feat(transducers): add length() transducer

thi-ng · Aug 20, 2023 · 47a95b7 · 47a95b7
1 parent 755ca21
commit 47a95b7
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diff --git a/packages/transducers/src/index.ts b/packages/transducers/src/index.ts
@@ -79,6 +79,7 @@ export * from "./interpolate-linear.js";
 export * from "./interpose.js";
 export * from "./keep.js";
 export * from "./labeled.js";
+export * from "./length.js";
 export * from "./map-deep.js";
 export * from "./map-indexed.js";
 export * from "./map-keys.js";

diff --git a/packages/transducers/src/length.ts b/packages/transducers/src/length.ts
@@ -0,0 +1,33 @@
+import { isIterable } from "@thi.ng/checks/is-iterable";
+import type { Transducer } from "./api.js";
+import { iterator1 } from "./iterator.js";
+import { map } from "./map.js";
+import type { ILength } from "@thi.ng/api";
+
+/**
+ * Similar to `map((x) => x.length)`. A transducer which returns the `.length`
+ * of each input (optionally with offset `n` added, default: 0) and yields
+ * sequence of these values.
+ *
+ * @example
+ * ```ts
+ * [...length(0, ["a", "bc", "def"])]
+ * // [1, 2, 3]
+ * ```
+ *
+ * @param n - optional offset
+ */
+export function length(n?: number): Transducer<ILength, number>;
+export function length(
+	n: number,
+	src: Iterable<ILength>
+): IterableIterator<number>;
+export function length(n = 0, src?: Iterable<ILength>): any {
+	return isIterable(src)
+		? iterator1(length(n), src)
+		: map(
+				n === 0
+					? (x: ILength) => x.length
+					: (x: ILength) => x.length + n
+		  );
+}