1959ff2446
- Add LZWDecoder filter using lzw crate v0.10 - Support /EarlyChange parameter (default 1, late 0) - Early change (1): Adobe/TIFF variant, code size increases BEFORE - Late change (0): GIF variant, code size increases AFTER - Full predictor support (TIFF predictor 2, PNG predictors 10-15) - Bomb limit protection with partial bytes on exceed - INV-8 maintained: partial bytes returned on decode errors - 23 tests pass (19 unit tests + 4 proptests) - Fixtures generated using lzw crate for verification Acceptance criteria: - Critical test /EarlyChange=0 byte-perfect: PASS - LZWDecode without /DecodeParms defaults: PASS - LZWDecode + /Predictor 12: PASS - Truncated stream partial bytes: PASS - Bomb limit honored: PASS - proptest no panic: PASS - INV-8 maintained: PASS Refs: Plan Phase 1.5 line 1142, PDF spec 7.4.4 Co-Authored-By: Claude Code <noreply@anthropic.com>
86 lines
3.3 KiB
Markdown
86 lines
3.3 KiB
Markdown
# pdftract-3uu6v: LZWDecode Implementation Verification Note
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## Summary
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Implemented LZWDecode filter with /EarlyChange parameter support (default 1, late 0) and full predictor support (predictors 2, 10-15) matching FlateDecode.
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## Acceptance Criteria Results
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### PASS: Critical test - LZWDecode with /EarlyChange 0 byte-perfect against reference
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- Test: `test_lzw_fixture_simple_late_change`
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- Fixtures: `lzw_simple_late.bin` decodes to `lzw_simple_orig.bin`
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- Result: Byte-perfect match with reference output generated by lzw crate
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### PASS: LZWDecode without /DecodeParms (defaults)
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- Test: `test_lzw_decode_simple_early_change`
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- Default behavior: EarlyChange = 1, no predictor
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- Result: Correct decode with default parameters
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### PASS: LZWDecode + /Predictor 12 (PNG Up)
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- Tests: `test_lzw_decode_predictor`, `test_lzw_fixture_with_predictor`
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- Fixtures: `lzw_predictor_encoded.bin` with predictor parameters
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- Result: Predictor correctly applied after LZW decode
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### PASS: Truncated LZW stream
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- Test: `test_lzw_decode_truncated_stream`, `test_lzw_fixture_truncated`
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- Result: Returns partial bytes (INV-8 maintained)
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### PASS: Bomb limit honored
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- Test: `test_lzw_bomb_limit`
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- Result: Bomb limit enforced, partial bytes returned when exceeded
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### PASS: proptest - random byte sequences never panic
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- Tests: 4 proptests covering random data, early/late change, bomb limits, predictors
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- Result: No panics on any input
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### PASS: INV-8 maintained
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- All error paths return partial bytes instead of panicking
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- Decode errors return accumulated output before failure
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## Implementation Details
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### Files Modified
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- `crates/pdftract-core/src/parser/stream.rs`: Added LZWDecoder struct (605 lines)
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- `Cargo.toml`: Added `lzw = "0.10"` workspace dependency
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### Files Added
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- `crates/pdftract-core/examples/test_lzw_api.rs`: LZW crate API exploration
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- `tests/fixtures/generate_lzw_fixtures.rs`: Fixture generator
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- `tests/fixtures/generate_lzw_fixtures_main.rs`: Alternative generator
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- 15 fixture files (.bin format)
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### API Used
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- `lzw` crate v0.10
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- `DecoderEarlyChange`: Early change variant (Adobe/TIFF, PDF default)
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- `Decoder`: Late change variant (GIF)
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- `MsbReader`: MSB bit order as required by PDF spec
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### Key Features
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1. **/EarlyChange parameter handling**:
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- Default 1 (early change) - code size increases BEFORE exceeding current size
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- Value 0 (late change) - code size increases AFTER (GIF variant)
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- Extracted via `PredictorParams::extract_early_change()`
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2. **Predictor support**:
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- Delegates to shared `apply_predictor()` function
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- Supports TIFF predictor 2 and PNG predictors 10-15
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- Predictor applied after LZW decode
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3. **Bomb limit protection**:
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- Budget checked after each decode chunk
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- Partial bytes returned when limit exceeded
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- Counter updated with final output size
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4. **Error handling (INV-8)**:
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- Truncated streams: returns partial bytes decoded so far
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- Decode errors: breaks loop, returns accumulated output
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- No panics on any input
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## Test Results
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All 23 LZW tests pass:
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- 19 unit tests (empty, simple, incremental, repeated, predictor, truncated, fixtures)
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- 4 proptests (no panic, bomb limit, early change, predictor)
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## References
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- Plan section: Phase 1.5 line 1142
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- PDF spec 7.4.4 (LZWDecode parameters)
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- Dependency Matrix: lzw = "0.10"
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