/** Slot 状态(REQ §6.1) */
export type SlotStatus =
  | "empty"
  | "reserved"
  | "downloading"
  | "verifying"
  | "satisfied"
  | "attention"
  | "upgrade_pending";

export interface SlotState {
  status: SlotStatus;
  /** 是否已有可用的当前资产(升级流程中旧资产仍在,部分覆盖的剧集也可能有资产) */
  hasAsset: boolean;
}

export type SlotEvent =
  | { type: "reserve" }
  | { type: "release" }
  | { type: "download_started" }
  | { type: "download_completed" }
  | { type: "download_failed"; reason?: string }
  | { type: "verify_passed"; complete: boolean }
  | { type: "verify_failed"; reason?: string }
  | { type: "upgrade_found" }
  | { type: "upgrade_cancelled" }
  | { type: "attention_resolved"; hasAsset: boolean }
  | { type: "asset_missing_confirmed" };

export class SlotTransitionError extends Error {
  override name = "SlotTransitionError";
  constructor(
    readonly from: SlotStatus,
    readonly event: SlotEvent["type"],
  ) {
    super(`非法的 Slot 状态迁移: ${from} --${event}-->`);
  }
}

/**
 * Slot 状态机(纯函数)。非法迁移抛 SlotTransitionError。
 * 升级流程:satisfied → upgrade_pending → reserved → downloading → verifying → satisfied;
 * 新文件验证通过前旧资产一直保留(hasAsset=true)。
 * 剧集部分覆盖:verify_passed(complete=false) 回到 empty 但 hasAsset=true,继续追更。
 */
export function transition(state: SlotState, event: SlotEvent): SlotState {
  const { status, hasAsset } = state;
  const bad = (): never => {
    throw new SlotTransitionError(status, event.type);
  };
  switch (event.type) {
    case "reserve":
      if (status === "empty" || status === "attention" || status === "upgrade_pending")
        return { status: "reserved", hasAsset };
      return bad();
    case "release":
      if (status === "reserved") return { status: hasAsset ? "satisfied" : "empty", hasAsset };
      return bad();
    case "download_started":
      if (status === "reserved") return { status: "downloading", hasAsset };
      return bad();
    case "download_completed":
      if (status === "downloading") return { status: "verifying", hasAsset };
      return bad();
    case "download_failed":
      if (status === "downloading" || status === "reserved") return { status: "attention", hasAsset };
      return bad();
    case "verify_passed":
      if (status === "verifying") return { status: event.complete ? "satisfied" : "empty", hasAsset: true };
      return bad();
    case "verify_failed":
      if (status === "verifying") return { status: "attention", hasAsset };
      return bad();
    case "upgrade_found":
      if (status === "satisfied") return { status: "upgrade_pending", hasAsset };
      return bad();
    case "upgrade_cancelled":
      if (status === "upgrade_pending") return { status: "satisfied", hasAsset };
      return bad();
    case "attention_resolved":
      if (status === "attention") return { status: event.hasAsset ? "satisfied" : "empty", hasAsset: event.hasAsset };
      return bad();
    case "asset_missing_confirmed":
      // 只接受"确认缺失"(对账多次观察后);单次观察失败不得触发(失败 ≠ 不存在)
      if (status === "satisfied" || status === "upgrade_pending") return { status: "attention", hasAsset: false };
      return bad();
  }
}

export function canTransition(state: SlotState, event: SlotEvent): boolean {
  try {
    transition(state, event);
    return true;
  } catch {
    return false;
  }
}