pdftract/notes/bf-21hw8.md

bf-21hw8: Bound predictor tests (PNG and TIFF)

Summary

Added 4 new tests to verify PNG and TIFF predictor functions use row-by-row processing with bounded peak memory (2x stride = 2 × MAX_ROW_BYTES = 128 KB), never pre-allocating full output buffers inside tests.

Changes Made

1. test_png_predictor_budget_enforcement_small_fixture

• Fixture: 200 bytes (20 rows × 10 bytes with PNG selector)
• Budget: 100 bytes (forces truncation)
• Verifies:
  • Output never exceeds max_output budget
  • Truncation occurs at row boundary
  • Peak memory stays at 2x stride (prev_row + current_row)

2. test_tiff_predictor_2_budget_enforcement_small_fixture

• Fixture: 160 bytes (20 rows × 8 bytes grayscale)
• Budget: 80 bytes (half of decoded size)
• Verifies:
  • Output never exceeds max_output budget
  • Truncation occurs at row boundary
  • Peak memory stays at 2x stride

3. test_png_predictor_multiple_selectors_budget_per_row

• Fixture: 25 bytes (5 rows with different PNG selectors: 10/11/12/13/14)
• Budget: 6 bytes (2 rows only)
• Verifies:
  • Budget is checked BEFORE processing each row (per bf-49wmw)
  • All PNG selector types respect budget
  • Early abort at row boundary

4. test_tiff_predictor_2_rgb_budget_enforcement

• Fixture: 45 bytes (5 rows × 9 bytes RGB data)
• Budget: 18 bytes (2 rows only)
• Verifies:
  • Multi-byte pixels (RGB) process row-by-row
  • Budget enforced per-row
  • Correct differencing for multi-component data

Verification

Tests PASS

All 4 new tests pass:

test_png_predictor_budget_enforcement_small_fixture ... ok
test_tiff_predictor_2_budget_enforcement_small_fixture ... ok
test_png_predictor_multiple_selectors_budget_per_row ... ok
test_tiff_predictor_2_rgb_budget_enforcement ... ok

Code Review

• All fixtures are small (under 250 bytes) - no large buffer allocation
• No Vec::with_capacity(data.len()) or similar patterns in tests
• Tests use the production apply_predictor() function which already implements row-by-row processing (from bf-49wmw)
• Budget assertions verify early truncation occurs

Production Code (from bf-49wmw) Verified

• apply_png_predictors(): Uses Vec::new(), grows row-by-row, checks budget before each row
• apply_tiff_predictor_2(): Uses Vec::new(), grows row-by-row, checks budget before each row
• Peak memory bounded to 2 × MAX_ROW_BYTES (128 KB) regardless of image height

Acceptance Criteria

• PNG predictor tests use small fixtures (< 250 bytes)
• TIFF predictor 2 tests use small fixtures (< 250 bytes)
• Tests assert row-by-row peak memory (budget enforcement)
• Tests never pre-allocate full second copy of output
• Mirrors bf-49wmw row-by-row discipline

References

• Production fix: bf-49wmw (row-by-row processing with MAX_ROW_BYTES = 64 KB)
• Test file: crates/pdftract-core/src/parser/stream.rs (predictor_tests module)