feat(transducers): add mapcatIndexed() xform

packages/transducers/src/index.ts

@@ -64,6 +64,7 @@ export * from "./xform/map-nth";
 export * from "./xform/map-vals";
 export * from "./xform/map";
 export * from "./xform/mapcat";
+export * from "./xform/mapcat-indexed";
 export * from "./xform/match-first";
 export * from "./xform/match-last";
 export * from "./xform/moving-average";

packages/transducers/src/xform/mapcat-indexed.ts

@@ -0,0 +1,37 @@
+import type { Fn2 } from "@thi.ng/api";
+import type { Transducer } from "../api";
+import { comp } from "../func/comp";
+import { $iter, iterator } from "../iterator";
+import { cat } from "./cat";
+import { mapIndexed } from "./map-indexed";
+
+/**
+ * Transducer. Similar to {@link (mapcat:1)}, but given `fn` takes two
+ * arguments: `index` and `value` to transform.
+ *
+ * @remarks
+ * An optional start index `offset` can be provided (default 0). Also see
+ * {@link (mapIndexed:1)}.
+ *
+ * @param fn - transformation function
+ * @param offset - initial index
+ */
+export function mapcatIndexed<A, B>(
+    fn: Fn2<number, A, Iterable<B> | null | undefined>,
+    offset?: number
+): Transducer<A, B>;
+export function mapcatIndexed<A, B>(
+    fn: Fn2<number, A, Iterable<B> | null | undefined>,
+    src: Iterable<A>
+): IterableIterator<B>;
+export function mapcatIndexed<A, B>(
+    fn: Fn2<number, A, Iterable<B> | null | undefined>,
+    offset: number,
+    src: Iterable<A>
+): IterableIterator<B>;
+export function mapcatIndexed<A, B>(...args: any[]): any {
+    return (
+        $iter(mapcatIndexed, args, iterator) ||
+        comp(mapIndexed<A, Iterable<B>>(args[0], args[1]), cat())
+    );
+}