const INDEX_KEY = "bitview.ask.chats.v2";
const CHAT_KEY_PREFIX = "bitview.ask.chat.v2.";
const LEGACY_KEY = "bitview.ask.v1";
const NEW_CHAT_TITLE = "New chat";

/**
 * @typedef {Object} StoredMessage
 * @property {"user" | "assistant"} role
 * @property {string} content
 *
 * @typedef {Object} ChatMeta
 * @property {string} id
 * @property {string} title
 * @property {number} createdAt
 * @property {number} updatedAt
 * @property {number} messageCount
 *
 * @typedef {Object} StoredChat
 * @property {string} id
 * @property {string} title
 * @property {number} createdAt
 * @property {number} updatedAt
 * @property {number} messageCount
 * @property {StoredMessage[]} messages
 * @property {string} memory
 * @property {number} compactedCount
 *
 * @typedef {Object} ChatIndex
 * @property {string} activeChatId
 * @property {ChatMeta[]} chats
 */

/** @param {unknown} value */
function isStoredMessage(value) {
  if (!value || typeof value !== "object") return false;

  const message = /** @type {Record<string, unknown>} */ (value);
  return (
    (message.role === "user" || message.role === "assistant") &&
    typeof message.content === "string"
  );
}

/** @param {string} question */
function titleFromQuestion(question) {
  const title = question.replace(/\s+/g, " ").trim();
  return title.length > 44 ? `${title.slice(0, 43)}…` : title;
}

/** @returns {ChatMeta} */
function createMeta() {
  const now = Date.now();
  return {
    id: crypto.randomUUID(),
    title: NEW_CHAT_TITLE,
    createdAt: now,
    updatedAt: now,
    messageCount: 0,
  };
}

/** @param {ChatMeta} meta */
function emptyChat(meta) {
  return { ...meta, messages: [], memory: "", compactedCount: 0 };
}

/** @param {string} id */
function chatKey(id) {
  return `${CHAT_KEY_PREFIX}${id}`;
}

/** @param {ChatIndex} index */
function writeIndex(index) {
  localStorage.setItem(INDEX_KEY, JSON.stringify(index));
}

/** @param {StoredChat} chat */
function writeChat(chat) {
  localStorage.setItem(
    chatKey(chat.id),
    JSON.stringify({
      messages: chat.messages,
      memory: chat.memory,
      compactedCount: chat.compactedCount,
    }),
  );
}

/** @returns {ChatIndex | undefined} */
function readIndex() {
  const value = localStorage.getItem(INDEX_KEY);
  if (!value) return undefined;

  const index = /** @type {ChatIndex} */ (JSON.parse(value));
  let migrated = false;
  const chats = index.chats.map((meta) => {
    if (Number.isInteger(meta.messageCount)) return meta;

    migrated = true;
    return { ...meta, messageCount: readChat(meta).messages.length };
  });

  if (!migrated) return index;

  const next = { ...index, chats };
  writeIndex(next);
  return next;
}

/** @param {ChatMeta} meta */
function readChat(meta) {
  const value = localStorage.getItem(chatKey(meta.id));
  if (!value) return emptyChat(meta);

  const stored = JSON.parse(value);
  const messages = Array.isArray(stored.messages)
    ? stored.messages.filter(isStoredMessage)
    : [];
  const compactedCount = Math.min(
    Number(stored.compactedCount) || 0,
    messages.length,
  );

  return {
    ...meta,
    messageCount: messages.length,
    messages,
    memory: typeof stored.memory === "string" ? stored.memory : "",
    compactedCount,
  };
}

/** @returns {ChatIndex} */
function initialize() {
  const meta = createMeta();
  const legacy = localStorage.getItem(LEGACY_KEY);
  const messages = /** @type {StoredMessage[]} */ (
    legacy ? JSON.parse(legacy).filter(isStoredMessage) : []
  );

  if (messages.length) {
    meta.messageCount = messages.length;
    const firstQuestion = messages.find((message) => message.role === "user");
    if (firstQuestion) meta.title = titleFromQuestion(firstQuestion.content);
  }

  const index = { activeChatId: meta.id, chats: [meta] };
  writeChat({ ...emptyChat(meta), messages });
  writeIndex(index);
  localStorage.removeItem(LEGACY_KEY);
  return index;
}

function load() {
  const index = readIndex() ?? initialize();
  const activeMeta =
    index.chats.find((chat) => chat.id === index.activeChatId) ?? index.chats[0];

  return {
    chats: index.chats,
    activeChat: readChat(activeMeta),
  };
}

function create() {
  const index = readIndex() ?? initialize();
  const meta = createMeta();
  const next = { activeChatId: meta.id, chats: [meta, ...index.chats] };

  writeChat(emptyChat(meta));
  writeIndex(next);
  return { chats: next.chats, activeChat: emptyChat(meta) };
}

/** @param {string} id */
function select(id) {
  const index = readIndex() ?? initialize();
  const meta = index.chats.find((chat) => chat.id === id);
  if (!meta) throw new Error("Chat not found");

  writeIndex({ ...index, activeChatId: id });
  return { chats: index.chats, activeChat: readChat(meta) };
}

/** @param {StoredChat} chat */
function save(chat) {
  const index = readIndex() ?? initialize();
  const now = Date.now();
  const firstQuestion = chat.messages.find((message) => message.role === "user");
  const title =
    chat.title === NEW_CHAT_TITLE && firstQuestion
      ? titleFromQuestion(firstQuestion.content)
      : chat.title;
  const saved = {
    ...chat,
    title,
    updatedAt: now,
    messageCount: chat.messages.length,
  };
  const meta = {
    id: saved.id,
    title: saved.title,
    createdAt: saved.createdAt,
    updatedAt: saved.updatedAt,
    messageCount: saved.messages.length,
  };
  const chats = [meta, ...index.chats.filter((item) => item.id !== chat.id)];

  writeChat(saved);
  writeIndex({ activeChatId: saved.id, chats });
  return { chats, activeChat: saved };
}

/** @param {string} id */
function remove(id) {
  const index = readIndex() ?? initialize();
  const chats = index.chats.filter((chat) => chat.id !== id);

  if (!chats.length) {
    const next = createMeta();
    writeChat(emptyChat(next));
    writeIndex({ activeChatId: next.id, chats: [next] });
  } else {
    const activeChatId =
      index.activeChatId === id ? chats[0].id : index.activeChatId;
    writeIndex({ activeChatId, chats });
  }

  localStorage.removeItem(chatKey(id));
  return load();
}

export const askStorage = /** @type {const} */ ({
  load,
  create,
  select,
  save,
  remove,
});