import {LRUCache} from 'lru-cache';

const cache = new LRUCache({
  max: 128,
});

export async function computeMissing(current, manifest) {
  const missing = [];
  for (const path in manifest) {
    if (!current || !current[path] || current[path].hash !== manifest[path]) {
      missing.push(path);
    }
  }
  return missing;
}

const octets = [];
for (let i = 0; i < 256; ++i) {
  octets.push(i.toString(16).padStart(2, '0'));
}

export async function parseResources(resources, buffer) {
  const view = new DataView(buffer);
  let caret = 0;
  const count = view.getUint32(caret);
  caret += 4;
  const manifest = {};
  for (let i = 0; i < count; ++i) {
    let hash = '';
    for (let j = 0; j < 20; ++j) {
      hash += octets[view.getUint8(caret)]
      caret += 1;
    }
    const byteLength = view.getUint32(caret);
    caret += 4;
    const asset = new ArrayBuffer(byteLength);
    const assetView = new DataView(asset);
    for (let j = 0; j < asset.byteLength; ++j) {
      assetView.setUint8(j, view.getUint8(caret));
      caret += 1;
    }
    manifest[resources[i]] = {asset, hash};
  }
  return manifest;
}

export async function fetchResources(paths, {signal} = {}) {
  const response = await fetch('/resources/stream', {
    body: JSON.stringify(paths),
    method: 'post',
    signal,
  });
  if (signal.aborted) {
    return undefined;
  }
  return parseResources(paths, await response.arrayBuffer());
}

export async function get() {
  const {get} = await import('idb-keyval');
  const resources = await get('$$silphius_resources');
  return resources || {};
}

export async function set(resources) {
  const {set} = await import('idb-keyval');
  cache.clear();
  await set('$$silphius_resources', resources);
}

export async function readAsset(path) {
  if (!cache.has(path)) {
    const {pathname} = new URL(path, 'http://example.org');
    const resourcePath = pathname.slice('/resources/'.length);
    cache.set(path, get().then((resources) => resources[resourcePath]?.asset));
  }
  return cache.get(path);
}