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/** Async version of Array.prototype.reduce()
 *  await reduce(['/foo', '/bar', '/baz'], async (acc, v) => {
 *    acc[v] = await (await fetch(v)).json();
 *    return acc;
 *  }, {});
 */
export async function reduce(arr, fn, val, pure) {
  for (let i=0; i<arr.length; i++) {
    let v = await fn(val, arr[i], i, arr);
    if (pure!==false) val = v;
  }
  return val;
}

/** Async version of Array.prototype.map()
 *  await map(['foo', 'baz'], async v => await fetch(v) )
 */
export async function map(arr, fn) {
  return await reduce(arr, (acc, value, index, arr) => {
    acc.push(await fn(value, index, arr));
  }, [], false);
}

/** Async version of Array.prototype.filter()
 *  await filter(['foo', 'baz'], async v => (await fetch(v)).ok )
 */
export async function filter(arr, fn) {
  return await reduce(arr, (acc, value, index, arr) => {
    if (await fn(value, index, arr)) acc.push(value);
  }, [], false);
}

function identity(x) {
  return x;
}

function resolve(list) {
  let out = Array.isArray(list) ? [] : {};
  for (let i in list) if (list.hasOwnProperty(i)) out[i] = list[i]();
  return out;
}

/** Provided by standard lib, replaces async.parallel()
 *  await parallel([
 *    () => fetch('foo'),
 *    () => fetch('baz')
 *  ])
 */
export async function parallel(list) {
  return await Promise.all(resolve(list));
}

/** Replaces async.series()
 *  await series([
 *    () => fetch('foo'),
 *    () => fetch('baz')
 *  ])
 */
export async function series(list) {
  return await map(resolve(list), identity);
}