/**
 * Moves the chat's unread state to the server, so that it survives a reload
 * and agrees across tabs and devices. `users.chat_read_at` holds the time of
 * the newest message the user has seen.
 *
 * It also replaces the index on `created_at` with one on `(created_at, id)`,
 * which the history now pages on, so that two messages from the same
 * millisecond can't fall between two pages.
 *
 * @module
 */

import type { DatabaseSync } from "node:sqlite";

/**
 * Run once by `runMigrations`. Existing users start with every message read,
 * instead of the whole history unread. A user created later starts with a
 * NULL `chat_read_at`, which chat-service reads as the user's creation time.
 */
export function up(db: DatabaseSync): void {
  const cols = db.prepare(`PRAGMA table_info(users);`).all() as {
    name: string;
  }[];
  if (!cols.some((c) => c.name === "chat_read_at")) {
    db.exec(`ALTER TABLE users ADD COLUMN chat_read_at TEXT;`);
    db.prepare(`UPDATE users SET chat_read_at = ?;`).run(
      new Date().toISOString(),
    );
  }

  db.exec(`DROP INDEX IF EXISTS idx_chat_messages_created;`);
  db.exec(
    `CREATE INDEX IF NOT EXISTS idx_chat_messages_created_id ON chat_messages(created_at, id);`,
  );
}