feat(asyncScan1Once): add asyncScan1Once function

Author: djcsdy

index.test.ts

@@ -42,6 +42,7 @@ import {
     asyncPushOnce,
     asyncRemoveFirstOnce,
     asyncRemoveOnce,
+    asyncScan1Once,
     asyncScanOnce,
     asyncSliceOnce,
     asyncSumOnce,
@@ -603,3 +604,10 @@ test("asyncScanOnce", async t => {
         ["_ 0 a", "_ 0 a 1 b", "_ 0 a 1 b 2 c"]
     );
 });
+
+test("asyncScan1Once", async t => {
+    t.deepEqual(
+        await asyncToArrayOnce(asyncScan1Once(asyncIterator([1, 2, 3]), (a, e, i) => a + e * i)),
+        [1, 3, 9]
+    );
+});

index.ts

@@ -1101,3 +1101,41 @@ export function asyncScanOnceFn<T, U>(
 ): (iterator: AsyncIteratorLike<T>) => AsyncIterator<U> {
     return iterator => asyncScanOnce(iterator, f, initial);
 }
+
+export function asyncScan1Once<T>(
+    iterator: AsyncIteratorLike<T>,
+    f: (accumulator: T, element: T, index: number) => T | Promise<T>
+): AsyncIterator<T> {
+    const it = asyncIterator(iterator);
+    let i = 1;
+    const first = async (): Promise<IteratorResult<T>> => {
+        const element = await it.next();
+        if (element.done === true) {
+            next = after;
+            return after();
+        } else {
+            next = during(element.value);
+            return {value: element.value};
+        }
+    };
+    const during = (accumulator: T) => async (): Promise<IteratorResult<T>> => {
+        const element = await it.next();
+        if (element.done === true) {
+            next = after;
+            return after();
+        } else {
+            const value = await f(accumulator, element.value, i++);
+            next = during(value);
+            return {value};
+        }
+    };
+    const after = async (): Promise<IteratorResult<T>> => ({done: true, value: undefined});
+    let next = first;
+    return {next: async () => next()};
+}
+
+export function asyncScan1OnceFn<T>(
+    f: (accumulator: T, element: T, index: number) => T | Promise<T>
+): (iterator: AsyncIteratorLike<T>) => AsyncIterator<T> {
+    return iterator => asyncScan1Once(iterator, f);
+}