feat: continue nested markdown lists on return

Sources/Conjure/NotesController.swift

@@ -119,6 +119,77 @@ final class NotesController: NSViewController, NSTextViewDelegate {
         _ = save()
     }
 
+    private static let listItem = try! NSRegularExpression(
+        pattern: "^([ \\t]*)([-+*]|([0-9]+)([.)]))([ \\t]+|$)(?:\\[([ xX])\\]([ \\t]+|$))?"
+    )
+
+    private static func indentationWidth(_ indentation: String) -> Int {
+        indentation.reduce(0) { $1 == "\t" ? $0 + 4 - $0 % 4 : $0 + 1 }
+    }
+
+    private static func continuedPrefix(_ item: NSTextCheckingResult, in line: NSString) -> String? {
+        var marker = line.substring(with: item.range(at: 2))
+        if item.range(at: 3).location != NSNotFound {
+            guard let number = UInt64(line.substring(with: item.range(at: 3))), number < UInt64.max else { return nil }
+            marker = "\(number + 1)" + line.substring(with: item.range(at: 4))
+        }
+        var prefix = line.substring(with: item.range(at: 1)) + marker + line.substring(with: item.range(at: 5))
+        if item.range(at: 6).location != NSNotFound {
+            prefix += "[ ]" + line.substring(with: item.range(at: 7))
+        }
+        return prefix
+    }
+
+    func textView(_ textView: NSTextView, doCommandBy commandSelector: Selector) -> Bool {
+        guard textView === editor, editor.isEditable,
+              commandSelector == #selector(NSResponder.insertNewline(_:)) else { return false }
+        let source = editor.string as NSString
+        let selection = editor.selectedRange()
+        var start = 0, end = 0
+        source.getLineStart(&start, end: nil, contentsEnd: &end,
+                            for: NSRange(location: selection.location, length: 0))
+        let line = source.substring(with: NSRange(location: start, length: end - start)) as NSString
+        guard let item = Self.listItem.firstMatch(in: line as String, range: NSRange(location: 0, length: line.length)),
+              selection.location >= start + item.range.length, NSMaxRange(selection) <= end,
+              !MarkdownHighlighting.codeBlocks(in: editor.string).contains(where: {
+                  selection.location >= $0.body.location && selection.location <= NSMaxRange($0.body)
+              }) else { return false }
+        let indent = line.substring(with: item.range(at: 1))
+        let marker = line.substring(with: item.range(at: 2))
+        // Spaced thematic breaks are not list items.
+        if marker == "-" || marker == "*" {
+            let characters = (line as String).lazy.filter { !$0.isWhitespace }
+            let bullet = marker.first!
+            if characters.count >= 3, characters.allSatisfy({ $0 == bullet }) { return false }
+        }
+        let body = line.substring(from: item.range.length)
+        if body.trimmingCharacters(in: .whitespaces).isEmpty {
+            var replacement = ""
+            var previous = start
+            let depth = Self.indentationWidth(indent)
+            while depth > 0, previous > 0 {
+                let range = source.lineRange(for: NSRange(location: previous - 1, length: 0))
+                previous = range.location
+                let ancestor = source.substring(with: range) as NSString
+                if let parent = Self.listItem.firstMatch(in: ancestor as String, range: NSRange(location: 0, length: ancestor.length)) {
+                    let parentIndent = ancestor.substring(with: parent.range(at: 1))
+                    if Self.indentationWidth(parentIndent) < depth {
+                        replacement = Self.continuedPrefix(parent, in: ancestor) ?? ""
+                        break
+                    }
+                } else if !(ancestor as String).trimmingCharacters(in: .whitespacesAndNewlines).isEmpty {
+                    let leading = (ancestor as String).prefix { $0 == " " || $0 == "\t" }
+                    if Self.indentationWidth(String(leading)) < depth { break }
+                }
+            }
+            editor.insertText(replacement, replacementRange: NSRange(location: start, length: end - start))
+        } else {
+            guard let prefix = Self.continuedPrefix(item, in: line) else { return false }
+            editor.insertText("\n" + prefix, replacementRange: selection)
+        }
+        return true
+    }
+
     @discardableResult
     func save() -> Bool {
         guard isDirty, let store, let selectedID else { return true }
@@ -244,7 +315,7 @@ enum MarkdownHighlighting {
     }()
     private static let openingFence = try! NSRegularExpression(pattern: "(?m)^ {0,3}(`{3,}|~{3,})([^\\r\\n]*)\\r?\\n")
 
-    private static func codeBlocks(in text: String) -> [(range: NSRange, body: NSRange, language: String?)] {
+    fileprivate static func codeBlocks(in text: String) -> [(range: NSRange, body: NSRange, language: String?)] {
         let source = text as NSString
         var blocks: [(NSRange, NSRange, String?)] = []
         var cursor = 0