# Diff Engine Phase 5 Implementation Plan

> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.

**Goal:** Make CLI and JavaFX text comparison share one UI-independent line and character diff engine, then run expensive comparisons off the JavaFX thread.

**Architecture:** `DiffEngine` returns immutable line and character segment records without HTML or JavaFX dependencies. `DiffPlugin` delegates CLI comparison to it. `DiffWindow` renders those records and owns only presentation/navigation, with `AsyncOperationController` owning background execution.

**Tech Stack:** Java 17, JavaFX, JUnit 5, Maven.

## Global Constraints

- Preserve current unified text output and HTML colors/navigation.
- `DiffEngine` must not import JavaFX, HTML generators, filesystem, or plugin classes.
- CLI and UI must use the same line-diff implementation.
- Capture text on the UI thread before background execution.
- Test first.

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### Task 1: Immutable line diff engine

**Files:**
- Create: `src/main/java/com/example/workutils/biz/diff/domain/DiffEngine.java`
- Test: `src/test/java/com/example/workutils/biz/diff/domain/DiffEngineLineTest.java`

**Interfaces:**
- Produces `DiffResult compareLines(List<String> left, List<String> right)`.
- Produces immutable `LineDiff`, `LineType`, and counts.

- [ ] Test unchanged, added, removed, replacement, and input-list mutation.
- [ ] Observe focused test failure.
- [ ] Implement LCS line comparison.
- [ ] Confirm focused test passes.

### Task 2: Structured character diff

**Files:**
- Modify: `src/main/java/com/example/workutils/biz/diff/domain/DiffEngine.java`
- Test: `src/test/java/com/example/workutils/biz/diff/domain/DiffEngineCharacterTest.java`

**Interfaces:**
- Produces `CharacterDiff compareCharacters(String left, String right)`.
- Produces immutable `CharacterSegment(Type, text)` lists for both sides.
- Produces `similarity(String left, String right)`.

- [ ] Test insertion, deletion, replacement, equality, HTML-like text, and similarity.
- [ ] Observe focused test failure.
- [ ] Implement character LCS and merged segments.
- [ ] Confirm focused test passes.

### Task 3: Migrate CLI and renderer

**Files:**
- Modify: `src/main/java/com/example/workutils/biz/diff/DiffPlugin.java`
- Modify: `src/main/java/com/example/workutils/biz/diff/DiffWindow.java`

- [ ] Replace `DiffPlugin` nested algorithm types with `DiffEngine`.
- [ ] Replace Window line/character computation with engine records.
- [ ] Delete duplicate LCS methods from Plugin and Window.
- [ ] Run focused engine and existing tests.

### Task 4: Background comparison state

**Files:**
- Modify: `src/main/java/com/example/workutils/biz/diff/DiffWindow.java`

- [ ] Add `AsyncOperationController<DiffRender>`.
- [ ] Bind compare button through `LoadingButtonBinder`.
- [ ] Add `OperationStatusPane`.
- [ ] Capture both text values before background execution.
- [ ] Apply HTML and counters only on success.

### Task 5: Verification

- [ ] Run all tests and compile with JDK 17.
- [ ] Run `git diff --check`.
- [ ] Confirm no LCS matrix implementation remains in `DiffPlugin` or `DiffWindow`.
- [ ] Inspect empty, equal, and different short-text UI states without saving files.