name: Detect affected areas
description: >-
  Classify a PR's changed files into CI work categories (python, frontend,
  site) so heavy jobs can skip work they cannot be affected by. Outputs are
  always "true" on push/dispatch events and fail open (everything "true") when
  the diff cannot be computed — a skipped category must never be a false
  negative.

# The caller must check out the repo with `fetch-depth: 0` BEFORE using this
# action, so both the PR base and head commits are present for `git diff`.

outputs:
  python:
    description: Run Python tests / ruff / ty / windows-footguns.
    value: ${{ steps.classify.outputs.python }}
  frontend:
    description: Run the TypeScript typecheck matrix + desktop build.
    value: ${{ steps.classify.outputs.frontend }}
  site:
    description: Build the Docusaurus docs site.
    value: ${{ steps.classify.outputs.site }}

runs:
  using: composite
  steps:
    - name: Classify changed files
      id: classify
      shell: bash
      env:
        EVENT_NAME: ${{ github.event_name }}
        BASE_SHA: ${{ github.event.pull_request.base.sha }}
        HEAD_SHA: ${{ github.event.pull_request.head.sha }}
      run: |
        set -euo pipefail
        # Only pull_request events are gated. Other events (push, release,
        # dispatch) leave CHANGED empty, so the classifier fails open and every
        # lane runs — post-merge / on-demand validation is never weakened.
        if [ "$EVENT_NAME" = "pull_request" ]; then
          # Three-dot diff = what the PR introduces vs its merge base, matching
          # how a reviewer reads it. An uncomputable diff (shallow clone, etc.)
          # yields an empty list, which the classifier also fails open on.
          CHANGED="$(git diff --name-only "${BASE_SHA}...${HEAD_SHA}" || true)"
        fi
        echo "Changed files:"
        printf '%s\n' "${CHANGED:-(none)}"
        # Caller already checked out the repo, so the classifier is at its
        # repo-relative path. It is the single source of the fail-open default.
        printf '%s\n' "${CHANGED:-}" | python3 scripts/ci/classify_changes.py