feat(mapKeyLastBy): add mapKeyLastBy function

Author: djcsdy

index.test.ts

@@ -31,6 +31,7 @@ import {
     map,
     mapKeyBy,
     mapKeyFirstBy,
+    mapKeyLastBy,
     maximum,
     maximumBy,
     minimum,
@@ -446,3 +447,14 @@ test("mapKeyFirstBy", async t => {
     t.is(map.get("odd"), "1");
     t.deepEqual(Array.from(map.keys()), ["odd", "even"]);
 });
+
+test("mapKeyLastBy", async t => {
+    const map = await mapKeyLastBy(asyncIterable([1, 3, 4, 2, 5, 6]), e => [
+        e % 2 === 0 ? "even" : "odd",
+        String(e)
+    ]);
+    t.deepEqual(Array.from(map.entries()), [
+        ["odd", "5"],
+        ["even", "6"]
+    ]);
+});

index.ts

@@ -1185,3 +1185,32 @@ export function mapKeyFirstByFn<TKey, TElement, TNewElement>(
 }
 
 export const asyncMapKeyFirstByFn = mapKeyFirstByFn;
+
+export async function mapKeyLastBy<TKey, TElement, TNewElement>(
+    iterable: AsyncIterableLike<TElement>,
+    f: (
+        element: TElement,
+        index: number
+    ) => readonly [TKey, TNewElement] | Promise<readonly [TKey, TNewElement]>
+): Promise<Map<TKey, TNewElement>> {
+    const map = new Map<TKey, TNewElement>();
+    let i = 0;
+    for await (const element of await iterable) {
+        const [key, value] = await f(element, i++);
+        map.set(key, value);
+    }
+    return map;
+}
+
+export const asyncMapKeyLastBy = mapKeyLastBy;
+
+export function mapKeyLastByFn<TKey, TElement, TNewElement>(
+    f: (
+        element: TElement,
+        index: number
+    ) => readonly [TKey, TNewElement] | Promise<readonly [TKey, TNewElement]>
+): (iterable: AsyncIterableLike<TElement>) => Promise<Map<TKey, TNewElement>> {
+    return async iterable => mapKeyLastBy(iterable, f);
+}
+
+export const asyncMapKeyLastByFn = mapKeyLastByFn;