name: Check for empty PR

on:
  merge_group:
  pull_request:

jobs:
  check-empty-pr:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v6
      with:
        ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
        fetch-depth: 0
        filter: tree:0

    - name: Check for empty diff
      run: |
        if [[ "${{ github.event_name }}" == "pull_request" ]]; then
          base=$(git merge-base "origin/${{ github.base_ref }}" HEAD)
        else
          base=$(git rev-parse HEAD^1)
        fi
        if git diff --quiet "$base" HEAD --; then
          echo "This PR introduces no changes compared to its base branch." | tee "$GITHUB_STEP_SUMMARY"
          echo "It may be a duplicate of an already-merged PR." | tee -a "$GITHUB_STEP_SUMMARY"
          exit 1
        fi
      shell: bash