feat(asyncFoldOnce): add asyncFoldOnce function

Author: djcsdy

index.test.ts

@@ -10,6 +10,7 @@ import {
     asyncExcludeNullOnce,
     asyncExcludeOnce,
     asyncFilterOnce,
+    asyncFoldOnce,
     asyncInitialOnce,
     asyncIterator,
     asyncLastOnce,
@@ -405,3 +406,7 @@ test("asyncRemoveFirstOnce", async t => {
         [1, 2, 4, 3, 2, 1]
     );
 });
+
+test("asyncFoldOnce", async t => {
+    t.is(await asyncFoldOnce(asyncIterator([1, 2, 3]), (a, e, i) => a + e * i, 0), 8);
+});

index.ts

@@ -624,3 +624,27 @@ export function asyncRemoveFirstOnceFn<T>(
 ): (iterator: AsyncIteratorLike<T>) => AsyncIterator<T> {
     return iterator => asyncRemoveFirstOnce(iterator, value);
 }
+
+export async function asyncFoldOnce<T, U>(
+    iterator: AsyncIteratorLike<T>,
+    f: (accumulator: U, element: T, index: number) => U | Promise<U>,
+    initial: U | Promise<U>
+): Promise<U> {
+    const it = asyncIterator(iterator);
+    let [element, accumulator] = await Promise.all([it.next(), initial] as const);
+    let i = 0;
+    while (element.done !== true) {
+        [accumulator, element] = await Promise.all([
+            f(accumulator, element.value, i++),
+            it.next()
+        ] as const);
+    }
+    return accumulator;
+}
+
+export function asyncFoldOnceFn<T, U>(
+    f: (accumulator: U, element: T, index: number) => U | Promise<U>,
+    initial: U | Promise<U>
+): (iterator: AsyncIteratorLike<T>) => Promise<U> {
+    return async iterator => asyncFoldOnce(iterator, f, initial);
+}