jj-diff

jj-diff

Fast diff engine with histogram line matching, syntax highlighting, and context collapsing. Extracted from JayJay.

Zero dependency on jj-lib — usable in any Rust project that needs diff rendering.

Features

• Histogram line diff via similar — matches jj diff and reads well on code
• Word-level diff highlighting within changed lines
• Syntax highlighting — tree-sitter with 18 languages
• Context collapsing with display-to-full line index mapping and tiny-gap auto-expansion
• Side-by-side row building — pairs removed/added lines for two-column rendering
• Placeholder detection — Git LFS, submodule, binary file detection
• Skip highlighting for .lock/.csv/.svg files

Usage

use jj_diff::{compute_file_diff, collapse_context_with_mapping, build_side_by_side_rows};

// Compute diff
let diff = compute_file_diff("main.rs", &old_content, &new_content, false);

// Collapse context for display
let collapsed = collapse_context_with_mapping(&diff);

// Build side-by-side rows
let rows = build_side_by_side_rows(&collapsed.diff.lines);
for row in &rows {
    // row.old_spans, row.new_spans, row.old_style, row.new_style
}

API

Function	Description
compute_file_diff(path, old, new, ignore_ws)	Collapsed diff (3 lines context)
compute_file_diff_full(path, old, new, ignore_ws)	Full diff (all lines, for editing)
collapse_context_with_mapping(diff)	Collapse with display→full line mapping
build_side_by_side_rows(lines)	Pair lines into SideBySideRow for two-column view
is_git_lfs(text) / is_git_submodule(text)	Placeholder detection

tree-sitter syntax highlighting is applied internally by the compute_file_diff* functions; only the SyntaxToken span labels are exported.

Supported Languages

Bash, C, C++, CSS, Go, HTML, Java, JavaScript, JSON, Markdown, Python, Ruby, Rust, Swift, TOML, TypeScript, YAML

Platform Support

What's in this crate (cross-platform, pure Rust)

Everything needed to compute and structure diffs:

• Diff computation (histogram algorithm)
• Word-level diff within lines
• Syntax highlighting (tree-sitter)
• Context collapsing with index mapping
• Side-by-side row pairing
• Placeholder/binary file detection
• All diff types (FileDiff, DiffLine, DiffSpan, SideBySideRow, etc.)

What platform clients need to provide

The rendering layer is platform-specific:

macOS (done — JayJayDiffUI Swift package)

• NativeDiffView — NSTextView-based unified diff with gutter, find bar, line selection
• SideBySideDiffView — Two NSTextViews with synchronized scrolling
• DiffGutterTextView — Line numbers, +/- markers, checkbox support
• DiffColors — NSColor theme (dark/light)
• DiffStore — SwiftUI @Observable cache + preloading via JayJayCore FFI

Linux (planned — GTK via gtk-rs)

Needed:

• GtkDiffView — GtkTextView or GtkSourceView-based unified diff renderer
• GtkSideBySideDiffView — Two GtkTextViews with scroll sync (GtkScrolledWindow)
• Gutter widget — line numbers + markers (GtkTextView gutter or custom DrawingArea)
• Color theme — map DiffColors values to GdkRGBA
• Text tags — map DiffSpanStyle → GtkTextTag for background/foreground colors
• Font handling — monospace font configuration

Can reuse directly from Rust (no FFI needed):

• compute_file_diff / compute_file_diff_full
• collapse_context_with_mapping
• build_side_by_side_rows
• All placeholder detection
• All diff types

Estimated scope: ~500-800 lines of GTK widget code (vs ~1500 lines macOS AppKit).

iOS (future — UIKit/SwiftUI)

Needed:

• UIKitDiffView — UITextView-based renderer (or pure SwiftUI Text with AttributedString)
• Side-by-side — horizontal UIScrollView with two UITextViews
• Touch-friendly gutter — larger tap targets, no right-click context menu
• UIColor theme mapping

Can reuse via UniFFI (same as macOS):

• All jj-diff functions via JayJayCore FFI bridge
• DiffStore (SwiftUI @Observable — works on iOS)
• DiffPlaceholders, SideBySideRow types

Estimated scope: ~800-1000 lines of UIKit view code. Could share DiffColors and DiffStore with macOS via the JayJayDiffUI package if made cross-platform with #if os() guards.

License

Apache-2.0