# .github/workflows/pr-commit-check.yml
# This GitHub Action workflow checks if a pull request has more than one commit.
# If it does, it fails the check and instructs the user to squash their commits.

name: 'PR Commit Check'

# This workflow runs on pull request events.
# It's configured to run on any pull request that is opened or synchronized (new commits pushed).
on:
  pull_request:
    types: [opened, synchronize]

# Defines the jobs that will run as part of the workflow.
jobs:
  check-commit-count:
    # The type of runner that the job will run on. 'ubuntu-latest' is a good default.
    runs-on: ubuntu-latest

    # The steps that will be executed as part of the job.
    steps:
      # Step 1: Check out the code
      # This action checks out your repository under $GITHUB_WORKSPACE, so your workflow can access it.
      - name: Checkout Code
        uses: actions/checkout@v4
        with:
          # We need to fetch all commits to accurately count them.
          # '0' means fetch all history for all branches and tags.
          fetch-depth: 0

      # Step 2: Count the commits in the pull request
      # This step runs a script to get the number of commits in the PR.
      - name: Count Commits
        id: count_commits
        # We use `git rev-list --count` to count the commits.
        # ${{ github.event.pull_request.base.sha }} is the commit SHA of the base branch.
        # ${{ github.event.pull_request.head.sha }} is the commit SHA of the head branch (the PR branch).
        # The '..' syntax gives us the list of commits in the head branch that are not in the base branch.
        # The output of the command (the count) is stored in a step output variable named 'count'.
        run: |
          count=$(git rev-list --count ${{ github.event.pull_request.base.sha }}..${{ github.event.pull_request.head.sha }})
          echo "commit_count=$count" >> $GITHUB_OUTPUT

      # Step 3: Check if the commit count is greater than 1
      # This step uses the output from the previous step to decide whether to pass or fail.
      - name: Check Commit Count
        # This step only runs if the 'commit_count' output from the 'count_commits' step is greater than 1.
        if: steps.count_commits.outputs.commit_count > 1
        # If the condition is met, the workflow will exit with a failure status.
        run: |
          echo "This pull request has ${{ steps.count_commits.outputs.commit_count }} commits."
          echo "Please squash them into a single commit before merging."
          echo "You can use git rebase -i HEAD~N"
          echo "...where N is the number of commits you want to squash together. The PR check conveniently tells you this number! For example, if the check says you have 3 commits, you would run: git rebase -i HEAD~3."
          echo "Because you have rewritten the commit history, you must use the --force flag to update the pull request: git push --force"
          exit 1

      # Step 4: Success message
      # This step runs if the commit count is not greater than 1 (i.e., it's 1).
      - name: Success
        if: steps.count_commits.outputs.commit_count <= 1
        run: |
          echo "This pull request has a single commit. Great job!"