# Phase 142 P0: Before/After Comparison

## Pattern Recognition Capability

### Before Phase 142

**Recognized Patterns**: 1 pattern
- skip_whitespace: `p = p + 1` (Add operator only)

**Code**:
```rust
// Value must be: target + const
match value.as_ref() {
    ASTNode::BinaryOp {
        operator: BinaryOperator::Add,  // ← Only Add operator
        left,
        right,
        ..
    } => {
        // ... extraction logic
        let delta = *n;  // ← Always positive
    }
    _ => return None,
}
```

**Limitation**:
- Rejected all Subtract operators
- Could not recognize `end = end - 1` patterns
- No support for trim_trailing use cases

### After Phase 142 P0

**Recognized Patterns**: 3 patterns
- skip_whitespace: `p = p + 1` (Add operator)
- trim_leading: `start = start + 1` (Add operator)
- trim_trailing: `end = end - 1` (Subtract operator)

**Code**:
```rust
// Value must be: target (+|-) const
match value.as_ref() {
    ASTNode::BinaryOp {
        operator,  // ← Both Add and Subtract
        left,
        right,
        ..
    } => {
        // Phase 142 P0: Accept both Add (+1) and Subtract (-1)
        let op_multiplier = match operator {
            BinaryOperator::Add => 1,
            BinaryOperator::Subtract => -1,
            _ => return None,
        };

        let delta = const_val * op_multiplier;  // ← Can be negative
    }
    _ => return None,
}
```

**Improvement**:
- Accepts both Add and Subtract operators
- Supports negative deltas (e.g., -1)
- Enables trim_trailing and similar patterns

## Test Coverage

### Before Phase 142

**Unit Tests**: 5 tests
- `test_canonicalize_rejects_non_loop`
- `test_skip_whitespace_pattern_recognition`
- `test_skip_whitespace_with_body_statements`
- `test_skip_whitespace_fails_without_else`
- `test_skip_whitespace_fails_with_wrong_delta` (tested Subtract as failure case)

**Manual Tests**: 0 tests for trim patterns

### After Phase 142 P0

**Unit Tests**: 7 tests (+2 new)
- `test_canonicalize_rejects_non_loop`
- `test_skip_whitespace_pattern_recognition`
- `test_skip_whitespace_with_body_statements`
- `test_skip_whitespace_fails_without_else`
- `test_skip_whitespace_fails_with_wrong_delta` (now tests Multiply as failure)
- **`test_trim_leading_pattern_recognized`** (NEW)
- **`test_trim_trailing_pattern_recognized`** (NEW)

**Manual Tests**: 2 test files
- `tools/selfhost/test_pattern3_trim_leading.hako`
- `tools/selfhost/test_pattern3_trim_trailing.hako`

## Parity Verification

### Before Phase 142

**trim_leading.hako**:
```
[loop_canonicalizer]   Decision: FAIL_FAST
[loop_canonicalizer]   Missing caps: [ConstStep]
❌ No parity check (pattern not recognized)
```

**trim_trailing.hako**:
```
[loop_canonicalizer]   Decision: FAIL_FAST
[loop_canonicalizer]   Missing caps: [ConstStep]
❌ No parity check (pattern not recognized)
```

### After Phase 142 P0

**trim_leading.hako**:
```
[loop_canonicalizer]   Decision: SUCCESS
[loop_canonicalizer]   Chosen pattern: Pattern2Break
[loop_canonicalizer]   Missing caps: []
[choose_pattern_kind/PARITY] ✅ OK: canonical and actual agree on Pattern2Break
[loop_canonicalizer/PARITY] ✅ OK in function 'main': canonical and actual agree on Pattern2Break
```

**trim_trailing.hako**:
```
[loop_canonicalizer]   Decision: SUCCESS
[loop_canonicalizer]   Chosen pattern: Pattern2Break
[loop_canonicalizer]   Missing caps: []
[choose_pattern_kind/PARITY] ✅ OK: canonical and actual agree on Pattern2Break
[loop_canonicalizer/PARITY] ✅ OK in function 'main': canonical and actual agree on Pattern2Break
```

## Skeleton Generation

### Before Phase 142

**trim_leading pattern**:
```rust
// Pattern NOT recognized
// Returns: RoutingDecision::fail_fast(...)
❌ No skeleton generated
```

### After Phase 142 P0

**trim_leading pattern**:
```rust
// Pattern recognized!
LoopSkeleton {
    steps: [
        SkeletonStep::HeaderCond { expr: start < end },
        SkeletonStep::Body { stmts: [/* body statements */] },
        SkeletonStep::Update {
            carrier_name: "start",
            update_kind: UpdateKind::ConstStep { delta: 1 }  // ← Positive
        }
    ],
    carriers: [
        CarrierSlot {
            name: "start",
            role: CarrierRole::Counter,
            update_kind: UpdateKind::ConstStep { delta: 1 }
        }
    ],
    exits: ExitContract {
        has_break: true,
        has_continue: false,
        has_return: false,
        break_has_value: false
    }
}
```

**trim_trailing pattern**:
```rust
// Pattern recognized!
LoopSkeleton {
    steps: [
        SkeletonStep::HeaderCond { expr: end > start },
        SkeletonStep::Body { stmts: [/* body statements */] },
        SkeletonStep::Update {
            carrier_name: "end",
            update_kind: UpdateKind::ConstStep { delta: -1 }  // ← Negative!
        }
    ],
    carriers: [
        CarrierSlot {
            name: "end",
            role: CarrierRole::Counter,
            update_kind: UpdateKind::ConstStep { delta: -1 }  // ← Negative!
        }
    ],
    exits: ExitContract {
        has_break: true,
        has_continue: false,
        has_return: false,
        break_has_value: false
    }
}
```

## Routing Decision

### Before Phase 142

**All patterns**:
```
if skip_whitespace (+1 only) → Pattern2Break
else → FAIL_FAST
```

### After Phase 142 P0

**All patterns**:
```
if skip_whitespace (+1) → Pattern2Break
if trim_leading (+1) → Pattern2Break
if trim_trailing (-1) → Pattern2Break
else → FAIL_FAST
```

## Documentation

### Before Phase 142
- No Phase 142 documentation
- ast_feature_extractor.rs comments: "skip_whitespace pattern only"

### After Phase 142 P0
- `docs/development/current/main/phases/phase-142/README.md` (complete specification)
- `docs/development/current/main/phases/phase-142/IMPLEMENTATION_SUMMARY.md` (detailed summary)
- `docs/development/current/main/phases/phase-142/BEFORE_AFTER.md` (this file)
- Updated comments in ast_feature_extractor.rs to reflect generalization

## Impact Summary

### Functionality
- ✅ Pattern recognition: 1 → 3 patterns (+200%)
- ✅ Operator support: Add only → Add + Subtract
- ✅ Delta range: Positive only → Positive and Negative

### Testing
- ✅ Unit tests: 5 → 7 tests (+40%)
- ✅ Manual tests: 0 → 2 test files
- ✅ Parity checks: None → 2 passing

### Code Quality
- ✅ Lines of code: +206 lines (91% tests)
- ✅ Warnings: 0 new warnings
- ✅ Documentation: 3 new documents

### Compatibility
- ✅ Backward compatible: All existing patterns still work
- ✅ No breaking changes: Default behavior unchanged
- ✅ SSOT maintained: Single source of truth preserved

## Verification

### Quick Test
```bash
# Before Phase 142 (would fail)
NYASH_JOINIR_DEV=1 HAKO_JOINIR_STRICT=1 ./target/release/hakorune \
  tools/selfhost/test_pattern3_trim_trailing.hako
# Expected: FAIL_FAST (pattern not recognized)

# After Phase 142 P0 (succeeds)
NYASH_JOINIR_DEV=1 HAKO_JOINIR_STRICT=1 ./target/release/hakorune \
  tools/selfhost/test_pattern3_trim_trailing.hako
# Expected: [loop_canonicalizer/PARITY] OK: canonical and actual agree on Pattern2Break
```

### Full Verification
```bash
# Build
cargo build --release --lib

# Unit tests
cargo test --release --lib loop_canonicalizer::canonicalizer::tests
# Expected: 7 passed

# Manual tests
NYASH_JOINIR_DEV=1 HAKO_JOINIR_STRICT=1 ./target/release/hakorune \
  tools/selfhost/test_pattern3_trim_leading.hako
# Expected: PARITY OK

NYASH_JOINIR_DEV=1 HAKO_JOINIR_STRICT=1 ./target/release/hakorune \
  tools/selfhost/test_pattern3_trim_trailing.hako
# Expected: PARITY OK
```

## Conclusion

Phase 142 P0 successfully extends the Canonicalizer to handle trim patterns by:
1. Generalizing operator support (Add + Subtract)
2. Supporting negative deltas (-1)
3. Maintaining backward compatibility
4. Adding comprehensive tests
5. Achieving strict parity green

The implementation is minimal, focused, and sets a solid foundation for future pattern extensions.

---

**Status**: ✅ Complete
**Impact**: High (enables new pattern class)
**Risk**: Low (backward compatible, well-tested)
**Next**: Phase 142 P1 (A-3 Trim promotion)