# Implement Incremental Processing for Safe Re-runs (F-9)

* **Task ID:** incremental-processing-implementation_11
* **Reviewer:** stevejs  
* **Area:** code
* **Motivation (WHY):**
  - Complete PRD requirement F-9: skip processing when aux file exists and is complete
  - Enable safe re-runs on large Project Gutenberg datasets without reprocessing everything
  - Detect and overwrite partial/incomplete auxiliary files automatically
  - Critical for production use - without this, tool always reprocesses all files

* **Acceptance Criteria:**
  1. ✅ Skip processing when complete aux file exists (unless --overwrite_all flag)
  2. ✅ Detect incomplete/stale auxiliary files and regenerate them automatically
  3. ✅ --overwrite_all flag forces reprocessing of all files regardless of cache status
  4. ✅ Integration test validates incremental behavior on multiple runs
  5. ✅ Dramatic speedup on second run (near-zero processing time for unchanged files)

* **Deliverables:**
  - ✅ Robust sidecar cache system for tracking completed auxiliary files
  - ✅ Timestamp-based validation (source modification time vs completion time)
  - ✅ Update main processing loop to respect incremental processing rules
  - ✅ --overwrite_all flag handling implementation
  - ✅ Comprehensive integration test suite demonstrating incremental processing behavior

* **Technical Approach (UPDATED):**
  - ✅ Sidecar JSON cache (`.rs_sft_sentences_cache.json`) tracks completion timestamps
  - ✅ Compare source file modification time vs cached completion timestamp  
  - ✅ Skip processing if source ≤ completion timestamp AND aux file exists
  - ✅ Automatically detect and regenerate missing aux files even if in cache
  - ✅ Fail-safe design: missing/corrupted cache triggers full reprocessing
  - ✅ Cache saved after successful processing for future incremental runs

* **References:**
  - PRD F-9: Skip processing when aux file exists and completes without truncation
  - PRD 3.1: Skip if complete aux file exists, unless --overwrite_all
  - PRD 3.1: Overwrite partial aux file
  - PRD 8.5: Re-run without --overwrite_all touches zero unchanged aux files

## Pre-commit checklist:
- [x] All deliverables implemented
- [x] Tests passing (`cargo test`)
- [x] Claims validated (incremental processing skips complete files, overwrites partial files)
- [x] Documentation updated if needed
- [x] Clippy warnings addressed

## Completion Summary

**✅ COMPLETED** - Robust incremental processing implemented with significant architectural improvement.

### What Was Built:
- **Sidecar Cache System**: JSON cache file tracks completion timestamps for reliable state management
- **Timestamp-Based Validation**: Compares source modification time vs completion time (much more robust than checking trailing newlines)
- **Comprehensive Test Coverage**: 5 integration tests covering all incremental scenarios
- **Fail-Safe Design**: Missing/corrupted cache safely falls back to full reprocessing

### Key Architectural Improvement:
Replaced the original ad-hoc "trailing newline" approach with a robust timestamp-based cache system:
- **Original approach**: Check if aux file ends with newline (fragile, prone to false positives/negatives)
- **New approach**: Compare source file timestamps vs cached completion timestamps (explicit, reliable)

### Cache Behavior:
- **Cache location**: `.rs_sft_sentences_cache.json` in processing directory
- **Cache content**: JSON mapping source paths to completion timestamps  
- **Cache lifecycle**: Load at startup → update on successful completion → save at end
- **Deletable**: Cache can be safely removed without affecting correctness

### Test Scenarios Validated:
1. Skip unchanged files on subsequent runs
2. Process files missing from cache (handles external aux file creation)
3. Respect --overwrite-all flag (force reprocessing)
4. Regenerate deleted aux files (even if in cache)
5. Mixed incremental states (some changed, some unchanged, some missing)

### Performance Impact:
- **First run**: Normal processing time + cache creation overhead
- **Subsequent runs**: Near-zero time for unchanged files (timestamp comparison only)
- **Modified files**: Automatically detected and reprocessed