name: Stack Gate

on:
  workflow_call:
    inputs:
      paths:
        description: 'Newline-separated path prefixes matching the caller''s on.pull_request.paths.'
        type: string
        required: true
    outputs:
      run:
        description: 'false when the PR is below the top of a stack whose top PR also triggers this workflow.'
        value: ${{ jobs.gate.outputs.run }}

jobs:
  gate:
    runs-on: ubuntu-latest
    timeout-minutes: 2
    outputs:
      run: ${{ steps.stack.outputs.run }}
    steps:
      - name: Check stack position
        id: stack
        uses: actions/github-script@v9
        env:
          PATHS: ${{ inputs.paths }}
        with:
          script: |
            if (context.eventName !== 'pull_request') {
              core.setOutput('run', 'true');
              return;
            }

            try {
              const result = await github.graphql(`
                query($owner: String!, $repo: String!, $number: Int!) {
                  repository(owner: $owner, name: $repo) {
                    pullRequest(number: $number) {
                      stack {
                        size
                        entries(first: 50) {
                          nodes {
                            position
                            pullRequest { files(first: 100) { nodes { path } } }
                          }
                        }
                      }
                      stackEntry { position }
                    }
                  }
                }`, {
                owner: context.repo.owner,
                repo: context.repo.repo,
                number: context.payload.pull_request.number,
              });

              const pr = result.repository.pullRequest;
              if (!pr.stack || !pr.stackEntry || pr.stackEntry.position === pr.stack.size) {
                core.setOutput('run', 'true');
                return;
              }

              const prefixes = process.env.PATHS.split('\n').map(p => p.trim()).filter(Boolean);
              const files = pr.stack.entries.nodes.find(e => e.position === pr.stack.size)?.pullRequest?.files;
              // Missing data or no match in the first 100 files fails open
              const topRuns = files?.nodes.some(f => prefixes.some(p => f.path.startsWith(p))) ?? false;
              if (!topRuns) {
                core.info('Top PR does not trigger this workflow, running here.');
                core.setOutput('run', 'true');
                return;
              }

              core.info(`PR is at position ${pr.stackEntry.position} of ${pr.stack.size}, CI runs on the top PR only.`);
              core.setOutput('run', 'false');
            } catch (error) {
              // Fail open so a lookup error never silently drops CI
              core.warning(error.message);
              core.setOutput('run', 'true');
            }