {
                start
            } else {
                range.end.min(end)
            }
        })
    }

    pub(crate) fn snapshot_lines_at(&self, file_index: usize, focus: Focus) -> Vec<String> {
        let file = &self.comparison.as_ref().expect("snapshot comparison").files[file_index];
        let (source, eof, live) = match focus {
            Focus::Before => (&file.before, file.before_eof, false),
            Focus::After => (
                &file.after,
                file.after_eof,
                self.live_files.contains(&file_index),
            ),
            Focus::Files => unreachable!("snapshot belongs to a text pane"),
        };
        let mut lines = source.clone();
        if !live && (eof || lines.is_empty()) {
            lines.push(String::new());
        }
        lines
    }

    pub fn review_anchor_current(&self, anchor: &crate::review::Anchor) -> bool {
        use crate::review::Side;
        let Some(comparison) = &self.comparison else {
            return false;
        };
        if self.local_review.document.root != comparison.root {
            return false;
        }
        comparison.files.iter().enumerate().any(|(index, file)| {
            if !file.loaded || file.notice.is_some() {
                return false;
            }
            let (path, lines, eof) = match anchor.side {
                Side::Before => (
                    file.old_path.as_deref().unwrap_or(&file.path),
                    &file.before,
                    file.before_eof,
                ),
                Side::After => (
                    file.path.as_path(),
                    &file.after,
                    file.after_eof && !self.live_files.contains(&index),
                ),
            };
            anchor.matches_lines(
                &comparison.revisions[anchor.side.index()],
                path,
                anchor.side,
                lines,
                eof,
            )
        })
    }

    pub(crate) fn corresponding_row(&self, from: Focus, to: Focus, row: usize) -> Option<usize> {
        let aligned = self
            .source_display_row(from, row)
            .and_then(|display| match self.display_rows[display] {
                DisplayRow::Line(index) => Some(index),
                DisplayRow::Fold { .. } => None,
            })
            .or_else(|| {
                self.rows.iter().position(|aligned| match from {
                    Focus::Before => aligned.before == Some(row),
                    Focus::After => aligned.after == Some(row),
                    Focus::Files => false,
                })
            })?;
        if to == Focus::Files {
            return None;
        }
        let mut nearest = None;
        for (range, display) in &self.source_ranges[usize::from(to == Focus::After)] {
            let visible = &self.display_rows[*display..*display + range.len()];
            let offset = visible
                .partition_point(|row| matches!(row, DisplayRow::Line(index) if *index < aligned));
            for offset in [offset.saturating_sub(1), offset.min(visible.len() - 1)] {
                let DisplayRow::Line(index) = visible[offset] else {
                    unreachable!()
                };
                let candidate = (index.abs_diff(aligned), range.start + offset);
                if nearest.is_none_or(|previous| candidate < previous) {
                    nearest = Some(candidate);
                }
            }
        }
        nearest.map(|(_, row)| row)
    }

    pub(crate) fn clamp_source_row(&self, focus: Focus, row: usize) -> Option<usize> {
        let ranges = &self.source_ranges[usize::from(focus == Focus::After)];
        if focus == Focus::Files {
            return None;
        }
        let index = ranges.partition_point(|(range, _)| range.end <= row);
        let next = ranges.get(index).map(|(range, _)| row.max(range.start));
        let previous = index.checked_sub(1).map(|index| ranges[index].0.end - 1);
        match (previous, next) {
            (Some(previous), Some(next)) if row - previous < next - row => Some(previous),
            (_, Some(next)) => Some(next),
            (previous, None) => previous,
        }
    }

    fn restore_top(&mut self, anchor: Option<DisplayRow>, side: Option<Focus>) {
        if let Some(anchor) = anchor {
            let index = match anchor {
                DisplayRow::Line(index) => index,
                DisplayRow::Fold { start, .. } => start,
            };
            self.top = self
                .display_rows
                .iter()
                .enumerate()
                .position(|(display, row)| match *row {
                    DisplayRow::Line(row) => {
                        row == index
                            && side.is_none_or(|side| {
                                self.display_focus(display)
                                    .is_none_or(|focus| focus == side)
                            })
                    }
                    DisplayRow::Fold { start, end } => start <= index && index < end,
                })
                .unwrap_or(0);
        }
    }

    pub fn refresh_after(&mut self, lines: &[String], revision: u64) {
        if !self.live_after_available() {
            return;
        }
        if self.live_revision == Some(revision) {
            return;
        }
        let file = &self.comparison.as_ref().unwrap().files[self.file];
        if self
            .after
            .as_ref()
            .is_some_and(|after| after.target != RevisionTarget::Working)
        {
            self.frozen_after
                .entry(self.file)
                .or_insert_with(|| (file.after.clone(), file.after_eof));
        }
        let was_live = self.live_files.contains(&self.file);
        let changed_lines = changed_line_range(&file.after, lines);
        let unchanged = if was_live {
            changed_lines.0 == changed_lines.1 && changed_lines.1 == changed_lines.2
        } else {
            let eof = lines.len() > 1 && lines.last().is_some_and(String::is_empty);
            file.after_eof == eof && file.after == live_content(lines, eof)
        };
        if !unchanged {
            self.modified_after.insert(self.file);
        }
        let comparison = self.comparison.as_ref().unwrap();
        let record = (self.local_review.document.root == comparison.root)
            .then(|| {
                self.local_review
                    .document
                    .reviewed_files
                    .iter()
                    .position(|record| {
                        record.revisions
                            == *file
                                .review_revisions
                                .as_ref()
                                .unwrap_or(&comparison.revisions)
                            && record.path == file.path
                            && record.old_path == file.old_path
                    })
            })
            .flatten();
        let matches = record
            .and_then(|position| {
                self.local_review.document.reviewed_files[position]
                    .after
                    .as_ref()
            })
            .map(|text| text.split('\n').eq(lines.iter().map(String::as_str)));
        if matches == Some(true) {
            self.reviewed_files.insert(self.file);
        }
        if self.reviewed_files.contains(&self.file) && !matches.unwrap_or(unchanged) {
            self.reviewed_files.remove(&self.file);
            if let Some(position) = record {
                self.local_review.document.reviewed_files.remove(position);
                self.local_review.dirty = true;
            }
        }
        if was_live && changed_lines.0 == changed_lines.1 && changed_lines.1 == changed_lines.2 {
            self.live_revision = Some(revision);
            return;
        }
        if was_live && stable_after_edit(file, lines, changed_lines) {
            let file = &mut self.comparison.as_mut().unwrap().files[self.file];
            replace_after_lines(
                &mut file.after,
                &mut self.after_widths,
                &mut self.max_after,
                lines,
                changed_lines,
            );
            refresh_character_changes(&mut self.rows, file, changed_lines.0..changed_lines.2);
            self.left_after = self.left_after.min(self.max_after);
            self.live_revision = Some(revision);
            return;
        }
        let mut old = file.after.iter().map(String::as_str).collect::<Vec<_>>();
        if !was_live && (file.after_eof || old.is_empty()) {
            old.push("");
        }
        let new = lines.iter().map(String::as_str).collect::<Vec<_>>();
        let ops = similar::capture_diff_slices_deadline(
            similar::Algorithm::Myers,
            &old,
            &new,
            Some(Instant::now() + Duration::from_millis(100)),
        );
        let mut mapping = vec![new.len().saturating_sub(1); old.len()];
        for op in &ops {
            let (_, old_range, new_range) = op.as_tag_tuple();
            for offset in 0..old_range.len() {
                mapping[old_range.start + offset] = (new_range.start
                    + offset.min(new_range.len().saturating_sub(1)))
                .min(new.len().saturating_sub(1));
            }
        }
        let map_after = |index: usize| mapping.get(index).copied().unwrap_or(0);
        let mut before_visible = BTreeSet::new();
        let mut after_visible = BTreeSet::new();
        for &(start, end) in &self.excerpt_ranges {
            for row in &self.rows[start..end] {
                before_visible.extend(row.before);
                after_visible.extend(row.after.map(map_after));
            }
        }
        let top_fold = self
            .display_rows
            .get(self.top)
            .and_then(|display| match *display {
                DisplayRow::Fold { start, .. } => Some(start),
                DisplayRow::Line(_) => None,
            });
        let top_before = self
            .display_source_row(Focus::Before, self.top)
            .or_else(|| top_fold.and_then(|index| self.rows[index].before));
        let top_after = self
            .display_source_row(Focus::After, self.top)
            .or_else(|| top_fold.and_then(|index| self.rows[index].after))
            .map(map_after);
        let was_changed = file_is_changed(file);
        let file = &mut self.comparison.as_mut().unwrap().files[self.file];
        replace_after_lines(
            &mut file.after,
            &mut self.after_widths,
            &mut self.max_after,
            lines,
            changed_lines,
        );
        file.after_eof = lines.len() > 1 && lines.last().is_some_and(String::is_empty);
        file.hunks = line_hunks(
            &file.before,
            file.before_eof,
            live_content(lines, file.after_eof),
            file.after_eof,
        );
        update_eof_notice(file);
        let changed = file_is_changed(file);
        self.live_files.insert(self.file);
        self.live_revision = Some(revision);
        let left_before = self.left_before;
        let left_after = self.left_after;
        self.rebuild_file(false);
        self.left_before = left_before.min(self.max_before);
        self.left_after = left_after.min(self.max_after);
        for (index, row) in self.rows.iter().enumerate() {
            if row
                .before
                .is_some_and(|index| before_visible.contains(&index))
                || row
                    .after
                    .is_some_and(|index| after_visible.contains(&index))
            {
                self.excerpt_ranges.push((index, index + 1));
            }
        }
        self.excerpt_ranges.sort_unstable();
        self.rebuild_display_rows();
        if let Some(row) = top_after {
            self.top = self.after_display_row(row).unwrap_or(0);
        } else if let Some(source) = top_before
            && let Some(index) = self.rows.iter().position(|row| row.before == Some(source))
        {
            self.restore_top(Some(DisplayRow::Line(index)), Some(Focus::Before));
        }
        if was_changed != changed {
            self.refresh_files();
        }
    }

    fn refresh_files(&mut self) {
        let Some(comparison) = &self.comparison else {
            return;
        };
        let paths = comparison
            .files
            .iter()
            .enumerate()
            .filter(|(_, file)| file_is_changed(file))
            .map(|(index, file)| (index, file.path.clone()));
        if let Some(files) = &mut self.files {
            files.replace_paths(paths);
        } else {
            self.files = Some(FilePicker::from_paths(comparison.root.clone(), paths));
        }
    }

    fn align_file(&mut self) {
        self.rebuild_file(true);
    }

    fn rebuild_file(&mut self, reset: bool) {
        self.normalize_focus();
        self.rows.clear();
        self.terminal_rows = [None; 2];
        self.display_rows.clear();
        self.display_sides.clear();
        self.source_ranges.iter_mut().for_each(Vec::clear);
        self.excerpt_ranges.clear();
        if reset {
            self.top = 0;
            self.left_before = 0;
            self.left_after = 0;
            self.lazy_changes = false;
            self.max_before = 0;
            self.max_after = 0;
            self.after_widths.clear();
        }
        let Some(file) = self
            .comparison
            .as_ref()
            .and_then(|comparison| comparison.files.get(self.file))
            .filter(|file| file.loaded)
        else {
            self.after_widths.clear();
            self.max_before = 0;
            self.max_after = 0;
            return;
        };
        self.lazy_changes = file.before.len().saturating_add(file.after.len()) > 20_000;
        if reset {
            self.max_before = file
                .before
                .iter()
                .map(|line| line_width(line))
                .max()
                .unwrap_or(0);
            self.after_widths
                .extend(file.after.iter().map(|line| line_width(line)));
            self.max_after = self.after_widths.iter().copied().max().unwrap_or(0);
        }
        let after_len = if self.live_files.contains(&self.file) {
            live_content(&file.after, file.after_eof).len()
        } else {
            file.after.len()
        };
        self.rows.reserve(file.before.len().max(file.after.len()));
        let mut before = 0;
        let mut after = 0;
        for hunk in &file.hunks {
            // Clamp invalid ranges rather than indexing historical text unchecked.
            let start_before = hunk.before_start.min(file.before.len()).max(before);
            let start_after = hunk.after_start.min(after_len).max(after);
            append_rows(
                &mut self.rows,
                before,
                start_before,
                after,
                start_after,
                false,
            );
            before = start_before;
            after = start_after;
            let end_before = before
                .saturating_add(hunk.before_len)
                .min(file.before.len());
            let end_after = after.saturating_add(hunk.after_len).min(after_len);
            append_rows(&mut self.rows, before, end_before, after, end_after, true);
            before = end_before;
            after = end_after;
        }
        append_rows(
            &mut self.rows,
            before,
            file.before.len(),
            after,
            after_len,
            false,
        );
        if after_len < file.after.len() {
            self.rows.push(AlignedRow {
                before: None,
                after: Some(after_len),
                kind: RowKind::Equal,
                changes: Vec::new(),
            });
        }
        refresh_character_changes(&mut self.rows, file, 0..file.after.len());
        self.reset_excerpts();
    }

    pub(crate) fn set_context_options(&mut self, context_lines: usize, expand_lines: usize) {
        if self.context_lines == context_lines && self.expand_lines == expand_lines {
            return;
        }
        let context_changed = self.context_lines != context_lines;
        self.context_lines = context_lines;
        self.expand_lines = expand_lines.max(1);
        if context_changed {
            self.reset_excerpts();
            self.top = 0;
        }
    }

    fn reset_excerpts(&mut self) {
        self.excerpt_ranges.clear();
        for (index, row) in self.rows.iter().enumerate() {
            if row.kind == RowKind::Equal {
                continue;
            }
            let start = index.saturating_sub(self.context_lines);
            let end = index
                .saturating_add(1)
                .saturating_add(self.context_lines)
                .min(self.rows.len());
            if let Some(last) = self.excerpt_ranges.last_mut()
                && start <= last.1
            {
                last.1 = end;
            } else {
                self.excerpt_ranges.push((start, end));
            }
        }
        self.rebuild_display_rows();
    }

    fn rebuild_display_rows(&mut self) {
        let mut count = 0;
        for index in 0..self.excerpt_ranges.len() {
            let range = self.excerpt_ranges[index];
            if count > 0 && range.0 <= self.excerpt_ranges[count - 1].1 {
                self.excerpt_ranges[count - 1].1 = self.excerpt_ranges[count - 1].1.max(range.1);
            } else {
                self.excerpt_ranges[count] = range;
                count += 1;
            }
        }
        self.excerpt_ranges.truncate(count);
        self.display_rows.clear();
        self.display_sides.clear();
        self.source_ranges.iter_mut().for_each(Vec::clear);
        if self.excerpt_ranges.is_empty() {
            return;
        }
        let visible = self
            .excerpt_ranges
            .iter()
            .map(|&(start, end)| end - start)
            .sum::<usize>();
        let capacity = visible
            .saturating_add(self.excerpt_ranges.len())
            .saturating_add(1);
        self.display_rows.reserve(capacity);
        if self.view_mode == ViewMode::Unified {
            self.display_sides.reserve(capacity);
        }
        let mut end = 0;
        for &(start, next_end) in &self.excerpt_ranges {
            if end < start {
                self.display_rows.push(DisplayRow::Fold {
                    start: end,
                    end: start,
                });
                if self.view_mode == ViewMode::Unified {
                    self.display_sides.push(None);
                }
            }
            let mut index = start;
            while index < next_end {
                if self.view_mode == ViewMode::Split || self.rows[index].kind == RowKind::Equal {
                    self.display_rows.push(DisplayRow::Line(index));
                    if self.view_mode == ViewMode::Unified {
                        self.display_sides.push(None);
                    }
                    index += 1;
                    continue;
                }
                let run_start = index;
                while index < next_end && self.rows[index].kind != RowKind::Equal {
                    index += 1;
                }
                // Unified hunks show all removals before all additions, without alignment holes.
                for side in [Focus::Before, Focus::After] {
                    for aligned in run_start..index {
                        if self.source_row(side, aligned).is_some() {
                            self.display_rows.push(DisplayRow::Line(aligned));
                            self.display_sides.push(Some(side));
                        }
                    }
                }
            }
            end = next_end;
        }
        if end < self.rows.len() {
            self.display_rows.push(DisplayRow::Fold {
                start: end,
                end: self.rows.len(),
            });
            if self.view_mode == ViewMode::Unified {
                self.display_sides.push(None);
            }
        }
        self.rebuild_source_ranges();
    }

    fn rebuild_source_ranges(&mut self) {
        self.source_ranges.iter_mut().for_each(Vec::clear);
        for display in 0..self.display_rows.len() {
            for (side, focus) in [Focus::Before, Focus::After].into_iter().enumerate() {
                let Some(source) = self.display_source_row(focus, display) else {
                    continue;
                };
                let ranges = &mut self.source_ranges[side];
                if let Some((range, first)) = ranges.last_mut()
                    && range.end == source
                    && *first + range.len() == display
                {
                    range.end += 1;
                } else {
                    ranges.push((source..source + 1, display));
                }
            }
        }
    }

    pub(crate) fn expand_source_excerpt(&mut self, focus: Focus, below: bool, source_row: usize) {
        let Some(display_row) = self.source_display_row(focus, source_row) else {
            return;
        };
        self.expand_excerpt_at(below, self.display_rows[display_row]);
    }

    fn expand_excerpt(&mut self, below: bool) {
        let Some(target) = self.display_rows.get(self.top).copied() else {
            return;
        };
        self.expand_excerpt_at(below, target);
    }

    fn expand_excerpt_at(&mut self, below: bool, target: DisplayRow) {
        let Some(top) = self.display_rows.get(self.top).copied() else {
            return;
        };
        let top_side = self.display_focus(self.top);
        let index = match target {
            DisplayRow::Line(row) => self
                .excerpt_ranges
                .iter()
                .position(|&(start, end)| start <= row && row < end),
            DisplayRow::Fold { end, .. } => self
                .excerpt_ranges
                .iter()
                .position(|&(start, _)| start == end)
                .or_else(|| self.excerpt_ranges.len().checked_sub(1)),
        };
        let Some(index) = index else {
            return;
        };
        let range = &mut self.excerpt_ranges[index];
        let previous = *range;
        if below {
            range.1 = range
                .1
                .saturating_add(self.expand_lines)
                .min(self.rows.len());
        } else {
            range.0 = range.0.saturating_sub(self.expand_lines);
        }
        if *range == previous {
            return;
        }
        let anchor = match top {
            DisplayRow::Line(row) => row,
            DisplayRow::Fold { end, .. } if !below && end == previous.0 => range.0,
            DisplayRow::Fold { start, .. } => start,
        };
        self.rebuild_display_rows();
        self.restore_top(Some(DisplayRow::Line(anchor)), top_side);
    }
}

fn append_rows(
    rows: &mut Vec<AlignedRow>,
    before: usize,
    before_end: usize,
    after: usize,
    after_end: usize,
    changed: bool,
) {
    for offset in 0..(before_end - before).max(after_end - after) {
        let before = (offset < before_end - before).then_some(before + offset);
        let after = (offset < after_end - after).then_some(after + offset);
        let kind = match (before, after) {
            (Some(_), Some(_)) if changed => RowKind::Changed,
            (Some(_), Some(_)) => RowKind::Equal,
            (Some(_), None) => RowKind::Removed,
            (None, Some(_)) => RowKind::Added,
            (None, None) => unreachable!("alignment has at least one line"),
        };
        rows.push(AlignedRow {
            before,
            after,
            kind,
            changes: Vec::new(),
        });
    }
}

fn refresh_character_changes(
    rows: &mut [AlignedRow],
    file: &ChangedFile,
    after_rows: std::ops::Range<usize>,
) {
    let deadline = Instant::now() + Duration::from_millis(20);
    for row in rows {
        if row.kind == RowKind::Changed
            && let Some(after) = row.after.filter(|index| after_rows.contains(index))
        {
            if file.before.len().saturating_add(file.after.len()) <= 20_000 {
                row.changes = character_changes_deadline(
                    &file.before[row.before.expect("paired changed row")],
                    &file.after[after],
                    deadline,
                );
            } else {
                row.changes.clear();
            }
        }
    }
}

fn character_changes(before: &str, after: &str) -> Vec<[std::ops::Range<usize>; 2]> {
    character_changes_deadline(before, after, Instant::now() + Duration::from_millis(2))
}

fn character_changes_deadline(
    before: &str,
    after: &str,
    deadline: Instant,
) -> Vec<[std::ops::Range<usize>; 2]> {
    if before == after {
        return Vec::new();
    }
    // Large/minified lines keep their complete text and line-level change shading,
    // without allocating a grapheme table proportional to their size.
    if before.len().saturating_add(after.len()) > 64 * 1024 || Instant::now() >= deadline {
        return vec![[0..usize::MAX, 0..usize::MAX]];
    }
    let before: Vec<_> = before.graphemes(true).collect();
    let after: Vec<_> = after.graphemes(true).collect();
    let mut changes: Vec<[std::ops::Range<usize>; 2]> = Vec::new();
    let (mut before_offset, mut after_offset) = (0, 0);
    for op in similar::capture_diff_slices_deadline(
        similar::Algorithm::Myers,
        &before,
        &after,
        if before.len().saturating_add(after.len()) <= 2048 {
            None
        } else {
            Some(deadline.min(Instant::now() + Duration::from_millis(2)))
        },
    ) {
        let (tag, before, after) = op.as_tag_tuple();
        // similar's optimized empty-range anchors can be stale; consume lengths in order.
        let before = before_offset..before_offset + before.len();
        let after = after_offset..after_offset + after.len();
        (before_offset, after_offset) = (before.end, after.end);
        if tag == similar::DiffTag::Equal {
            continue;
        }
        if let Some(previous) = changes.last_mut()
            && (before.start - previous[0].end <= 2 || after.start - previous[1].end <= 2)
        {
            previous[0].end = before.end;
            previous[1].end = after.end;
        } else {
            changes.push([before, after]);
        }
    }

    // A merge can enable an earlier gap; VS Code bounds this pass to 11 sweeps.
    for _ in 0..11 {
        let mut kept = 1;
        for index in 1..changes.len() {
            if short_match_between_large_changes(&before, &changes[kept - 1], &changes[index]) {
                changes[kept - 1][0].end = changes[index][0].end;
                changes[kept - 1][1].end = changes[index][1].end;
            } else {
                if kept != index {
                    changes[kept] = changes[index].clone();
                }
                kept += 1;
            }
        }
        if kept >= changes.len() {
            break;
        }
        changes.truncate(kept);
    }
    changes
}

fn short_match_between_large_changes(
    before: &[&str],
    previous: &[std::ops::Range<usize>; 2],
    next: &[std::ops::Range<usize>; 2],
) -> bool {
    let gap = &before[previous[0].end..next[0].start];
    if gap.len() > 500 {
        return false;
    }
    let start = gap
        .iter()
        .position(|grapheme| !grapheme.trim().is_empty())
        .unwrap_or(gap.len());
    let trimmed_len = gap[start..]
        .iter()
        .rposition(|grapheme| !grapheme.trim().is_empty())
        .map_or(0, |end| end + 1);
    if trimmed_len > 20 {
        return false;
    }

    // VS Code's size heuristic, specialized to single lines and normalized by 130^2.25:
    // https://github.com/microsoft/vscode/blob/main/src/vs/editor/common/diff/defaultLinesDiffComputer/heuristicSequenceOptimizations.ts
    let score = |change: &[std::ops::Range<usize>; 2]| {
        let weight = change
            .iter()
            .map(|range| {
                let length = range.len().min(130) as f64 / 130.0;
                length * length.sqrt()
            })
            .sum::<f64>();
        weight * weight.sqrt()
    };
    score(previous) + score(next) > 1.3
}

fn line_width(line: &str) -> usize {
    #[cfg(test)]
    tests::WIDTH_SCANS.with(|scans| scans.set(scans.get() + 1));
    if line.is_ascii() {
        if !line.bytes().any(|byte| byte < 32 || byte == 127) {
            return line.len();
        }
        return line.bytes().fold(0, |column, byte| {
            column
                + match byte {
                    b'\t' => 4 - column % 4,
                    0..=31 | 127 => 0,
                    _ => 1,
                }
        });
    }
    line.graphemes(true).fold(0, |column, grapheme| {
        column
            + if grapheme == "\t" {
                4 - column % 4
            } else {
                UnicodeWidthStr::width(grapheme)
            }
    })
}

fn changed_line_range(old: &[String], new: &[String]) -> (usize, usize, usize) {
    let start = old
        .iter()
        .zip(new)
        .take_while(|(old, new)| old == new)
        .count();
    let suffix = old[start..]
        .iter()
        .rev()
        .zip(new[start..].iter().rev())
        .take_while(|(old, new)| old == new)
        .count();
    (start, old.len() - suffix, new.len() - suffix)
}

fn stable_after_edit(
    file: &ChangedFile,
    lines: &[String],
    (start, old_end, new_end): (usize, usize, usize),
) -> bool {
    if old_end != start + 1
        || new_end != start + 1
        || file.after_eof != (lines.len() > 1 && lines.last().is_some_and(String::is_empty))
        || live_content(&file.after, file.after_eof).len()
            != live_content(lines, file.after_eof).len()
    {
        return false;
    }
    let old = &file.after[start];
    let new = &lines[start];
    // With neither token matching another line, the equality graph seen by Myers
    // is unchanged. Hunks, alignment, excerpts and scroll anchors remain exact.
    !file.before.iter().any(|line| line == old || line == new)
        && !file
            .after
            .iter()
            .enumerate()
            .any(|(index, line)| index != start && (line == old || line == new))
}

fn replace_after_lines(
    after: &mut Vec<String>,
    widths: &mut Vec<usize>,
    max_width: &mut usize,
    lines: &[String],
    (start, old_end, new_end): (usize, usize, usize),
) {
    let removed_max = widths[start..old_end].iter().copied().max().unwrap_or(0);
    if old_end == new_end {
        for index in start..new_end {
            if after[index] != lines[index] {
                after[index].clone_from(&lines[index]);
                widths[index] = line_width(&lines[index]);
            }
        }
    } else {
        after.splice(start..old_end, lines[start..new_end].iter().cloned());
        widths.splice(
            start..old_end,
            lines[start..new_end].iter().map(|line| line_width(line)),
        );
    }
    let added_max = widths[start..new_end].iter().copied().max().unwrap_or(0);
    *max_width = if removed_max == *max_width && added_max < removed_max {
        widths.iter().copied().max().unwrap_or(0)
    } else {
        (*max_width).max(added_max)
    };
}

fn live_content(lines: &[String], eof: bool) -> &[String] {
    if eof || (lines.len() == 1 && lines[0].is_empty()) {
        &lines[..lines.len().saturating_sub(1)]
    } else {
        lines
    }
}

pub(crate) fn line_hunks(
    before: &[String],
    before_eof: bool,
    after: &[String],
    after_eof: bool,
) -> Vec<Hunk> {
    // Empty sides need neither a token table nor a Myers frontier.
    if before.is_empty() || after.is_empty() {
        return if before.is_empty() && after.is_empty() {
            Vec::new()
        } else {
            vec![Hunk {
                before_start: 0,
                before_len: before.len(),
                after_start: 0,
                after_len: after.len(),
            }]
        };
    }
    let same = |old: usize, new: usize| {
        before[old] == after[new]
            && (old + 1 < before.len() || before_eof) == (new + 1 < after.len() || after_eof)
    };
    let large = before.len().saturating_add(after.len()) > 20_000;
    // Keep ordinary Myers/Compact anchoring unchanged; large comparisons avoid
    // tokenizing their already-equal prefix and suffix.
    let (start, suffix) = if large {
        let start = (0..before.len().min(after.len()))
            .take_while(|&index| same(index, index))
            .count();
        let suffix = (0..(before.len() - start).min(after.len() - start))
            .take_while(|&offset| same(before.len() - 1 - offset, after.len() - 1 - offset))
            .count();
        (start, suffix)
    } else {
        (0, 0)
    };
    let before_end = before.len() - suffix;
    let after_end = after.len() - suffix;
    if start == before_end || start == after_end {
        return if start == before_end && start == after_end {
            Vec::new()
        } else {
            vec![Hunk {
                before_start: start,
                before_len: before_end - start,
                after_start: start,
                after_len: after_end - start,
            }]
        };
    }
    fn tokens(lines: &[String], eof: bool, range: std::ops::Range<usize>) -> Vec<(&str, bool)> {
        range
            .map(|index| (lines[index].as_str(), index + 1 < lines.len() || eof))
            .collect()
    }
    let old = tokens(before, before_eof, start..before_end);
    let new = tokens(after, after_eof, start..after_end);
    // ponytail: after 100ms Myers may conservatively replace unresolved lines;
    // every source line remains present, never mislabeled as equal.
    let mut old_offset = start;
    let mut new_offset = start;
    similar::capture_diff_slices_deadline(
        similar::Algorithm::Myers,
        &old,
        &new,
        Some(Instant::now() + Duration::from_millis(100)),
    )
    .into_iter()
    .filter_map(|op| {
        let (tag, old, new) = op.as_tag_tuple();
        let hunk = Hunk {
            before_start: if large { old_offset } else { old.start },
            before_len: old.len(),
            after_start: if large { new_offset } else { new.start },
            after_len: new.len(),
        };
        old_offset += old.len();
        new_offset += new.len();
        (tag != similar::DiffTag::Equal).then_some(hunk)
    })
    .collect()
}

fn file_is_changed(file: &ChangedFile) -> bool {
    !file.loaded
        || file.status != FileStatus::Modified
        || !file.hunks.is_empty()
        || file.notice.as_ref().is_some_and(|notice| {
            notice
                .split("; ")
                .any(|part| !part.starts_with("EOF newline:"))
        })
}

fn update_eof_notice(file: &mut ChangedFile) {
    let mut notices = file
        .notice
        .as_deref()
        .unwrap_or("")
        .split("; ")
        .filter(|part| !part.is_empty() && !part.starts_with("EOF newline:"))
        .map(str::to_owned)
        .collect::<Vec<_>>();
    let before_empty = file.before.is_empty();
    let after_empty = live_content(&file.after, file.after_eof).is_empty();
    if (!before_empty && !file.before_eof) || (!after_empty && !file.after_eof) {
        let label = |absent, empty, eof| {
            if absent {
                "absent"
            } else if empty {
                "empty file"
            } else if eof {
                "present"
            } else {
                "missing"
            }
        };
        notices.push(format!(
            "EOF newline: Before {}, After {}",
            label(
                file.status == FileStatus::Added,
                before_empty,
                file.before_eof
            ),
            label(false, after_empty, file.after_eof)
        ));
    }
    file.notice = (!notices.is_empty()).then(|| notices.join("; "));
}

#[cfg(test)]
#[path = "../tests/unit/diff.rs"]
mod tests;