/**
 * 考试状态机（纯函数）
 *
 * 状态流转（ISSUE-003 仲裁 scheme A）：
 *   draft → published → in_progress → grading → graded → archived
 *                                                    └→ cancelled（终态）
 *   cancelled 可从 published/in_progress/grading 流入（异常终止）
 *
 * archived 为软删除终态（ISSUE-006 决策 #7：archived 仅做软删除，不物理删除）
 */
export type ExamStatus =
  | "draft"
  | "published"
  | "in_progress"
  | "grading"
  | "graded"
  | "archived"
  | "cancelled";

export type ExamAction =
  "publish" | "start" | "submit" | "grade" | "archive" | "cancel";

const TRANSITIONS: Record<
  ExamStatus,
  Partial<Record<ExamAction, ExamStatus>>
> = {
  draft: { publish: "published", cancel: "cancelled" },
  published: { start: "in_progress", cancel: "cancelled" },
  in_progress: { submit: "grading", cancel: "cancelled" },
  grading: { grade: "graded", cancel: "cancelled" },
  graded: { archive: "archived" },
  archived: {},
  cancelled: {},
};

export function canTransition(from: ExamStatus, action: ExamAction): boolean {
  return TRANSITIONS[from]?.[action] !== undefined;
}

export function transition(from: ExamStatus, action: ExamAction): ExamStatus {
  const next = TRANSITIONS[from]?.[action];
  if (!next) {
    throw new Error(`Invalid exam state transition: ${from} --${action}-->`);
  }
  return next;
}

export function isTerminal(status: ExamStatus): boolean {
  return status === "archived" || status === "cancelled";
}

export const EXAM_STATUSES: readonly ExamStatus[] = [
  "draft",
  "published",
  "in_progress",
  "grading",
  "graded",
  "archived",
  "cancelled",
] as const;