feat(collections): add findLastIndex (#1062)

collections/find_last_index.ts

@@ -0,0 +1,32 @@
+// Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
+
+import { Predicate } from "./types.ts";
+
+/**
+ * Returns the index of the last element in the given array matching the given predicate
+ *
+ * Example:
+ *
+ * ```ts
+ * import { findLastIndex } from "./find_last_index.ts";
+ *
+ * const numbers = [ 4, 2, 7 ]
+ * const lastIndexNumber = findLastIndex(numbers, it => it % 2 === 0)
+ *
+ * console.assert(lastIndexNumber === 1)
+ * ```
+ */
+export function findLastIndex<T>(
+  array: Array<T>,
+  predicate: Predicate<T>,
+): number {
+  for (let i = array.length - 1; i >= 0; i -= 1) {
+    const element = array[i];
+
+    if (predicate(element)) {
+      return i;
+    }
+  }
+
+  return -1;
+}

collections/find_last_index_test.ts

@@ -0,0 +1,47 @@
+// Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
+
+import { assertEquals } from "../testing/asserts.ts";
+import { findLastIndex } from "./find_last_index.ts";
+
+function findLastIndexTest<I>(
+  input: [Array<I>, (element: I) => boolean],
+  expected: number,
+  message?: string,
+) {
+  const actual = findLastIndex(...input);
+  assertEquals(actual, expected, message);
+}
+
+Deno.test({
+  name: "[collections/findLastIndex] empty input",
+  fn() {
+    findLastIndexTest([[], (_) => true], -1);
+  },
+});
+
+Deno.test({
+  name: "[collections/findLastIndex] no matches",
+  fn() {
+    findLastIndexTest([[9, 11, 13], (it) => it % 2 === 0], -1);
+    findLastIndexTest([["foo", "bar"], (it) => it.startsWith("z")], -1);
+  },
+});
+
+Deno.test({
+  name: "[collections/findLastIndex] only match",
+  fn() {
+    findLastIndexTest([[9, 12, 13], (it) => it % 2 === 0], 1);
+    findLastIndexTest([["zap", "foo", "bar"], (it) => it.startsWith("z")], 0);
+  },
+});
+
+Deno.test({
+  name: "[collections/findLastIndex] multiple matches",
+  fn() {
+    findLastIndexTest([[0, 1, 2, 3, 4, 5, 6], (it) => it % 2 === 0], 6);
+    findLastIndexTest(
+      [[0, 1, 2, 3, 4, 5, 6, 7, 8, 9], (it) => it % 2 === 0],
+      8,
+    );
+  },
+});