feat: Add stepBy

mod.ts

@@ -14,6 +14,7 @@ export * from "./src/product.ts";
 export * from "./src/range.ts";
 export * from "./src/reduce.ts";
 export * from "./src/skip.ts";
+export * from "./src/stepBy.ts";
 export * from "./src/sum.ts";
 export * from "./src/take.ts";
 export * from "./src/toArray.ts";

src/stepBy.ts

@@ -0,0 +1,45 @@
+export function stepBy<T>(n: number): (_: Iterator<T>) => Iterator<T> {
+  return (iter) => {
+    let isFirstTake = true;
+
+    return {
+      next() {
+        if (isFirstTake) {
+          isFirstTake = false;
+          return iter.next();
+        } else {
+          for (let i = 0; i < n - 1; i++) {
+            const result = iter.next();
+            if (result.done) return result;
+          }
+
+          return iter.next();
+        }
+      },
+    };
+  };
+}
+
+export function asyncStepBy<T>(
+  n: number,
+): (_: AsyncIterator<T>) => AsyncIterator<T> {
+  return (iter) => {
+    let isFirstTake = true;
+
+    return {
+      async next() {
+        if (isFirstTake) {
+          isFirstTake = false;
+          return iter.next();
+        } else {
+          for (let i = 0; i < n - 1; i++) {
+            const result = await iter.next();
+            if (result.done) return result;
+          }
+
+          return iter.next();
+        }
+      },
+    };
+  };
+}

src/stepBy_test.ts

@@ -0,0 +1,24 @@
+import { assertObjectMatch } from "../dev_deps.ts";
+import { toAsync } from "./toAsync.ts";
+
+import { asyncStepBy, stepBy } from "./stepBy.ts";
+
+Deno.test("skip", () => {
+  const a = [0, 1, 2, 3, 4, 5];
+
+  const iter = stepBy(2)(a.values());
+  assertObjectMatch(iter.next(), { value: 0 });
+  assertObjectMatch(iter.next(), { value: 2 });
+  assertObjectMatch(iter.next(), { value: 4 });
+  assertObjectMatch(iter.next(), { done: true });
+});
+
+Deno.test("asyncSkip", async () => {
+  const a = [0, 1, 2, 3, 4, 5];
+
+  const iter = asyncStepBy(2)(toAsync(a.values()));
+  assertObjectMatch(await iter.next(), { value: 0 });
+  assertObjectMatch(await iter.next(), { value: 2 });
+  assertObjectMatch(await iter.next(), { value: 4 });
+  assertObjectMatch(await iter.next(), { done: true });
+});