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running_reduce.ts
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running_reduce.ts
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// Copyright 2018-2022 the Deno authors. All rights reserved. MIT license.
// This module is browser compatible.
/**
* Calls the given reducer on each element of the given collection, passing it's
* result as the accumulator to the next respective call, starting with the given
* initialValue. Returns all intermediate accumulator results.
*
* Example:
*
* ```ts
* import { runningReduce } from "https://deno.land/std@$STD_VERSION/collections/mod.ts";
* import { assertEquals } from "https://deno.land/std@$STD_VERSION/testing/asserts.ts";
*
* const numbers = [1, 2, 3, 4, 5];
* const sumSteps = runningReduce(numbers, (sum, current) => sum + current, 0);
*
* assertEquals(sumSteps, [1, 3, 6, 10, 15]);
* ```
*/
export function runningReduce<T, O>(
array: readonly T[],
reducer: (accumulator: O, current: T, currentIndex: number) => O,
initialValue: O,
): O[] {
let currentResult = initialValue;
return array.map((el, currentIndex) =>
currentResult = reducer(currentResult, el, currentIndex)
);
}