aggregate_groups.ts

// Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.

import { mapEntries } from "./map_entries.ts";

/**
 * Applies the given aggregator to each group in the given Grouping, returning the results together with the respective group keys
 *
 * ```ts
 * import { aggregateGroups } from "https://deno.land/std@$STD_VERSION/collections/mod.ts";
 * import { assertEquals } from "https://deno.land/std@$STD_VERSION/testing/asserts.ts";
 *
 * const foodProperties = {
 *     'Curry': [ 'spicy', 'vegan' ],
 *     'Omelette': [ 'creamy', 'vegetarian' ],
 * }
 * const descriptions = aggregateGroups(foodProperties,
 *     (current, key, first, acc) => {
 *         if (first)
 *             return `${key} is ${current}`
 *
 *         return `${acc} and ${current}`
 *     },
 * )
 *
 * assertEquals(descriptions, {
 *     'Curry': 'Curry is spicy and vegan',
 *     'Omelette': 'Omelette is creamy and vegetarian',
 * })
 * ```
 */
export function aggregateGroups<T, A>(
  record: Readonly<Record<string, Array<T>>>,
  aggregator: (current: T, key: string, first: boolean, accumulator?: A) => A,
): Record<string, A> {
  return mapEntries(
    record,
    ([key, values]) => [
      key,
      // Need the type assertions here because the reduce type does not support the type transition we need
      values.reduce(
        (accumulator, current, currentIndex) =>
          aggregator(current, key, currentIndex === 0, accumulator),
        undefined as A | undefined,
      ) as A,
    ],
  );
}