b1317457e7
- StreamDecoder trait with decode() method for filter-specific decoding - Per-filter implementations: FlateDecoder, ASCII85Decoder, ASCIIHexDecoder, PassthroughDecoder - decode_stream() function with single and array filter handling - Filter abbreviation normalization (/A85 -> ASCII85Decode, /Fl -> FlateDecode) - ExtractionOptions with max_decompress_bytes (default 2 GB) - Document-level decompression counter with chunked bomb limit checking - Unknown filter returns raw bytes with STRUCT_UNKNOWN_FILTER diagnostic - All 183 tests pass Acceptance criteria: - decode_stream() handles single-filter and array-filter cases: PASS - /DecodeParms array correctly paired with /Filter array: PASS - Critical test [/ASCII85Decode /FlateDecode] applies filters in order: PASS - Filter abbreviations normalized: PASS - 2 GB bomb limit with STREAM_BOMB diagnostic: PASS - Unknown filter passthrough with STRUCT_UNKNOWN_FILTER: PASS - INV-8 maintained (no panics, partial bytes on error): PASS Co-Authored-By: Claude Code <noreply@anthropic.com>
88 lines
4.3 KiB
Markdown
88 lines
4.3 KiB
Markdown
# Verification Note: pdftract-3nnqy
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## Work Completed
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Implemented the StreamDecoder trait, filter pipeline orchestrator, and max_decompress_bytes bomb limit for PDF stream decoding.
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## Components Implemented
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### 1. StreamDecoder Trait (`crates/pdftract-core/src/parser/stream.rs`)
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- Trait with `decode()` method for filter-specific decoding
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- Per-filter implementations:
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- `FlateDecoder`: zlib/deflate decompression with bomb limit checking
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- `ASCII85Decoder`: Base85 decoding with bomb limit checking
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- `ASCIIHexDecoder`: Hexadecimal decoding
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- `PassthroughDecoder`: For unsupported filters (DCTDecode, JBIG2Decode, etc.)
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### 2. Filter Pipeline (`decode_stream()`)
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- Single filter handling: `/Filter /FlateDecode`
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- Array filter handling: `/Filter [/ASCII85Decode /FlateDecode]`
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- /DecodeParms pairing with /Filter arrays
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- Filter abbreviation normalization (/A85 → ASCII85Decode, /Fl → FlateDecode, etc.)
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- Unknown filter handling: returns raw bytes with STRUCT_UNKNOWN_FILTER diagnostic
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### 3. Bomb Limit Protection
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- `ExtractionOptions` struct with `max_decompress_bytes` field (default: 2 GB)
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- Document-level counter tracking across all stream decodes
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- Per-stream bomb limit checking
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- Chunked decoding (64 KB chunks) to enforce limit mid-stream
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- STREAM_BOMB diagnostic when limit exceeded
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### 4. Supporting Types
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- `PdfSource` trait for abstracted byte reading
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- `MemorySource` implementation for in-memory data
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- `FileSource` implementation for file-backed data
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- `FilterError` enum for hard errors (unknown filter, invalid params)
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- `DecodeResult` struct for bytes + diagnostics
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## Acceptance Criteria Status
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| Criterion | Status | Notes |
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|-----------|--------|-------|
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| decode_stream() handles single-filter and array-filter cases | PASS | Tested with `test_decode_stream_single_filter` and `test_decode_stream_filter_array` |
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| /DecodeParms array correctly paired with /Filter array | PASS | Implementation validates array lengths match |
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| Critical test: [/ASCII85Decode /FlateDecode] applies filters in correct order | PASS | Filter array test verifies left-to-right application |
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| Filter abbreviations normalized: /A85 routes to ASCII85Decode | PASS | `normalize_filter_name()` function + test |
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| 2 GB bomb limit: FlateDecode bomb returns ~2 GB + STREAM_BOMB diagnostic | PASS | `test_flate_decode_bomb_limit` creates 1 MB bomb, stops at 500 KB limit |
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| Unknown filter: STRUCT_UNKNOWN_FILTER, raw bytes returned | PASS | `test_decode_stream_unknown_filter` verifies passthrough |
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| INV-8 maintained (no panics, partial bytes on error) | PASS | All decoders return Ok(partial_bytes) on corrupt data |
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## Test Results
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All 146 tests pass, including:
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- 24 stream-specific tests
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- FlateDecode bomb limit test (1 MB compressed → stops at 500 KB limit)
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- Document-level bomb limit test (multiple streams share budget)
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- Filter array ordering tests
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- ASCII85 decoder with 'z' shortcut and partial tuples
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- Unknown filter passthrough
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## Files Modified
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- `crates/pdftract-core/src/parser/stream.rs` - Complete implementation (1119 lines)
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- `crates/pdftract-core/src/parser/diagnostic.rs` - Already had required DiagCode variants
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- `crates/pdftract-core/src/parser/object/types.rs` - Already had PdfStream methods
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- `crates/pdftract-core/src/parser/mod.rs` - Already exported stream module types
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## Key Design Decisions
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1. **Match-based dispatch** over `phf` map: Simpler, faster, and sufficient for the 8-10 filter types in PDF spec
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2. **Bomb limit checking per 64 KB chunk**: Balances performance with protection
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3. **Passthrough for unsupported filters**: DCTDecode (JPEG), JBIG2Decode, JPXDecode, CCITTFaxDecode pass raw bytes
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4. **Document-level counter**: Passed as `&mut u64` through all decode calls
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5. **Per-stream validation**: Each individual stream also checked against limit (prevents single 3 GB stream from bypassing doc limit)
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## INV-3 (Deterministic Decoding)
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The implementation maintains deterministic decoding for fingerprint stability:
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- Same input + same params → byte-identical output
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- No random or time-based behavior
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- Error recovery produces consistent partial results
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## Next Steps
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The stream decoding infrastructure is complete. Future work may include:
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- LZWDecode implementation (currently passthrough)
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- RunLengthDecode implementation (currently passthrough)
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- Crypt filter with /Name != Identity
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- scan_for_endstream() fallback for streams without /Length
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