- name: Restore baseline
  uses: dawidd6/action-download-artifact@v3
  with:
    workflow: coverage.yml
    branch: main
    name: coverage-baseline
    path: baseline/

- name: Parse current
  run: python scripts/parse_lcov.py coverage/lcov.info > current.json

- name: Parse baseline
  run: python scripts/parse_lcov.py baseline/lcov.info > baseline.json

- name: Generate diff
  run: python scripts/coverage_diff.py current.json baseline.json > diff.md

- name: Checkout main
  run: git fetch origin main && git checkout origin/main

- name: Run tests on main
  run: npm test -- --coverage && cp coverage/lcov.info baseline.lcov

- name: Checkout PR head
  run: git checkout ${{ github.event.pull_request.head.sha }}

- name: Run tests on PR head
  run: npm test -- --coverage

# scripts/coverage_diff.py
def compute_diff(current, baseline):
    base_idx = {f['path']: f for f in baseline}
    rows = []
    for f in current:
        b = base_idx.get(f['path'])
        line_now    = pct(f.get('lh', 0), f.get('lf', 0))
        branch_now  = pct(f.get('brh', 0), f.get('brf', 0))
        line_then   = pct(b.get('lh', 0), b.get('lf', 0)) if b else None
        branch_then = pct(b.get('brh', 0), b.get('brf', 0)) if b else None
        rows.append({
            'path': f['path'],
            'is_new': b is None,
            'line_now': line_now,    'line_delta':   delta(line_now, line_then),
            'branch_now': branch_now,'branch_delta': delta(branch_now, branch_then),
        })
    # Also catch deletions - files in baseline but not current.
    for path, b in base_idx.items():
        if path not in {f['path'] for f in current}:
            rows.append({'path': path, 'is_deleted': True, 'line_now': None, 'line_then': pct(b.get('lh', 0), b.get('lf', 0))})
    return rows

def classify(row):
    if row.get('is_deleted'):                      return 'deleted'
    if row.get('is_new') and row['line_now'] < 80: return 'new_below_threshold'
    if row.get('is_new'):                          return 'new_ok'
    if row['line_delta'] is not None and row['line_delta'] <= -5:   return 'regressed'
    if row['line_delta'] is not None and row['line_delta'] <  0:    return 'declined'
    if row['line_delta'] is not None and row['line_delta'] >  0:    return 'improved'
    return 'unchanged'

## Coverage diff - `<sha>` vs `main` `<base-sha>`

**Overall:** line 84.2% (-0.3pp) | branch 71.5% (-0.1pp)
**Files changed:** 7 (3 regressed, 1 new, 2 improved, 1 deleted)

### ⚠ Regressions (4)

| File                                  | Line%       | Branch%     |
|---------------------------------------|-------------|-------------|
| `src/checkout/cart.ts`                | 65.4 (-12.8 ⬇) | 50.0 (-25.0 ⬇) |
| `src/checkout/promo.ts`               | 78.0 (-8.5 ⬇)  | 60.0 (-15.0 ⬇) |

### 🆕 New files (1)

| File                                  | Line%       | Branch%     |
|---------------------------------------|-------------|-------------|
| `src/checkout/discount-stack.ts`      | 35.0 (NEW, below 80% threshold) | 25.0 |

### ✅ Improvements (2)

| File                                  | Line%       | Branch%     |
|---------------------------------------|-------------|-------------|
| `src/orders/list.ts`                  | 92.0 (+4.5 ⬆) | 85.0 (+10.0 ⬆) |

### 🗑 Deleted (1)

| File                                  | Was line%   |
|---------------------------------------|-------------|
| `src/legacy/old-checkout.ts`          | 22.0        |

📉 Coverage 84.2% (-0.3pp) - 3 files regressed, 1 new file below threshold. See full report below.

✅ Coverage 84.5% (+0.2pp) - no regressions, 2 files improved.

- name: Generate diff report
  run: python scripts/coverage_diff.py current.json baseline.json > diff.md

- name: Post / update PR comment
  uses: marocchino/sticky-pull-request-comment@v2
  with:
    header: coverage-diff
    path: diff.md