feat(mapKeyFirstBy): add mapKeyFirstBy function

Author: djcsdy

index.test.ts

@@ -30,6 +30,7 @@ import {
     last,
     map,
     mapKeyBy,
+    mapKeyFirstBy,
     maximum,
     maximumBy,
     minimum,
@@ -435,3 +436,13 @@ test("mapKeyBy", async t => {
     t.deepEqual(map.get("odd"), ["1", "3", "5"]);
     t.deepEqual(Array.from(map.keys()), ["odd", "even"]);
 });
+
+test("mapKeyFirstBy", async t => {
+    const map = await mapKeyFirstBy(asyncIterable([1, 3, 4, 2, 5, 6]), e => [
+        e % 2 === 0 ? "even" : "odd",
+        String(e)
+    ]);
+    t.is(map.get("even"), "4");
+    t.is(map.get("odd"), "1");
+    t.deepEqual(Array.from(map.keys()), ["odd", "even"]);
+});

index.ts

@@ -1154,3 +1154,34 @@ export function mapKeyByFn<TKey, TElement, TNewElement>(
 }
 
 export const asyncMapKeyByFn = mapKeyByFn;
+
+export async function mapKeyFirstBy<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++);
+        if (!map.has(key)) {
+            map.set(key, value);
+        }
+    }
+    return map;
+}
+
+export const asyncMapKeyFirstBy = mapKeyFirstBy;
+
+export function mapKeyFirstByFn<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 => mapKeyFirstBy(iterable, f);
+}
+
+export const asyncMapKeyFirstByFn = mapKeyFirstByFn;