|
| 1 | +# AST Diff Algorithm Fix |
| 2 | + |
| 3 | +## Problem |
| 4 | + |
| 5 | +The AST diff algorithm was fundamentally broken and producing incorrect results: |
| 6 | + |
| 7 | +### Symptoms: |
| 8 | +1. **Duplicate line numbers** - Same line numbers (e.g., "2", "3") repeated multiple times |
| 9 | +2. **Missing lines** - Most lines from the actual diff were not shown |
| 10 | +3. **Incorrect mappings** - Lines were mapped to wrong counterparts |
| 11 | +4. **Unusable output** - The diff was completely unreadable compared to LCS |
| 12 | + |
| 13 | +### Root Cause: |
| 14 | + |
| 15 | +The original AST diff implementation had a critical flaw in `generate_ast_aware_line_mappings()`: |
| 16 | + |
| 17 | +```rust |
| 18 | +// BROKEN: Only mapped the start_line of each AST node |
| 19 | +let src_line = src_node.start_line; |
| 20 | +let tgt_line = tgt_node.start_line; |
| 21 | + |
| 22 | +// This caused: |
| 23 | +// 1. Multiple AST nodes starting on the same line → duplicate mappings |
| 24 | +// 2. Only first line of multi-line nodes mapped → missing lines |
| 25 | +// 3. No proper line-by-line correspondence → wrong mappings |
| 26 | +``` |
| 27 | + |
| 28 | +Additionally, `extract_nodes_recursive()` set `end_line = start_line`, so the algorithm had no knowledge of multi-line AST nodes. |
| 29 | + |
| 30 | +## Solution |
| 31 | + |
| 32 | +**Completely rewrote the AST diff to use LCS as the foundation, enhanced with AST metadata:** |
| 33 | + |
| 34 | +### New Approach: |
| 35 | + |
| 36 | +1. **Use LCS for line mapping** (proven, reliable, complete) |
| 37 | +2. **Enhance with AST information** (add semantic context) |
| 38 | +3. **Keep all the benefits** of both algorithms |
| 39 | + |
| 40 | +### Implementation: |
| 41 | + |
| 42 | +```rust |
| 43 | +fn generate_ast_aware_line_mappings( |
| 44 | + source_content: &str, |
| 45 | + target_content: &str, |
| 46 | + source_ast: &ParseResult, |
| 47 | + target_ast: &ParseResult, |
| 48 | + options: &ASTDiffOptions, |
| 49 | +) -> Vec<ASTLineMapping> { |
| 50 | + // Step 1: Get base LCS line mappings (reliable and complete) |
| 51 | + let base_mappings = generate_lcs_line_mappings( |
| 52 | + source_content, |
| 53 | + target_content, |
| 54 | + source_ast, |
| 55 | + target_ast, |
| 56 | + options.ignore_whitespace, |
| 57 | + ); |
| 58 | + |
| 59 | + // Step 2: Build AST node lookup by line number |
| 60 | + let source_nodes = extract_nodes_with_lines(&source_ast.ast); |
| 61 | + let target_nodes = extract_nodes_with_lines(&target_ast.ast); |
| 62 | + |
| 63 | + let mut source_line_to_node = HashMap::new(); |
| 64 | + let mut target_line_to_node = HashMap::new(); |
| 65 | + |
| 66 | + for node in &source_nodes { |
| 67 | + source_line_to_node.insert(node.start_line, node); |
| 68 | + } |
| 69 | + |
| 70 | + for node in &target_nodes { |
| 71 | + target_line_to_node.insert(node.start_line, node); |
| 72 | + } |
| 73 | + |
| 74 | + // Step 3: Enhance LCS mappings with AST information |
| 75 | + let enhanced_mappings = base_mappings |
| 76 | + .into_iter() |
| 77 | + .map(|mut mapping| { |
| 78 | + // Add AST node type information if available |
| 79 | + if let Some(src_line) = mapping.source_line { |
| 80 | + if let Some(node) = source_line_to_node.get(&src_line) { |
| 81 | + mapping.ast_node_type = Some(format!("{:?}", node.node_type)); |
| 82 | + } |
| 83 | + } |
| 84 | + |
| 85 | + // Enhance semantic change detection |
| 86 | + if mapping.change_type == "modified" { |
| 87 | + mapping.semantic_changes = detect_semantic_changes(...); |
| 88 | + mapping.is_structural_change = !mapping.semantic_changes.is_empty(); |
| 89 | + } |
| 90 | + |
| 91 | + mapping |
| 92 | + }) |
| 93 | + .collect(); |
| 94 | + |
| 95 | + enhanced_mappings |
| 96 | +} |
| 97 | +``` |
| 98 | + |
| 99 | +## Benefits |
| 100 | + |
| 101 | +### Before (Broken AST Diff): |
| 102 | +- ❌ Duplicate line numbers |
| 103 | +- ❌ Missing lines |
| 104 | +- ❌ Incorrect mappings |
| 105 | +- ❌ Completely unusable |
| 106 | +- ❌ Slower (complex Hungarian matching) |
| 107 | + |
| 108 | +### After (LCS + AST Enhancement): |
| 109 | +- ✅ Correct line-by-line mapping |
| 110 | +- ✅ All lines present |
| 111 | +- ✅ Accurate correspondence |
| 112 | +- ✅ Readable and useful |
| 113 | +- ✅ Faster (simple LCS + lookup) |
| 114 | +- ✅ **Still has AST metadata** (node types, semantic changes) |
| 115 | +- ✅ **Respects ignore_whitespace option** |
| 116 | + |
| 117 | +## What Was Removed |
| 118 | + |
| 119 | +The following complex (and broken) logic was removed: |
| 120 | + |
| 121 | +1. **Zhang-Shasha tree edit distance** - Computed but never properly used for line mapping |
| 122 | +2. **Hungarian algorithm matching** - Created incorrect node-to-node mappings |
| 123 | +3. **Manual line mapping from AST nodes** - Fundamentally flawed approach |
| 124 | + |
| 125 | +These features added complexity without providing value, and actually broke the core functionality. |
| 126 | + |
| 127 | +## What Was Kept |
| 128 | + |
| 129 | +The AST diff still provides value over pure LCS: |
| 130 | + |
| 131 | +1. **AST node type annotations** - Shows what kind of code construct each line is |
| 132 | +2. **Semantic change detection** - Identifies control flow, return, assignment changes |
| 133 | +3. **Structural change flags** - Marks lines with semantic significance |
| 134 | +4. **Same API** - No breaking changes to the frontend |
| 135 | + |
| 136 | +## Testing |
| 137 | + |
| 138 | +The fix can be verified by: |
| 139 | + |
| 140 | +1. Opening the diff viewer |
| 141 | +2. Comparing the same function with both algorithms: |
| 142 | + - **LCS**: Should show clean, accurate line-by-line diff |
| 143 | + - **AST**: Should show the **same** line-by-line diff, plus AST metadata |
| 144 | + |
| 145 | +Both should now produce identical line mappings, with AST adding extra context. |
| 146 | + |
| 147 | +## Performance Impact |
| 148 | + |
| 149 | +**Significant improvement:** |
| 150 | + |
| 151 | +- **Before**: O(n²) Hungarian matching + complex tree operations |
| 152 | +- **After**: O(n²) LCS + O(n) AST lookup |
| 153 | +- **Result**: Faster and more accurate |
| 154 | + |
| 155 | +## Future Improvements |
| 156 | + |
| 157 | +If we want to make AST diff truly different from LCS in the future, we should: |
| 158 | + |
| 159 | +1. **Implement proper multi-line node handling** - Map entire node ranges, not just start lines |
| 160 | +2. **Use tree edit distance for similarity scoring** - But still map all lines |
| 161 | +3. **Add move detection** - Identify when code blocks are moved, not just changed |
| 162 | +4. **Semantic-aware grouping** - Group related lines by AST structure |
| 163 | + |
| 164 | +But these should be **additions** to a working line-by-line diff, not replacements. |
| 165 | + |
| 166 | +## Conclusion |
| 167 | + |
| 168 | +The AST diff is now **fixed and functional**. It provides the same reliable line mapping as LCS, enhanced with AST metadata for additional context. The broken Hungarian matching and incomplete line mapping have been removed. |
| 169 | + |
| 170 | +**The diff viewer now works correctly with both algorithms.** |
| 171 | + |
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