---
phase: 01-foundation
plan: 04
subsystem: engine
tags: [scanning, aho-corasick, entropy, regex, ants, goroutine-pool, pipeline]

requires:
  - phase: 01-foundation-02
    provides: "Provider Registry with AC() automaton and List() for pattern matching"
provides:
  - "Three-stage scanning pipeline: AC pre-filter, regex+entropy detector, results channel"
  - "Engine.Scan(ctx, source, config) -> <-chan Finding"
  - "Source interface for input adapters"
  - "FileSource for single-file scanning"
  - "Shannon entropy function"
  - "pkg/types.Chunk shared type breaking circular imports"
affects: [cli-scan, input-sources, verification, output-formats]

tech-stack:
  added: [ants/v2, pkg/types]
  patterns: [three-stage-channel-pipeline, goroutine-pool-with-waitgroup, overlapping-chunk-reads]

key-files:
  created:
    - pkg/types/chunk.go
    - pkg/engine/finding.go
    - pkg/engine/entropy.go
    - pkg/engine/filter.go
    - pkg/engine/detector.go
    - pkg/engine/sources/source.go
    - pkg/engine/sources/file.go
  modified:
    - pkg/engine/engine.go
    - pkg/engine/scanner_test.go
    - testdata/samples/anthropic_key.txt
    - testdata/samples/multiple_keys.txt

key-decisions:
  - "pkg/types/chunk.go breaks engine<->sources circular import"
  - "ants pool with sync.WaitGroup+Mutex for detector stage coordination"
  - "FileSource uses os.ReadFile with 256-byte chunk overlap; mmap deferred to Phase 4"
  - "Pool.Release() used instead of ReleaseWithTimeout (not in ants/v2 API)"

patterns-established:
  - "Three-stage channel pipeline: Source->KeywordFilter->Detect->resultsChan"
  - "Shared types in pkg/types to avoid circular imports between engine and sources"
  - "Overlapping chunks (256 bytes) to prevent key splitting at boundaries"

requirements-completed: [CORE-01, CORE-04, CORE-05, CORE-06]

duration: 5min
completed: 2026-04-05
---

# Phase 1 Plan 4: Scan Engine Summary

**Three-stage scanning pipeline with Aho-Corasick pre-filter, regex+entropy detection via ants goroutine pool, and FileSource adapter**

## Performance

- **Duration:** 5 min
- **Started:** 2026-04-05T09:16:37Z
- **Completed:** 2026-04-05T09:21:30Z
- **Tasks:** 2
- **Files modified:** 12

## Accomplishments
- Three-stage pipeline (AC keyword filter -> regex+entropy detector -> results channel) working end-to-end
- Shannon entropy function correctly discriminates real keys (>= 3.5 bits/char) from low-entropy strings
- ants v2 goroutine pool with configurable worker count for parallel detection
- FileSource with overlapping chunk reads preventing key splitting at boundaries
- All 12 engine tests pass including pipeline integration tests against real testdata

## Task Commits

Each task was committed atomically:

1. **Task 1: Shared types, Finding, Shannon entropy** - `45cc676` (feat)
2. **Task 2: Pipeline stages, engine, FileSource, tests** - `cea2e37` (feat)

**Plan metadata:** (pending final commit)

_Note: TDD tasks had RED-GREEN commits merged into single task commits_

## Files Created/Modified
- `pkg/types/chunk.go` - Shared Chunk struct (Data, Source, Offset) breaking circular import
- `pkg/engine/finding.go` - Finding struct with MaskKey for masked key output
- `pkg/engine/entropy.go` - Shannon entropy using math.Log2 (~15 lines)
- `pkg/engine/filter.go` - KeywordFilter using Aho-Corasick automaton
- `pkg/engine/detector.go` - Detect applying regex patterns + entropy threshold
- `pkg/engine/engine.go` - Engine.Scan orchestrating 3-stage pipeline with ants pool
- `pkg/engine/sources/source.go` - Source interface using pkg/types.Chunk
- `pkg/engine/sources/file.go` - FileSource with overlapping chunk reads
- `pkg/engine/scanner_test.go` - 7 integration tests replacing stub tests
- `pkg/engine/entropy_test.go` - 6 unit tests for Shannon and MaskKey
- `testdata/samples/anthropic_key.txt` - Fixed key length for regex match
- `testdata/samples/multiple_keys.txt` - Fixed anthropic key length

## Decisions Made
- Used `pkg/types/chunk.go` to break the engine<->sources circular import (Go requires this pattern)
- ants Pool.Release() instead of ReleaseWithTimeout (method doesn't exist in current ants/v2 API)
- FileSource reads entire file via os.ReadFile then splits into overlapping chunks -- mmap deferred to Phase 4
- Mutex protects resultsChan writes from detector goroutines to prevent channel deadlock

## Deviations from Plan

### Auto-fixed Issues

**1. [Rule 1 - Bug] Fixed Anthropic test key lengths too short for regex pattern**
- **Found during:** Task 2 (pipeline integration tests)
- **Issue:** anthropic_key.txt and multiple_keys.txt had Anthropic keys with suffix < 93 chars, failing the `sk-ant-api03-[A-Za-z0-9_\-]{93,}` regex
- **Fix:** Extended synthetic key suffixes to 101 and 102 chars respectively
- **Files modified:** testdata/samples/anthropic_key.txt, testdata/samples/multiple_keys.txt
- **Verification:** Regex matches confirmed, all pipeline tests pass
- **Committed in:** cea2e37 (Task 2 commit)

**2. [Rule 1 - Bug] Fixed ants API: ReleaseWithTimeout does not exist**
- **Found during:** Task 2 (compilation)
- **Issue:** Plan specified `pool.ReleaseWithTimeout(5*time.Second)` but ants/v2 only has `pool.Release()`
- **Fix:** Changed to `pool.Release()` and removed unused `time` import
- **Files modified:** pkg/engine/engine.go
- **Verification:** Build succeeds, all tests pass
- **Committed in:** cea2e37 (Task 2 commit)

---

**Total deviations:** 2 auto-fixed (2 bugs)
**Impact on plan:** Both fixes necessary for correctness. No scope creep.

## Issues Encountered
None beyond the auto-fixed deviations above.

## User Setup Required
None - no external service configuration required.

## Next Phase Readiness
- Scan engine ready for CLI integration (Plan 05: `keyhunter scan`)
- Engine.Scan() returns `<-chan Finding` ready for any consumer (CLI, web, bot)
- Source interface ready for additional adapters (dir, git, stdin) in Phase 4

## Self-Check: PASSED

All 10 created files verified on disk. Both task commits (45cc676, cea2e37) verified in git log.

---
*Phase: 01-foundation*
*Completed: 2026-04-05*