/**
 * Stores the site-wide chat's messages and each user's read marker.
 *
 * The WebSocket route (`routes/ws.ts`) writes messages and moves read markers,
 * then broadcasts the result. `routes/chat.ts` reads the history, a page at a
 * time.
 *
 * @module
 */

import { APIException } from "../lib/api-exception.ts";
import type { ChatMessage } from "../../shared/chat.ts";
import { chatMessageRowToApi, isChatMessageRow } from "../model/chat.ts";
import { db } from "../db/connection.ts";
import { linkAttachments } from "./attachment-service.ts";
import { VALIDATION } from "../../shared/validation.ts";

const DEFAULT_LIMIT = 50;
const MAX_LIMIT = 100;

/**
 * How much of the replied-to message a reply carries. The client shows one line
 * of it, once stripped of its Markdown, so the cut leaves room for a long link
 * or image URL before the text.
 */
const REPLY_SNIPPET_LEN = 300;

/** The columns of a `ChatMessageRow`, to select over {@link FROM_JOINS}. */
const SELECT_COLS =
  `m.id, m.user_id, m.body, m.created_at, m.updated_at, m.reply_to_id,
  u.username as author_username, u.avatar_mime as author_avatar_mime,
  ru.username as reply_to_author, substr(rm.body, 1, ${REPLY_SNIPPET_LEN}) as reply_to_body`;

/**
 * Joins a message to its author, and to the message it replies to and that
 * message's author. Those two are left joins: a reply outlives its target.
 */
const FROM_JOINS = `FROM chat_messages m
   JOIN users u ON m.user_id = u.id
   LEFT JOIN chat_messages rm ON m.reply_to_id = rm.id
   LEFT JOIN users ru ON rm.user_id = ru.id`;

/** Returns message `id` as the broadcast sends it, or throws a 404. */
function fetchMessage(id: string): ChatMessage {
  const row = db.prepare(
    `SELECT ${SELECT_COLS} ${FROM_JOINS} WHERE m.id = ?;`,
  ).get(id);
  if (!row || !isChatMessageRow(row)) {
    throw new APIException(404, "Message not found");
  }
  return chatMessageRowToApi(row);
}

/**
 * Stores a message from `userId` on a `chat_send`, and claims the pasted images
 * it links to. A `replyToId` naming no message is dropped, and the message is
 * posted as no reply. The caller broadcasts the result.
 *
 * @throws {APIException} 400 when the body is blank or longer than
 * `VALIDATION.CHAT_MESSAGE_MAX`.
 */
export function createChatMessage(
  userId: string,
  body: string,
  replyToId?: string,
): ChatMessage {
  const trimmed = body.trim();
  if (!trimmed) {
    throw new APIException(400, "Message cannot be empty");
  }
  if (trimmed.length > VALIDATION.CHAT_MESSAGE_MAX) {
    throw new APIException(
      400,
      `Message exceeds ${VALIDATION.CHAT_MESSAGE_MAX} characters`,
    );
  }
  let replyTo: string | null = null;
  if (replyToId) {
    const exists = db.prepare(`SELECT 1 FROM chat_messages WHERE id = ?;`).get(
      replyToId,
    );
    if (exists) replyTo = replyToId;
  }
  const id = crypto.randomUUID();
  const createdAt = new Date().toISOString();
  db.prepare(
    `INSERT INTO chat_messages (id, user_id, body, created_at, reply_to_id) VALUES (?, ?, ?, ?, ?);`,
  ).run(id, userId, trimmed, createdAt, replyTo);
  // Unclaimed attachments are deleted as orphans at startup.
  linkAttachments(trimmed, "chat", id);
  return fetchMessage(id);
}

/** Returns the id of message `id`'s author, or throws a 404. */
function getMessageOwner(id: string): string {
  const row = db.prepare(`SELECT user_id FROM chat_messages WHERE id = ?;`).get(
    id,
  ) as { user_id: string } | undefined;
  if (!row) {
    throw new APIException(404, "Message not found");
  }
  return row.user_id;
}

/**
 * Replaces a message's body on a `chat_edit`, and claims any image the new body
 * links to. `canModerate` lets a moderator edit anyone's message. The caller
 * broadcasts the result.
 *
 * @throws {APIException} 404 when there is no such message, 403 when the user
 * is neither its author nor a moderator, 400 when the body is blank or too
 * long.
 */
export function updateChatMessage(
  id: string,
  body: string,
  requestingUserId: string,
  canModerate: boolean,
): ChatMessage {
  const ownerId = getMessageOwner(id);
  if (ownerId !== requestingUserId && !canModerate) {
    throw new APIException(403, "Not authorized to edit this message");
  }
  const trimmed = body.trim();
  if (!trimmed) {
    throw new APIException(400, "Message cannot be empty");
  }
  if (trimmed.length > VALIDATION.CHAT_MESSAGE_MAX) {
    throw new APIException(
      400,
      `Message exceeds ${VALIDATION.CHAT_MESSAGE_MAX} characters`,
    );
  }
  db.prepare(`UPDATE chat_messages SET body = ?, updated_at = ? WHERE id = ?;`)
    .run(trimmed, new Date().toISOString(), id);
  linkAttachments(trimmed, "chat", id);
  return fetchMessage(id);
}

/**
 * Deletes a message on a `chat_delete`. `canModerate` lets a moderator delete
 * anyone's message. Replies to it keep their `replyToId`, without its preview.
 *
 * @throws {APIException} 404 when there is no such message, 403 when the user
 * is neither its author nor a moderator.
 */
export function deleteChatMessage(
  id: string,
  requestingUserId: string,
  canModerate: boolean,
): void {
  const ownerId = getMessageOwner(id);
  if (ownerId !== requestingUserId && !canModerate) {
    throw new APIException(403, "Not authorized to delete this message");
  }
  db.prepare(`DELETE FROM chat_messages WHERE id = ?;`).run(id);
}

/**
 * Passed by `routes/chat.ts` to page through the history, from the message the
 * client already has at one end. Messages are ordered by `createdAt`, then by
 * id, so a page boundary between two messages sent in the same millisecond
 * skips neither.
 */
export interface ChatCursor {
  /** An ISO timestamp, compared as a string with `created_at`. */
  createdAt: string;
  /**
   * Without it, the cursor takes in every message at `createdAt`: they are
   * all older for {@link getRecentChatMessages} and all newer for
   * {@link getChatMessagesAfter}.
   */
  id?: string;
}

/** Converts query rows to messages, or throws a 500 if one doesn't parse. */
function toMessages(rows: unknown[]): ChatMessage[] {
  if (!rows.every(isChatMessageRow)) {
    throw new APIException(500, "Malformed chat message data");
  }
  return rows.map(chatMessageRowToApi);
}

function capLimit(limit: number): number {
  return Math.min(Math.max(Math.floor(limit), 1), MAX_LIMIT);
}

/**
 * Returns the latest `limit` messages, or with `before` the `limit` messages
 * just older than it, oldest first either way. `limit` is clamped to 1–100.
 *
 * @throws {APIException} 500 when a row doesn't parse as a message.
 */
export function getRecentChatMessages(
  limit = DEFAULT_LIMIT,
  before?: ChatCursor,
): ChatMessage[] {
  const capped = capLimit(limit);
  const rows = before
    ? db.prepare(
      `SELECT ${SELECT_COLS} ${FROM_JOINS}
       WHERE m.created_at < ? OR (m.created_at = ? AND m.id < ?)
       ORDER BY m.created_at DESC, m.id DESC LIMIT ?;`,
    ).all(
      before.createdAt,
      before.createdAt,
      // Nothing sorts below "", so no message at that timestamp is taken.
      before.id ?? "",
      capped,
    )
    : db.prepare(
      `SELECT ${SELECT_COLS} ${FROM_JOINS}
       ORDER BY m.created_at DESC, m.id DESC LIMIT ?;`,
    ).all(capped);
  return toMessages(rows).reverse();
}

/**
 * Returns the first `limit` messages after `after`, oldest first. The chatbox
 * uses it to fetch what it missed while the socket was down. `limit` is
 * clamped to 1–100.
 *
 * @throws {APIException} 500 when a row doesn't parse as a message.
 */
export function getChatMessagesAfter(
  after: ChatCursor,
  limit = DEFAULT_LIMIT,
): ChatMessage[] {
  const rows = db.prepare(
    `SELECT ${SELECT_COLS} ${FROM_JOINS}
     WHERE m.created_at > ? OR (m.created_at = ? AND m.id > ?)
     ORDER BY m.created_at ASC, m.id ASC LIMIT ?;`,
  ).all(after.createdAt, after.createdAt, after.id ?? "", capLimit(limit));
  return toMessages(rows);
}

/**
 * Returned to the WebSocket route by {@link getChatReadState} and
 * {@link markChatRead}. The route sends it in `welcome` and in
 * `chat_read_update`, and the client shows `unread` on the chat's badge.
 */
export interface ChatReadState {
  /** The `created_at` of the newest message the user has read. */
  readAt: string;
  /** Counts only the messages of other users after `readAt`. */
  unread: number;
}

/**
 * Returns how far `userId` has read the chat. A user who never opened it has
 * read up to their registration, so the history from before it isn't unread.
 *
 * @throws {APIException} 404 when there is no such user.
 */
export function getChatReadState(userId: string): ChatReadState {
  const row = db.prepare(
    `SELECT COALESCE(u.chat_read_at, u.created_at) AS read_at,
       (SELECT COUNT(*) FROM chat_messages m
         WHERE m.user_id != u.id
           AND m.created_at > COALESCE(u.chat_read_at, u.created_at)) AS unread
     FROM users u WHERE u.id = ?;`,
  ).get(userId) as { read_at: string; unread: number } | undefined;
  if (!row) {
    throw new APIException(404, "User not found");
  }
  return { readAt: row.read_at, unread: row.unread };
}

/**
 * Moves `userId`'s read marker to `at`, a message's `created_at`, on a
 * `chat_read`. It never moves back, so a lagging tab can't undo a newer tab's
 * read. It never passes the newest message, so a timestamp from the future
 * can't mark messages read before they are sent.
 *
 * @throws {APIException} 400 when `at` isn't a date, 404 when there is no
 * such user.
 */
export function markChatRead(userId: string, at: string): ChatReadState {
  const parsed = new Date(at);
  if (Number.isNaN(parsed.getTime())) {
    throw new APIException(400, "Invalid read marker");
  }
  const newest = db.prepare(
    `SELECT MAX(created_at) AS newest FROM chat_messages;`,
  ).get() as { newest: string | null };
  // With no message to cap it at, a marker from the future would stay stuck.
  if (!newest.newest) return getChatReadState(userId);
  const iso = parsed.toISOString();
  const target = iso > newest.newest ? newest.newest : iso;
  db.prepare(
    `UPDATE users SET chat_read_at = ?
     WHERE id = ? AND (chat_read_at IS NULL OR chat_read_at < ?);`,
  ).run(target, userId, target);
  return getChatReadState(userId);
}