name: Merge Conflict Detector

on:
  pull_request:
    types: [opened, synchronize, reopened]
    branches: [main]
  push:
    branches: [main]

# Cancel in-progress runs when a new commit lands — no point checking stale state
concurrency:
  group: merge-conflict-${{ github.event_name == 'pull_request' && github.ref || github.sha }}
  cancel-in-progress: true

jobs:
  detect-and-label:
    runs-on: ubuntu-latest
    permissions:
      contents: read
      pull-requests: write
      issues: write

    steps:
      - name: Checkout
        uses: actions/checkout@v7
        with:
          fetch-depth: 0

      - name: Ensure label exists
        uses: actions/github-script@v9
        with:
          script: |
            const LABEL = 'has-conflicts';
            const { data: labels } = await github.rest.issues.listLabelsForRepo({
              owner: context.repo.owner,
              repo: context.repo.repo,
            });
            if (!labels.some(l => l.name === LABEL)) {
              await github.rest.issues.createLabel({
                owner: context.repo.owner,
                repo: context.repo.repo,
                name: LABEL,
                color: 'd73a4a',
                description: 'This PR has merge conflicts that need to be resolved',
              });
            }

      - name: Check PRs and label conflicts
        uses: actions/github-script@v9
        with:
          script: |
            const LABEL = 'has-conflicts';

            async function checkAndLabel(prNumber) {
              // Fetch PR with mergeable status
              const { data: pr } = await github.rest.pulls.get({
                owner: context.repo.owner,
                repo: context.repo.repo,
                pull_number: prNumber,
              });

              // mergeable: true = clean, false = conflicts, null = still computing
              const hasConflicts = pr.mergeable === false;
              const currentLabels = pr.labels.map(l => l.name);
              const hasLabel = currentLabels.includes(LABEL);

              core.info(
                `PR #${prNumber}: mergeable=${pr.mergeable}, ` +
                `hasLabel=${hasLabel}, conflicts=${hasConflicts}`
              );

              if (hasConflicts && !hasLabel) {
                await github.rest.issues.addLabels({
                  owner: context.repo.owner,
                  repo: context.repo.repo,
                  issue_number: prNumber,
                  labels: [LABEL],
                });
                core.info(`✅ Added "${LABEL}" to PR #${prNumber}`);
              } else if (!hasConflicts && hasLabel) {
                await github.rest.issues.removeLabel({
                  owner: context.repo.owner,
                  repo: context.repo.repo,
                  issue_number: prNumber,
                  name: LABEL,
                });
                core.info(`✅ Removed "${LABEL}" from PR #${prNumber}`);
              } else if (pr.mergeable === null) {
                core.warning(
                  `PR #${prNumber}: mergeable is null (still computing). ` +
                  `Label left as-is. The next event will recheck.`
                );
              }
            }

            // ── Single PR (pull_request event) ─────────────────────────
            if (context.eventName === 'pull_request') {
              await checkAndLabel(context.issue.number);
              return;
            }

            // ── Bulk check all open PRs (push to main event) ───────────
            if (context.eventName === 'push') {
              const { data: prs } = await github.rest.pulls.list({
                owner: context.repo.owner,
                repo: context.repo.repo,
                state: 'open',
                base: 'main',
                per_page: 100,
              });
              core.info(`Checking ${prs.length} open PR(s) base on main...`);
              for (const pr of prs) {
                await checkAndLabel(pr.number);
              }
            }