import { loadImage } from '@napi-rs/canvas';

async function boundedBytes(response: Response) {
  const reader = response.body?.getReader();
  if (!reader) return null;
  const chunks: Uint8Array[] = []; let size = 0;
  try {
    while (true) {
      const { value, done } = await reader.read();
      if (done) return Buffer.concat(chunks);
      size += value.byteLength;
      if (size > 1024 * 1024) { await reader.cancel(); return null; }
      chunks.push(value);
    }
  } finally { reader.releaseLock(); }
}

type Avatar = Awaited<ReturnType<typeof loadImage>>;
type Options = {
  request?: typeof fetch;
  now?: () => number;
  maxEntries?: number;
  maxPending?: number;
  successTtlMs?: number;
  failureTtlMs?: number;
};
/** Hash-based Discord URLs invalidate naturally; bound both cached images and downloads. */
export function createAvatarCache({ request = ((url, init) => fetch(url, init)) as typeof fetch, now = Date.now, maxEntries = 128, maxPending = 4, successTtlMs = 3600000, failureTtlMs = 30000 }: Options = {}) {
  const cache = new Map<string, { image: Avatar | null; expires: number }>();
  const pending = new Map<string, Promise<Avatar | null>>();
  return async (url: string): Promise<Avatar | null> => {
    const cached = cache.get(url);
    if (cached && cached.expires > now()) {
      cache.delete(url); cache.set(url, cached);
      return cached.image;
    }
    cache.delete(url);
    const existing = pending.get(url);
    if (existing) return existing;
    if (pending.size >= maxPending) return null;
    const job = (async () => {
      let image: Avatar | null = null;
      try {
        const response = await request(url, { redirect: 'error', signal: AbortSignal.timeout(3000) });
        if (response.ok) {
          const bytes = await boundedBytes(response);
          if (bytes) {
            const decoded = await loadImage(Buffer.from(bytes));
            if (decoded.width <= 256 && decoded.height <= 256) image = decoded;
          }
        }
      } catch { /* A brief negative cache keeps an unavailable CDN out of the hot path. */ }
      cache.set(url, { image, expires: now() + (image ? successTtlMs : failureTtlMs) });
      while (cache.size > maxEntries) cache.delete(cache.keys().next().value!);
      return image;
    })().finally(() => { pending.delete(url); });
    pending.set(url, job);
    return job;
  };
}
export const cachedAvatar = createAvatarCache();