feat (collections): add runningReduce

collections/README.md

@@ -834,3 +834,21 @@ const random = sample(numbers);
 
 assert(numbers.includes(random as number));
 ```
+
+### runningReduce
+
+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]);
+```

collections/mod.ts

@@ -41,3 +41,4 @@ export * from "./first_not_nullish_of.ts";
 export * from "./drop_last_while.ts";
 export * from "./reduce_groups.ts";
 export * from "./sample.ts";
+export * from "./running_reduce.ts";

collections/running_reduce.ts

@@ -0,0 +1,27 @@
+// Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
+
+/**
+ * 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) => O,
+  initialValue: O,
+): O[] {
+  let currentResult = initialValue;
+  return array.map((el) => currentResult = reducer(currentResult, el));
+}

collections/running_reduce_test.ts

@@ -0,0 +1,43 @@
+// Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
+
+import { assertEquals } from "../testing/asserts.ts";
+import { runningReduce } from "./running_reduce.ts";
+
+Deno.test({
+  name: "[collections/runningReduce] no mutation",
+  fn() {
+    const numbers = [1, 2, 3, 4, 5];
+    runningReduce(numbers, (sum, current) => sum + current, 0);
+
+    assertEquals(numbers, [1, 2, 3, 4, 5]);
+  },
+});
+
+Deno.test({
+  name: "[collections/runningReduce] array of numbers",
+  fn() {
+    const numbers = [1, 2, 3, 4, 5];
+    const result = runningReduce(numbers, (sum, current) => sum + current, 0);
+
+    assertEquals(result, [1, 3, 6, 10, 15]);
+  },
+});
+
+Deno.test({
+  name: "[collections/runningReduce] array of strings",
+  fn() {
+    const strings = ["a", "b", "c", "d", "e"];
+    const result = runningReduce(strings, (str, current) => str + current, "");
+
+    assertEquals(result, ["a", "ab", "abc", "abcd", "abcde"]);
+  },
+});
+
+Deno.test({
+  name: "[collections/runningReduce] empty input",
+  fn() {
+    const result = runningReduce([], (sum, current) => sum + current, 0);
+
+    assertEquals(result, []);
+  },
+});