// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
// This module is browser compatible.

import { mapValues } from "./map_values.ts";

/**
 * Applies the given reducer to each group in the given grouping, returning the
 * results together with the respective group keys.
 *
 * @template T input type of an item in a group in the given grouping.
 * @template A type of the accumulator value, which will match the returned record's values.
 * @example
 * ```ts
 * import { reduceGroups } from "@std/collections/reduce_groups";
 * import { assertEquals } from "@std/assert/assert_equals";
 *
 * const votes = {
 *   "Woody": [2, 3, 1, 4],
 *   "Buzz": [5, 9],
 * };
 *
 * const totalVotes = reduceGroups(votes, (sum, it) => sum + it, 0);
 *
 * assertEquals(totalVotes, {
 *   "Woody": 10,
 *   "Buzz": 14,
 * });
 * ```
 */
export function reduceGroups<T, A>(
  record: Readonly<Record<string, ReadonlyArray<T>>>,
  reducer: (accumulator: A, current: T) => A,
  initialValue: A,
): Record<string, A> {
  return mapValues(record, (it) => it.reduce(reducer, initialValue));
}