#!/usr/bin/env bash
set -euo pipefail

# Enforce: if a PR adds migration files under backend/migrations/versions/, it must add <= 1.
# Rationale: keeps review/rollback straightforward and avoids surprise multiple-head merges.

if [ "${GITHUB_EVENT_NAME:-}" != "pull_request" ]; then
  echo "Not a pull_request event; skipping one-migration-per-PR gate."
  exit 0
fi

BASE_SHA="${GITHUB_BASE_SHA:-${GITHUB_EVENT_PULL_REQUEST_BASE_SHA:-}}"
HEAD_SHA="${GITHUB_SHA:-}"

if [ -z "$BASE_SHA" ] || [ -z "$HEAD_SHA" ]; then
  echo "Missing BASE_SHA/HEAD_SHA (BASE_SHA='$BASE_SHA', HEAD_SHA='$HEAD_SHA')"
  exit 2
fi

# Ensure base is present in shallow clones.
git fetch --no-tags --depth=1 origin "$BASE_SHA" || true

CHANGED=$(git diff --name-only "$BASE_SHA" "$HEAD_SHA" || true)
if [ -z "$CHANGED" ]; then
  echo "No changed files detected."
  exit 0
fi

MIGRATIONS=$(echo "$CHANGED" | grep -E '^backend/migrations/versions/.*\.py$' || true)
COUNT=$(echo "$MIGRATIONS" | sed '/^$/d' | wc -l | tr -d ' ')

if [ "$COUNT" -le 1 ]; then
  echo "Migration gate OK (migrations_added=$COUNT)."
  exit 0
fi

echo "Migration gate FAILED: this PR adds $COUNT migration files; policy allows at most 1."
echo

echo "Migrations detected:"
echo "$MIGRATIONS"
echo

echo "How to fix:"
echo "- Consolidate schema changes into a single migration file (squash)."
echo "- If you have multiple Alembic heads, create ONE merge migration instead of several."
echo "- If you truly need multiple migrations, split into multiple PRs."

exit 1