/**
 * Home bootstrap reveal policy.
 *
 * The loading screen owns every wait before the page appears. Phase "account"
 * fetches account, model catalog and the session list; phase "prepare" fetches
 * today's starlanguage and the rectification entry summary in parallel. The
 * page is revealed once every applicable item has settled, or when the prepare
 * budget runs out — never with a spinner inside.
 */

import { parseSessionUrlQuery } from "./chat-session-url.ts";

export type BootstrapPhase = "account" | "prepare";

export type LandingSession = Readonly<{
  id: string;
  sessionType: string;
  messages: readonly unknown[];
  /** False for a list row whose messages were not loaded; absent means loaded. */
  messagesHydrated?: boolean;
  archivedAt?: string | null;
}>;

export type LandingUrlAction = "keep" | "replace-clear" | "none" | "lookup" | "new-chat";

/**
 * A rectification Case is opened only when the address bar names that session
 * (`?c=`), including refresh. Bare `/` after login must not open the latest
 * history item in the background.
 */
export function shouldAutoOpenRectificationSession(input: {
  readonly sessionType: string | undefined;
  readonly sessionId: string | undefined;
  readonly search: string;
}): boolean {
  if (input.sessionType !== "birth_time_rectification" || !input.sessionId) return false;
  return parseSessionUrlQuery(input.search).sessionId === input.sessionId;
}

/** An unsaved empty consultation: the blank starter home, not a conversation. */
export function isStarterHomeDraft(session: LandingSession): boolean {
  return session.sessionType === "consultation"
    && !session.archivedAt
    && session.messagesHydrated !== false
    && session.messages.length === 0;
}

/**
 * Bare `/` (and a dead `?c=`) lands on the blank starter home, never on an old
 * conversation (BUG-1052; replaces the BUG-599 rule that only swapped away
 * from a rectification session and otherwise opened the latest chat). An
 * empty draft already in memory is reused; otherwise
 * `starterHomeLandingNeedsConsultation` asks for a new local one. `?c=`,
 * its lookup and `?new=1` keep the requested session.
 */
export function resolveStarterHomeLandingSessionId(
  sessions: readonly LandingSession[],
  selectedId: string,
  urlAction: LandingUrlAction,
): string {
  if (urlAction === "keep" || urlAction === "lookup" || urlAction === "new-chat") return selectedId;
  return sessions.find(isStarterHomeDraft)?.id ?? selectedId;
}

export function starterHomeLandingNeedsConsultation(
  sessions: readonly LandingSession[],
  landingId: string,
  urlAction: LandingUrlAction,
): boolean {
  if (urlAction === "new-chat") return true;
  if (urlAction === "keep" || urlAction === "lookup") return false;
  const landing = sessions.find((session) => session.id === landingId);
  return !landing || !isStarterHomeDraft(landing);
}

/**
 * Whether a background answer being recovered may take the screen. Only when
 * the address bar named a conversation; the blank home and an explicit new
 * chat stay put while the older answer keeps recovering (BUG-1015/1052).
 */
export function landingYieldsToRecovery(urlAction: LandingUrlAction): boolean {
  return urlAction === "keep" || urlAction === "lookup";
}

export const HOME_BOOTSTRAP_SLOW_MS = 20_000;
export const BOOTSTRAP_PREPARE_TIMEOUT_MS = 4000;
export const SESSION_PREFETCH_COUNT = 5;

export function retryHomeBootstrap(
  setAccountError: (value: string) => void,
  setHydrated: (value: boolean) => void,
  setBootstrapPhase: (value: BootstrapPhase) => void,
  prepareStartedAt: { current: number | null },
  reloadSessionList: () => void,
): void {
  setAccountError("");
  setHydrated(false);
  setBootstrapPhase("account");
  prepareStartedAt.current = null;
  reloadSessionList();
}

export type BootstrapPrepareState = Readonly<{
  dailyStarlanguageApplicable: boolean;
  dailyStarlanguageSettled: boolean;
  entrySummarySettled: boolean;
  /** A URL-named rectification session must be opened and hydrated before reveal. */
  rectificationApplicable?: boolean;
  rectificationSettled?: boolean;
}>;

export function bootstrapPrepareSettled(state: BootstrapPrepareState): boolean {
  if (!state.entrySummarySettled) return false;
  if (state.rectificationApplicable && !state.rectificationSettled) return false;
  if (state.dailyStarlanguageApplicable && !state.dailyStarlanguageSettled) return false;
  return true;
}

export function bootstrapRevealDelayMs(now: number, prepareStartedAt: number | null, settled: boolean): number {
  if (settled) return 0;
  if (prepareStartedAt === null) return BOOTSTRAP_PREPARE_TIMEOUT_MS;
  return Math.max(0, BOOTSTRAP_PREPARE_TIMEOUT_MS - (now - prepareStartedAt));
}

export function bootstrapLoadingCopy(phase: BootstrapPhase): Readonly<{ title: string; detail: string }> {
  return phase === "prepare"
    ? { title: "正在准备对话", detail: "整理今日星语与对话记录" }
    : { title: "正在载入账户", detail: "同步个人资料与对话记录" };
}

export type PrefetchableSession = Readonly<{
  id: string;
  sessionType: string;
  messagesHydrated?: boolean;
}>;

export function sessionIdsToPrefetch(
  sessions: readonly PrefetchableSession[],
  activeSessionId: string,
  limit = SESSION_PREFETCH_COUNT,
): string[] {
  const ids: string[] = [];
  for (const session of sessions) {
    if (ids.length >= limit) break;
    if (session.id === activeSessionId) continue;
    if (session.messagesHydrated) continue;
    if (session.sessionType !== "consultation") continue;
    ids.push(session.id);
  }
  return ids;
}