gerbeur/api/remote/bandcamp-tralbum.ts

/**
 * Parser for Bandcamp's `data-tralbum` attribute.
 *
 * Every Bandcamp track and album page carries the whole release — titles,
 * durations and signed mp3 stream URLs — as one HTML-escaped JSON blob on a
 * `data-tralbum` attribute. One fetch of an album page therefore yields every
 * track's stream URL; there is no per-track request.
 *
 * The stream URLs are signed and expire 24h after the page was fetched, which
 * is why nothing here is ever persisted to `rich_content` — see
 * `api/routes/bandcamp.ts` for the resolve-at-play-time endpoint.
 *
 * The shape is undocumented, so every field is treated as optional and the
 * whole parse is best-effort: a `null` return means "not a Bandcamp page",
 * which callers turn into a fallback to the iframe embed.
 *
 * @module
 */

import type { Tralbum, TralbumTrack } from "../../shared/player.ts";

/**
 * The hostnames Bandcamp serves its own sites from, anchored at the end so that `bandcamp.com.example.org` cannot pass. Artists on custom
 * domains do not match this and keep using the embed.
 */
export const BANDCAMP_HOST_RE = /(?:^|\.)bandcamp\.com$/;

/**
 * Decodes HTML entities in a single pass.
 *
 * The single pass is the point: `decodeHtmlEntities` in remote/html.ts
 * replaces `&` before `"`, so an escaped-ampersand-then-quot sequence
 * (`"`, i.e. the literal text `"` inside a title) decodes twice
 * and yields a bare `"` — which lands inside the JSON string and breaks the
 * parse. Matching every entity form in one regex means each is replaced once.
 */
function decodeEntitiesOnce(input: string): string {
  const named: Record<string, string> = {
    quot: '"',
    apos: "'",
    amp: "&",
    lt: "<",
    gt: ">",
    nbsp: " ",
  };
  return input.replace(
    /&(?:#(\d+)|#[xX]([0-9a-fA-F]+)|([a-zA-Z]+));/g,
    (match, dec: string, hex: string, name: string) => {
      if (dec) return String.fromCodePoint(Number(dec));
      if (hex) return String.fromCodePoint(parseInt(hex, 16));
      return named[name.toLowerCase()] ?? match;
    },
  );
}

/** Returns the URL of the large square artwork numbered `artId`. */
function artworkUrl(artId: unknown): string | undefined {
  if (typeof artId !== "number" && typeof artId !== "string") return undefined;
  const id = String(artId).trim();
  if (!/^\d+$/.test(id)) return undefined;
  // _16 is the large square art Bandcamp's own player uses.
  return `https://f4.bcbits.com/img/a${id}_16.jpg`;
}

/**
 * Pulls the `data-tralbum` blob out of a Bandcamp page and normalises it.
 * Returns null when the page has no blob, no audio, or nothing parseable.
 */
export function parseTralbum(html: string, sourceUrl: string): Tralbum | null {
  const match = html.match(/\bdata-tralbum=(["'])([\s\S]*?)\1/);
  if (!match) return null;

  let raw: Record<string, unknown>;
  try {
    raw = JSON.parse(decodeEntitiesOnce(match[2]));
  } catch {
    return null;
  }
  if (!raw || typeof raw !== "object") return null;
  if (raw.hasAudio === false) return null;

  const rawTracks = Array.isArray(raw.trackinfo) ? raw.trackinfo : [];
  const tracks: TralbumTrack[] = rawTracks.map((entry, index) => {
    const t = (entry ?? {}) as Record<string, unknown>;
    const file = (t.file ?? {}) as Record<string, unknown>;
    const mp3 = typeof file["mp3-128"] === "string" ? file["mp3-128"] : null;
    // Only ever hand out bcbits URLs. Without this the endpoint would relay
    // whatever host a hostile page put in the blob straight to the browser.
    const streamUrl = mp3 && isBcbitsUrl(mp3) ? mp3 : null;
    return {
      index,
      trackNum: typeof t.track_num === "number" ? t.track_num : undefined,
      title: typeof t.title === "string" && t.title.trim()
        ? t.title
        : `Track ${index + 1}`,
      duration: typeof t.duration === "number" && t.duration > 0
        ? t.duration
        : undefined,
      streamUrl,
      streamable: streamUrl !== null && t.streaming === 1 &&
        t.unreleased_track !== true && !t.encoding_pending,
      capped: t.is_capped === true,
    };
  });

  if (!tracks.some((t) => t.streamable)) return null;

  const current = (raw.current ?? {}) as Record<string, unknown>;
  return {
    sourceUrl,
    itemType: raw.item_type === "album" ? "album" : "track",
    title: typeof current.title === "string" ? current.title : undefined,
    artist: typeof raw.artist === "string" ? raw.artist : undefined,
    artworkUrl: artworkUrl(raw.art_id),
    tracks,
    resolvedAt: Date.now(),
  };
}

/** Checks whether `url` is an https URL on Bandcamp's bcbits.com CDN. */
function isBcbitsUrl(url: string): boolean {
  try {
    const { protocol, hostname } = new URL(url);
    return protocol === "https:" && /(?:^|\.)bcbits\.com$/.test(hostname);
  } catch {
    return false;
  }
}