feat(collections): add dropLastWhile (#1193)

collections/README.md

@@ -605,3 +605,24 @@ const personWithMinAge = minBy(people, (i) => i.age);
 
 assertEquals(personWithMinAge, { name: "John", age: 23 });
 ```
+
+### dropLastWhile
+
+Returns a new array that drops all elements in the given collection until the
+last element that does not match the given predicate
+
+Example:
+
+```ts
+import { dropLastWhile } from "https://deno.land/std@$STD_VERSION/collections/mod.ts";
+import { assertEquals } from "https://deno.land/std@$STD_VERSION/testing/asserts.ts";
+
+const numbers = [22, 30, 44];
+
+const notFourtyFour = dropLastWhile(numbers, (i) => i != 44);
+
+assertEquals(
+  notFourtyFour,
+  [22, 30],
+);
+```

collections/drop_last_while.ts

@@ -0,0 +1,29 @@
+/**
+ * Returns a new array that drops all elements in the given collection until the
+ * last element that does not match the given predicate
+ *
+ * Example:
+ * ```ts
+ * import { dropLastWhile } from "./drop_last_while.ts";
+ * import { assertEquals } from "../testing/asserts.ts";
+ *
+ * const numbers = [22, 30, 44];
+ *
+ * const notFourtyFour = dropLastWhile(numbers, i => i != 44);
+ *
+ * assertEquals(
+ *   notFourtyFour,
+ *   [22, 30]
+ * );
+ * ```
+ *
+ */
+export function dropLastWhile<T>(
+  array: readonly T[],
+  predicate: (el: T) => boolean,
+): T[] {
+  let offset = array.length;
+  while (0 < offset && predicate(array[offset - 1])) offset--;
+
+  return array.slice(0, offset);
+}

collections/drop_last_while_test.ts

@@ -0,0 +1,51 @@
+import { dropLastWhile } from "./drop_last_while.ts";
+import { assertEquals } from "../testing/asserts.ts";
+
+Deno.test("[collections/dropLastWhile] Num array", () => {
+  const values = [20, 33, 44];
+
+  const actual = dropLastWhile(values, (i) => i > 30);
+
+  assertEquals(actual, [20]);
+});
+
+Deno.test("[collections/dropLastWhile] No mutation", () => {
+  const array = [1, 2, 3, 4, 5, 6];
+
+  const actual = dropLastWhile(array, (i) => i > 4);
+
+  assertEquals(actual, [1, 2, 3, 4]);
+  assertEquals(array, [1, 2, 3, 4, 5, 6]);
+});
+
+Deno.test("[collections/dropLastWhile] Negatives", () => {
+  const array = [-1, -2, -3, -4, -5, -6];
+
+  const actual = dropLastWhile(array, (i) => i < -4);
+
+  assertEquals(actual, [-1, -2, -3, -4]);
+});
+
+Deno.test("[collections/dropLastWhile] Empty input returns empty array", () => {
+  const array: number[] = [];
+
+  const actual = dropLastWhile(array, (i) => i > 4);
+
+  assertEquals(actual, []);
+});
+
+Deno.test("[collections/dropLastWhile] Returns same array when the last element doesn't get dropped", () => {
+  const array = [40, 30, 20];
+
+  const actual = dropLastWhile(array, (i) => i > 40);
+
+  assertEquals(actual, [40, 30, 20]);
+});
+
+Deno.test("[collections/dropLastWhile] Returns empty array when all elements get dropped", () => {
+  const array = [20, 30, 20];
+
+  const actual = dropLastWhile(array, (i) => i < 40);
+
+  assertEquals(actual, []);
+});

collections/mod.ts

@@ -31,3 +31,4 @@ export * from "./zip.ts";
 export * from "./take_last_while.ts";
 export * from "./take_while.ts";
 export * from "./first_not_nullish_of.ts";
+export * from "./drop_last_while.ts";