refactor(transducers): update labeled(), mapIndexed(), partition()

- labeled() - add support for label fn instead of just static label
- mapIndexed() - add optional start index arg
- partition() - minor update in completing reducer

packages/transducers/src/xform/labeled.ts

@@ -1,6 +1,12 @@
+import { isFunction } from "@thi.ng/checks/is-function";
+
 import { Transducer } from "../api";
 import { map } from "./map";
 
-export function labeled<L, T>(id: L): Transducer<T, [L, T]> {
-    return map((x) => [id, x]);
+export function labeled<L, T>(id: L | ((x: T) => L)): Transducer<T, [L, T]> {
+    return map(
+        isFunction(id) ?
+            (x) => [id(x), x] :
+            (x) => [id, x]
+    );
 }

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

@@ -1,10 +1,10 @@
 import { Reducer, Transducer } from "../api";
 import { compR } from "../func/compr";
 
-export function mapIndexed<A, B>(fn: (i: number, x: A) => B): Transducer<A, B> {
+export function mapIndexed<A, B>(fn: (i: number, x: A) => B, offset = 0): Transducer<A, B> {
     return (rfn: Reducer<any, B>) => {
         const r = rfn[2];
-        let i = 0;
+        let i = offset;
         return compR(rfn,
             (acc, x: A) => r(acc, fn(i++, x)));
     };

packages/transducers/src/xform/partition.ts

@@ -19,7 +19,7 @@ export function partition<T>(...args: any[]): Transducer<T, T[]> {
         return [
             init,
             (acc) => {
-                if (buf.length && (all || buf.length === size)) {
+                if (all && buf.length > 0) {
                     acc = reduce(acc, buf);
                     buf = [];
                 }