feat(pairwise): add pairwise function

Author: djcsdy

index.test.ts

@@ -40,6 +40,7 @@ import {
     notEmpty,
     only,
     or,
+    pairwise,
     prefixMatch,
     prepend,
     product,
@@ -394,6 +395,18 @@ test("scan1", async t => {
     t.deepEqual(await toArray(scan1(asyncIterable([1, 2, 3]), (a, e, i) => a + e * i)), [1, 3, 9]);
 });
 
+test("pairwise", async t => {
+    t.deepEqual(await toArray(pairwise(asyncIterable([]))), []);
+    t.deepEqual(await toArray(pairwise(asyncIterable([1]))), []);
+    t.deepEqual(await toArray(pairwise(asyncIterable([1, 2]))), [[1, 2]]);
+    t.deepEqual(await toArray(pairwise(asyncIterable([1, 2, 3, 4, 5]))), [
+        [1, 2],
+        [2, 3],
+        [3, 4],
+        [4, 5]
+    ]);
+});
+
 test("zip", async t => {
     t.deepEqual(await toArray(zip(asyncIterable([1, 2, 3]), asyncIterable([6, 5, 4, 3, 2, 1]))), [
         [1, 6],

index.ts

@@ -1028,6 +1028,22 @@ export function scan1Fn<T>(
     return iterable => scan1(iterable, f);
 }
 
+export async function* pairwise<T>(iterable: AsyncIterableLike<T>): AsyncIterable<readonly [T, T]> {
+    const iterator = asyncIterator(iterable);
+    let prev = await iterator.next();
+
+    if (prev.done === true) {
+        return;
+    }
+
+    let element = await iterator.next();
+    while (element.done !== true) {
+        yield [prev.value, element.value];
+        prev = element;
+        element = await iterator.next();
+    }
+}
+
 export async function* zip<T, U>(
     a: AsyncIterableLike<T>,
     b: AsyncIterableLike<U>