import { TaskStatus } from './task-state.js';

/**
 * Legal state transitions for the 10-state task FSM.
 *
 * Terminal states (COMPLETED, FAILED, CANCELLED, TIMED_OUT, UNKNOWN) are
 * absorbing — they have no outgoing transitions.
 *
 * UNKNOWN is reachable from every non-terminal state as a crash-recovery
 * fallback.
 *
 * Spec reference: §4.1
 */
export const VALID_TRANSITIONS: Record<TaskStatus, ReadonlySet<TaskStatus>> = {
  [TaskStatus.WAITING]: new Set([
    TaskStatus.PENDING,
    TaskStatus.CANCELLED,
    TaskStatus.FAILED,
    TaskStatus.TIMED_OUT,
    TaskStatus.UNKNOWN,
  ]),

  [TaskStatus.PENDING]: new Set([
    TaskStatus.RUNNING,
    TaskStatus.WAITING,
    TaskStatus.COMPLETED,
    TaskStatus.CANCELLED,
    TaskStatus.FAILED,
    TaskStatus.TIMED_OUT,
    TaskStatus.UNKNOWN,
  ]),

  [TaskStatus.RUNNING]: new Set([
    TaskStatus.COMPLETED,
    TaskStatus.FAILED,
    TaskStatus.CANCELLED,
    TaskStatus.TIMED_OUT,
    TaskStatus.RATE_LIMITED,
    TaskStatus.WAITING_ANSWER,
    TaskStatus.UNKNOWN,
  ]),

  [TaskStatus.RATE_LIMITED]: new Set([
    TaskStatus.RUNNING,
    TaskStatus.FAILED,
    TaskStatus.CANCELLED,
    TaskStatus.TIMED_OUT,
    TaskStatus.UNKNOWN,
  ]),

  [TaskStatus.WAITING_ANSWER]: new Set([
    TaskStatus.RUNNING,
    TaskStatus.FAILED,
    TaskStatus.CANCELLED,
    TaskStatus.TIMED_OUT,
    TaskStatus.UNKNOWN,
  ]),

  // Terminal states — no outgoing transitions
  [TaskStatus.COMPLETED]: new Set<TaskStatus>(),
  [TaskStatus.FAILED]: new Set<TaskStatus>(),
  [TaskStatus.CANCELLED]: new Set<TaskStatus>(),
  [TaskStatus.TIMED_OUT]: new Set<TaskStatus>(),
  [TaskStatus.UNKNOWN]: new Set<TaskStatus>(),
};

/**
 * Returns whether `from → to` is a legal FSM transition.
 */
export function isValidTransition(from: TaskStatus, to: TaskStatus): boolean {
  return VALID_TRANSITIONS[from].has(to);
}