feat(mapkeylastby): add mapKeyLastBy function

Author: djcsdy

index.test.ts

@@ -33,6 +33,7 @@ import {
     map,
     mapKeyBy,
     mapKeyFirstBy,
+    mapKeyLastBy,
     maximum,
     minimum,
     only,
@@ -527,6 +528,14 @@ test("mapKeyFirstBy", t => {
     t.deepEqual(Array.from(map.keys()), ["odd", "even"]);
 });
 
+test("mapKeyLastBy", t => {
+    const map = mapKeyLastBy([1, 3, 4, 2, 5, 6], e => [e % 2 === 0 ? "even" : "odd", String(e)]);
+    t.deepEqual(Array.from(map.entries()), [
+        ["odd", "5"],
+        ["even", "6"]
+    ]);
+});
+
 test("groupByIdentity", t => {
     t.deepEqual(
         groupByIdentity(["abc", "adef", "bghi"], a => a.substr(0, 1)),

index.ts

@@ -1096,6 +1096,26 @@ export function mapKeyFirstByFn<TKey, TElement, TNewElement>(
     return array => mapKeyFirstBy(array, f);
 }
 
+export function mapKeyLastBy<TKey, TElement, TNewElement>(
+    array: ArrayLike<TElement>,
+    f: (element: TElement, index: number) => readonly [TKey, TNewElement]
+): Map<TKey, TNewElement> {
+    const result = new Map<TKey, TNewElement>();
+
+    for (let i = 0; i < array.length; ++i) {
+        const [key, element] = f(array[i], i);
+        result.set(key, element);
+    }
+
+    return result;
+}
+
+export function mapKeyLastByFn<TKey, TElement, TNewElement>(
+    f: (element: TElement, index: number) => readonly [TKey, TNewElement]
+): (array: ArrayLike<TElement>) => Map<TKey, TNewElement> {
+    return array => mapKeyLastBy(array, f);
+}
+
 export interface IdentityGrouping<T> {
     readonly identity: (element: T) => unknown;
 }