/**
 * Makes the server notify the same users the chat highlights and the editor
 * suggests, by defining once what counts as an @mention.
 *
 * @module
 */

/**
 * Matches an @username, with the name as its first group. The `@` must not
 * follow a word character, so an email address mentions no one. Nor may it
 * follow `[` or `(`, which leaves out a Markdown link's text or URL starting
 * with `@`, and also a plain `(@name)`. The editor's suggestions match its
 * source at the caret; the rest goes through {@link mentionedUsernames}.
 * `Markdown` links mentions with a looser pattern of its own, on text already
 * parsed, where code and URLs are out of the way.
 */
export const MENTION_RE: RegExp = /(?<![[(\w])@(\w+)/g;

/**
 * Returns the names `body` mentions, in lower case, each once. Code, fenced
 * or inline, and URLs mention no one, as `Markdown` shows them. The server
 * notifies by it, and the chat highlights and chimes by it.
 */
export function mentionedUsernames(body: string): string[] {
  const prose = body
    // A fence left open runs to the end, as Markdown renders it.
    .replace(/```[\s\S]*?(?:```|$)/g, " ")
    .replace(/`[^`\n]*`/g, " ")
    .replace(/https?:\/\/\S+/g, " ");
  return [
    ...new Set([...prose.matchAll(MENTION_RE)].map((m) => m[1].toLowerCase())),
  ];
}

/**
 * The name `@everyone` reaches. Said by a role with `chat:everyone` in the
 * chat, it notifies every user, so no account may take it
 * (`assertUsernameAvailable`).
 */
export const EVERYONE = "everyone";

/**
 * Checks whether `body` says `@everyone`, in any case, by the rule of
 * {@link mentionedUsernames}. Only the chat service reads it, to set a
 * message's `mentionsEveryone` when its author may; the client reads that
 * flag, not the text.
 */
export function mentionsEveryone(body: string): boolean {
  return mentionedUsernames(body).includes(EVERYONE);
}