/**
 * A restore set (ADR 0015) is one nightly `pg_dump` plus the matching
 * uploads/plugins archive, tied together by a shared backup id derived from
 * the run's UTC start time. This module owns the naming scheme and the pure
 * prune decision; the sidecar's runner applies it to
 * the filesystem, and the api reads it to list local sets for the in-app
 * restore (issue #103).
 */

export interface BackupSet {
  id: string;
  /** File names (not paths) present in the backups directory. */
  files: string[];
  /** Complete = both the dump and the volume archive exist. */
  complete: boolean;
}

const ID_PATTERN = /^(\d{4})(\d{2})(\d{2})-(\d{2})(\d{2})(\d{2})$/;
const SET_FILE_PATTERN = /^(?:db-|files-)(\d{8}-\d{6})\.(?:dump|tar\.gz)$/;

/** Backup id for a run starting now: UTC timestamp, filesystem-safe. */
export function newBackupId(now: Date): string {
  const pad = (value: number): string => String(value).padStart(2, '0');
  return (
    `${now.getUTCFullYear()}${pad(now.getUTCMonth() + 1)}${pad(now.getUTCDate())}` +
    `-${pad(now.getUTCHours())}${pad(now.getUTCMinutes())}${pad(now.getUTCSeconds())}`
  );
}

/** The UTC time encoded in a backup id, or null for a malformed id. */
export function backupIdTime(id: string): Date | null {
  const match = ID_PATTERN.exec(id);
  if (!match) return null;
  const [, year, month, day, hour, minute, second] = match;
  return new Date(
    Date.UTC(
      Number(year),
      Number(month) - 1,
      Number(day),
      Number(hour),
      Number(minute),
      Number(second),
    ),
  );
}

export function dumpFileName(id: string): string {
  return `db-${id}.dump`;
}

export function archiveFileName(id: string): string {
  return `files-${id}.tar.gz`;
}

/**
 * Groups the backup directory's file names into sets, oldest first. Files
 * that do not belong to the naming scheme (status.json, `.partial` staging
 * files of a running or crashed run) are ignored — prune never touches them.
 */
export function listSets(fileNames: string[]): BackupSet[] {
  const byId = new Map<string, string[]>();
  for (const name of fileNames) {
    const match = SET_FILE_PATTERN.exec(name);
    if (!match || !backupIdTime(match[1]!)) continue;
    const files = byId.get(match[1]!) ?? [];
    files.push(name);
    byId.set(match[1]!, files);
  }
  return [...byId.entries()]
    .sort(([a], [b]) => a.localeCompare(b))
    .map(([id, files]) => ({
      id,
      files: files.sort(),
      complete: files.includes(dumpFileName(id)) && files.includes(archiveFileName(id)),
    }));
}

/**
 * The sets prune may delete: older than the retention cutoff — but never
 * the newest complete set, even when it is expired. A stalled instance must
 * always keep one restorable set (issue #83 acceptance criteria).
 */
export function expiredSets(sets: BackupSet[], now: Date, retentionDays: number): BackupSet[] {
  const cutoff = now.getTime() - retentionDays * 24 * 60 * 60 * 1000;
  const newestComplete = [...sets].reverse().find((set) => set.complete);
  return sets.filter((set) => {
    if (set === newestComplete) return false;
    const time = backupIdTime(set.id);
    return time !== null && time.getTime() < cutoff;
  });
}