import { existsSync } from 'node:fs';
import { join } from 'node:path';
import { getAttachmentsDir, getTmpAttachmentsDir } from '../../data-path';
import { getUserById } from 'officerdb';

/**
 * Where an attached file ended up, so the agent can be told to read it.
 *
 * ── Why a path and not the bytes ──
 *
 * An image goes to the model as base64 inside the message. A PDF cannot: it would be megabytes of base64
 * in the turn, re-sent for the rest of the conversation, to produce text the agent's own tools extract
 * better. Reading a document is something `Read` already does well — page ranges, and re-reading a
 * section later without paying for it twice.
 *
 * So the file stays where `/upload` put it and the prompt names it. This is a machine where the agent
 * runs as the person who attached it, so it can simply open it.
 *
 * ── The ids are checked, not trusted ──
 *
 * An id arrives in a websocket message, and it is used to build a path. `/upload` generates them as
 * `<uuid>.<ext>`, so anything that is not that shape did not come from us — and a `..` inside one would
 * name a file outside the attachment directory entirely. Only the two directories uploads actually write
 * to are searched, and only ids matching the shape uploads actually produce.
 */
const ATTACHMENT_ID = /^[a-f0-9-]{36}\.[A-Za-z0-9]{1,8}$/;

export async function attachedFilePaths(
  userId: number,
  sessionId: string,
  attachmentIds: string[] | undefined,
): Promise<string[]> {
  if (!attachmentIds?.length) return [];

  const user = await getUserById(userId).catch(() => null);
  if (!user?.email) return [];

  const paths: string[] = [];
  for (const id of attachmentIds) {
    if (!ATTACHMENT_ID.test(id)) continue;
    // Uploads before a session exists land in `tmp/`; everything after lands under the session. Both are
    // checked because a first message carries ids from the first shape and later ones from the second.
    const candidates = [join(getAttachmentsDir(user.email, sessionId), id), join(getTmpAttachmentsDir(user.email), id)];
    const found = candidates.find((path) => existsSync(path));
    if (found) paths.push(found);
  }
  return paths;
}