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

ROOT=${3:-$(CDPATH='' cd -- "$(dirname -- "$0")/.." && pwd)}
previous_commit=${1:-}
target_commit=${2:-HEAD}
policy_path=priv/repo/migration_application_compatibility.tsv

usage() {
  echo "Usage: $0 PREVIOUS_COMMIT TARGET_COMMIT [REPOSITORY_ROOT]" >&2
}

if [[ -z "$previous_commit" || -z "$target_commit" ]]; then
  usage
  exit 2
fi

for commit in "$previous_commit" "$target_commit"; do
  git -C "$ROOT" cat-file -e "$commit^{commit}" 2>/dev/null || {
    echo "Commit is unavailable in the repository: $commit" >&2
    exit 2
  }
done

git -C "$ROOT" merge-base --is-ancestor "$previous_commit" "$target_commit" || {
  echo "The target commit is not a descendant of the previous commit." >&2
  exit 2
}

mapfile -t migration_changes < <(
  git -C "$ROOT" diff --name-status "$previous_commit..$target_commit" -- \
    'priv/repo/migrations/[0-9]*.exs'
)

if [[ ${#migration_changes[@]} -eq 0 ]]; then
  echo "migration_policy=application_safe"
  echo "migration_versions=none"
  echo "migration_count=0"
  exit 0
fi

registry=$(git -C "$ROOT" show "$target_commit:$policy_path" 2>/dev/null) || {
  echo "The target commit does not contain $policy_path." >&2
  exit 2
}

overall_policy=application_safe
versions=()

for change in "${migration_changes[@]}"; do
  status=${change%%$'\t'*}
  path=${change#*$'\t'}

  if [[ "$status" != A ]]; then
    echo "Applied migration files must not be modified, renamed, or deleted: $change" >&2
    exit 2
  fi

  filename=${path##*/}
  version=${filename%%_*}
  [[ "$version" =~ ^[0-9]{14}$ ]] || {
    echo "Migration does not have a 14-digit Ecto version: $path" >&2
    exit 2
  }

  mapfile -t matches < <(
    awk -F '\t' -v version="$version" '
      $0 !~ /^#/ && $1 == version {print $2 "\t" $3}
    ' <<<"$registry"
  )

  if [[ ${#matches[@]} -ne 1 ]]; then
    echo "Migration $version must have exactly one entry in $policy_path." >&2
    exit 2
  fi

  policy=${matches[0]%%$'\t'*}
  reason=${matches[0]#*$'\t'}

  case "$policy" in
    application_safe | forward_only) ;;
    *)
      echo "Migration $version has an invalid application rollback policy: $policy" >&2
      exit 2
      ;;
  esac

  [[ -n "$reason" && "$reason" != "$policy" ]] || {
    echo "Migration $version must document why its policy is correct." >&2
    exit 2
  }

  if [[ "$policy" == forward_only ]]; then
    overall_policy=forward_only
  fi

  versions+=("$version:$policy")
done

printf 'migration_policy=%s\n' "$overall_policy"
printf 'migration_versions=%s\n' "$(IFS=,; echo "${versions[*]}")"
printf 'migration_count=%s\n' "${#versions[@]}"