fix: highlight fenced code by language

Sources/Conjure/NotesController.swift

@@ -1,6 +1,7 @@
 import AppKit
 import ConjureKit
 import UniformTypeIdentifiers
+import Highlighter
 
 @MainActor
 final class NotesController: NSViewController, NSTextViewDelegate {
@@ -208,9 +209,36 @@ enum MarkdownHighlighting {
         (try! NSRegularExpression(pattern: "\\*\\*[^\\n]+?\\*\\*|__[^\\n]+?__"), [.font: NSFont.boldSystemFont(ofSize: 14)]),
         (try! NSRegularExpression(pattern: "(?<!\\*)\\*(?!\\*)[^*\\n]+\\*(?!\\*)|(?<!_)_(?!_)[^_\\n]+_(?!_)"), [.font: NSFontManager.shared.convert(.systemFont(ofSize: 14), toHaveTrait: .italicFontMask)]),
         (try! NSRegularExpression(pattern: "!?\\[[^\\]\\n]+\\]\\([^\\n)]+\\)|(?m)^\\s*(?:[-+*]|[0-9]+\\.)\\s"), [.foregroundColor: SunsetPalette.accent]),
-        (try! NSRegularExpression(pattern: "`[^`\\n]+`|(?ms)^```[^\\n]*\\n.*?(?:^```[^\\n]*$|\\z)"), [.font: NSFont.monospacedSystemFont(ofSize: 14, weight: .regular), .foregroundColor: NSColor.systemGreen]),
+        (try! NSRegularExpression(pattern: "`[^`\\n]+`"), [.font: NSFont.monospacedSystemFont(ofSize: 14, weight: .regular), .foregroundColor: NSColor.systemGreen]),
     ]
 
+    private static let highlighter: Highlighter = {
+        guard let value = Highlighter() else { preconditionFailure("Missing bundled syntax highlighter") }
+        value.setTheme("atom-one-dark", withFont: "Menlo-Regular", ofSize: 14)
+        return value
+    }()
+    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?)] {
+        let source = text as NSString
+        var blocks: [(NSRange, NSRange, String?)] = []
+        var cursor = 0
+        while let opening = openingFence.firstMatch(in: text, range: NSRange(location: cursor, length: source.length - cursor)) {
+            let fence = source.substring(with: opening.range(at: 1))
+            let marker = NSRegularExpression.escapedPattern(for: String(fence.first!))
+            let closing = try! NSRegularExpression(pattern: "(?m)^ {0,3}\(marker){\(fence.count),}[ \\t]*\\r?$")
+            let start = NSMaxRange(opening.range)
+            let end = closing.firstMatch(in: text, range: NSRange(location: start, length: source.length - start))
+            let bodyEnd = end?.range.location ?? source.length
+            cursor = end.map { NSMaxRange($0.range) } ?? source.length
+            let language = source.substring(with: opening.range(at: 2)).split(whereSeparator: { $0.isWhitespace }).first.map(String.init)
+            blocks.append((NSRange(location: opening.range.location, length: cursor - opening.range.location),
+                           NSRange(location: start, length: bodyEnd - start), language))
+            if cursor == source.length { break }
+        }
+        return blocks
+    }
+
     static func apply(to editor: NSTextView) {
         guard let storage = editor.textStorage else { return }
         let text = editor.string
@@ -226,11 +254,27 @@ enum MarkdownHighlighting {
             if scale < 1 { image.size = NSSize(width: size.width * scale, height: size.height * scale) }
         }
         storage.addAttributes([.font: NSFont.systemFont(ofSize: 14), .foregroundColor: SunsetPalette.text], range: range)
+        let blocks = codeBlocks(in: text)
         for (expression, attributes) in rules {
             for match in expression.matches(in: text, range: range) {
+                guard !blocks.contains(where: { NSIntersectionRange($0.range, match.range).length > 0 }) else { continue }
                 storage.addAttributes(attributes, range: match.range)
             }
         }
+        for block in blocks {
+            storage.addAttributes([.font: NSFont.monospacedSystemFont(ofSize: 14, weight: .regular),
+                                   .foregroundColor: SunsetPalette.secondaryText], range: block.range)
+            storage.addAttribute(.foregroundColor, value: SunsetPalette.text, range: block.body)
+            let code = (text as NSString).substring(with: block.body)
+            guard let highlighted = highlighter.highlight(code, as: block.language),
+                  highlighted.string == code else { continue }
+            highlighted.enumerateAttribute(.foregroundColor, in: NSRange(location: 0, length: highlighted.length)) { color, range, _ in
+                if let color {
+                    storage.addAttribute(.foregroundColor, value: color,
+                                         range: NSRange(location: block.body.location + range.location, length: range.length))
+                }
+            }
+        }
         editor.typingAttributes = [.font: NSFont.systemFont(ofSize: 14), .foregroundColor: SunsetPalette.text]
     }
 }