on: issue_comment

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/github-script@0.3.0
        if: github.event.action == 'created'
        with:
          github-token: ${{ secrets.GITHUB_TOKEN }}
          script: |
            const isValidSignOff = (
              context.payload.action === 'created' &&
              context.payload.issue.pull_request &&
              context.payload.comment.user.id === context.payload.issue.user.id &&
              context.payload.comment.body === '/signoff'
            )
            if (!isValidSignOff) return
            const pr = await github.pulls.get({
              ...context.repo,
              pull_number: context.payload.issue.number
            })
            const commits = await github.pulls.listCommits({
              ...context.repo,
              pull_number: context.payload.issue.number
            })
            const baseCommit = await github.git.getCommit({
              ...context.repo,
              commit_sha: pr.data.head.sha
            })
            const tree = await github.git.getTree({
              ...context.repo,
              tree_sha: baseCommit.data.tree.sha
            })
            const commitLines = commits.data.map(item => `- ${item.sha.slice(0, 6)}: ${item.commit.message}`).join('\n')
            const header = `I, @${context.payload.comment.user.login}, hereby signoff on these commits:`
            const newCommit = await github.git.createCommit({
              ...context.repo,
              message: `${header}\n\n${commitLines}`,
              tree: tree.data.sha,
              parents: [pr.data.head.sha]
            })
            await github.git.updateRef({
              ...context.repo,
              ref: `heads/${pr.data.head.ref}`,
              sha: newCommit.data.sha
            })