determine-changes.yml

name: Determine Changes

on:
  workflow_call:
    inputs:
      path-filters:
        description: >
          JSON object mapping flag names to arrays of paths.
          Example: {"unit_tests": ["src/", "tests/"], "lint": ["src/"]}
        type: string
        required: true
    outputs:
      flags:
        description: >
          JSON object mapping flag names to "true"/"false" strings.
          On push/workflow_dispatch, all flags are "true".
          On pull_request, flags reflect which paths changed.
        value: ${{ jobs.detect.outputs.flags }}

jobs:
  detect:
    runs-on: ubuntu-latest
    permissions:
      contents: read
    outputs:
      flags: ${{ steps.result.outputs.flags }}
    steps:
      - uses: actions/checkout@v6
        if: github.event_name == 'pull_request'
        with:
          fetch-depth: 0

      - name: Determine merge base
        if: github.event_name == 'pull_request'
        id: merge_base
        run: |
          sha=$(git merge-base HEAD "origin/${{ github.event.pull_request.base.ref }}")
          echo "sha=${sha}" >> "$GITHUB_OUTPUT"

      - name: Check changed paths
        id: result
        run: |
          FILTERS='${{ inputs.path-filters }}'

          if [ "${{ github.event_name }}" != "pull_request" ]; then
            FLAGS=$(echo "$FILTERS" | jq -c 'to_entries | map({key: .key, value: "true"}) | from_entries')
          else
            FLAGS="{}"
            for key in $(echo "$FILTERS" | jq -r 'keys[]'); do
              PATHS=$(echo "$FILTERS" | jq -r --arg k "$key" '.[$k][]')
              # shellcheck disable=SC2086
              if ! git diff --quiet "${{ steps.merge_base.outputs.sha }}...HEAD" -- $PATHS 2>/dev/null; then
                FLAGS=$(echo "$FLAGS" | jq -c --arg k "$key" '. + {($k): "true"}')
              else
                FLAGS=$(echo "$FLAGS" | jq -c --arg k "$key" '. + {($k): "false"}')
              fi
            done
          fi

          echo "flags=$FLAGS" >> "$GITHUB_OUTPUT"