"use client";

import localforage from "localforage";

import { fetchSkillRuntimeConditional, type SkillRuntime } from "@/lib/api";

/**
 * A Skill runtime is a pure function of the files inside its directory, so the
 * previous result stays valid until those files change. The catalog already
 * carries each Skill's `guide_version`, which lets a cached entry be checked
 * against the installed files without an extra round trip.
 *
 * Opens are served in this order:
 *   1. memory hit that still matches the catalog version -> no request at all
 *   2. IndexedDB hit -> paint immediately, then revalidate with If-None-Match
 *   3. network fetch -> store for the next open
 */

const MEMORY_LIMIT = 24;
const STORAGE_LIMIT = 24;
const FRESH_MS = 60_000;
const STORAGE_NAME = "chatgpt2api";
const STORE_NAME = "skill-runtime";
const INDEX_KEY = "__index__";

export type SkillRuntimeCacheEntry = {
  version: string;
  etag: string;
  savedAt: number;
  runtime: SkillRuntime;
};

const memory = new Map<string, SkillRuntimeCacheEntry>();
const inflight = new Map<string, Promise<SkillRuntime>>();

const storage = localforage.createInstance({ name: STORAGE_NAME, storeName: STORE_NAME });
let storageReady: Promise<void> | null = null;
let storageDisabled = false;

function ensureStorage() {
  if (storageDisabled) return Promise.resolve();
  if (!storageReady) {
    storageReady = storage.ready().then(() => undefined).catch(() => {
      // Private browsing or a blocked storage backend: keep working in memory.
      storageDisabled = true;
    });
  }
  return storageReady;
}

async function readIndex(): Promise<string[]> {
  try {
    const value = await storage.getItem<string[]>(INDEX_KEY);
    if (!Array.isArray(value)) return [];
    return value.filter((item): item is string => typeof item === "string");
  } catch {
    return [];
  }
}

async function writeIndex(ids: string[]) {
  try {
    await storage.setItem(INDEX_KEY, ids);
  } catch {
    // A full quota must never break the detail dialog.
  }
}

function rememberInMemory(skillId: string, entry: SkillRuntimeCacheEntry) {
  memory.delete(skillId);
  memory.set(skillId, entry);
  while (memory.size > MEMORY_LIMIT) {
    const oldest = memory.keys().next().value;
    if (oldest === undefined) break;
    memory.delete(oldest);
  }
}

async function readEntry(skillId: string): Promise<SkillRuntimeCacheEntry | null> {
  const inMemory = memory.get(skillId);
  if (inMemory) return inMemory;
  await ensureStorage();
  if (storageDisabled) return null;
  try {
    const value = await storage.getItem<SkillRuntimeCacheEntry>(skillId);
    if (value && value.runtime) {
      rememberInMemory(skillId, value);
      return value;
    }
  } catch {
    return null;
  }
  return null;
}

async function writeEntry(skillId: string, entry: SkillRuntimeCacheEntry) {
  rememberInMemory(skillId, entry);
  await ensureStorage();
  if (storageDisabled) return;
  try {
    await storage.setItem(skillId, entry);
    const ids = await readIndex();
    const next = [skillId, ...ids.filter((item) => item !== skillId)];
    const dropped = next.slice(STORAGE_LIMIT);
    await writeIndex(next.slice(0, STORAGE_LIMIT));
    await Promise.all(dropped.map((id) => storage.removeItem(id)));
  } catch {
    // Ignore quota errors: the in-memory copy still serves this session.
  }
}

/** Cached runtime for instant paint, without touching the network. */
export function peekSkillRuntime(skillId: string): Promise<SkillRuntimeCacheEntry | null> {
  return readEntry(skillId);
}

/**
 * True when a cached entry matches the version the catalog reports and was
 * written recently enough that no revalidation is expected at all.
 */
export function isSkillRuntimeFresh(entry: SkillRuntimeCacheEntry | null, expectedVersion = "") {
  if (!entry || !expectedVersion || entry.version !== expectedVersion) return false;
  return Date.now() - entry.savedAt < FRESH_MS;
}

export function forgetSkillRuntime(skillId: string) {
  memory.delete(skillId);
  void (async () => {
    await ensureStorage();
    if (storageDisabled) return;
    try {
      await storage.removeItem(skillId);
      const ids = await readIndex();
      await writeIndex(ids.filter((item) => item !== skillId));
    } catch {
      // Nothing to clean up.
    }
  })();
}

async function revalidateSkillRuntime(skillId: string, expectedVersion: string): Promise<SkillRuntime> {
  const cached = await readEntry(skillId);
  const versionMatches = Boolean(cached && expectedVersion && cached.version === expectedVersion);
  if (cached && versionMatches && Date.now() - cached.savedAt < FRESH_MS) {
    return cached.runtime;
  }

  const result = await fetchSkillRuntimeConditional(skillId, cached?.etag || "");
  if (result.notModified) {
    if (!cached) throw new Error("Skill 数据读取失败");
    // The server confirmed the files are unchanged; refresh recency and adopt
    // the catalog version so the next open can skip the request entirely.
    await writeEntry(skillId, {
      ...cached,
      version: expectedVersion || cached.version,
      etag: result.etag || cached.etag,
      savedAt: Date.now(),
    });
    return cached.runtime;
  }

  const runtime = result.runtime;
  if (!runtime) throw new Error("Skill 数据读取失败");
  await writeEntry(skillId, {
    version: expectedVersion,
    etag: result.etag,
    savedAt: Date.now(),
    runtime,
  });
  return runtime;
}

/** Load a Skill runtime, reusing the cache and de-duplicating concurrent opens. */
export function loadSkillRuntime(skillId: string, expectedVersion = ""): Promise<SkillRuntime> {
  const pending = inflight.get(skillId);
  if (pending) return pending;
  const task = revalidateSkillRuntime(skillId, expectedVersion).finally(() => {
    if (inflight.get(skillId) === task) inflight.delete(skillId);
  });
  inflight.set(skillId, task);
  return task;
}