From 9d83255a6be245260da58fdcfcfd95e8fd00cb05 Mon Sep 17 00:00:00 2001
From: Karsten Schmidt <k@postspectacular.com>
Date: Wed, 22 Aug 2018 23:22:35 +0100
Subject: [PATCH] perf(transducers): add IReducible, update reduce()

- add IReducible interface for custom/optimized iteration
  and source value provision for reduce()
- add array-like check to reduce() and switch to version without
  forcing array iterator (`for..of..`)
---
 packages/transducers/src/api.ts      |  4 ++++
 packages/transducers/src/iterator.ts |  3 ++-
 packages/transducers/src/reduce.ts   | 30 +++++++++++++++++++++-------
 3 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/packages/transducers/src/api.ts b/packages/transducers/src/api.ts
index 819fa24302..83fed71217 100644
--- a/packages/transducers/src/api.ts
+++ b/packages/transducers/src/api.ts
@@ -12,6 +12,10 @@ export interface Reducer<A, B> extends Array<any> {
     [2]: ReductionFn<A, B>;
 };
 
+export interface IReducible<A, B> {
+    $reduce(rfn: ReductionFn<A, B>, acc: A): A | Reduced<A>;
+}
+
 export type TransformFn = (x: any) => any;
 export type TransformSubSpec = IObjectOf<TransformSpec | TransformFn>;
 export interface TransformSpec extends Array<any> {
diff --git a/packages/transducers/src/iterator.ts b/packages/transducers/src/iterator.ts
index 50b0aaac59..7768f05830 100644
--- a/packages/transducers/src/iterator.ts
+++ b/packages/transducers/src/iterator.ts
@@ -28,7 +28,8 @@ export function* iterator<A, B>(xform: Transducer<A, B>, xs: Iterable<A>): Itera
 }
 
 /**
- * Optimized version of `iterator()` for transducers which are guaranteed to:
+ * Optimized version of `iterator()` for transducers which are
+ * guaranteed to:
  *
  * 1) Only produce none or a single result per input
  * 2) Do not require a `completion` reduction step
diff --git a/packages/transducers/src/reduce.ts b/packages/transducers/src/reduce.ts
index a20795bfa3..dc5999beb5 100644
--- a/packages/transducers/src/reduce.ts
+++ b/packages/transducers/src/reduce.ts
@@ -1,12 +1,16 @@
+import { implementsFunction } from "@thi.ng/checks/implements-function";
+import { isArrayLike } from "@thi.ng/checks/is-arraylike";
 import { illegalArity } from "@thi.ng/errors/illegal-arity";
 
-import { Reducer } from "./api";
+import { Reducer, IReducible } from "./api";
 import { isReduced, unreduced, Reduced } from "./reduced";
 
 export function reduce<A, B>(rfn: Reducer<A, B>, xs: Iterable<B>): A;
 export function reduce<A, B>(rfn: Reducer<A, B>, acc: A, xs: Iterable<B>): A;
+export function reduce<A, B>(rfn: Reducer<A, B>, xs: IReducible<A, B>): A;
+export function reduce<A, B>(rfn: Reducer<A, B>, acc: A, xs: IReducible<A, B>): A;
 export function reduce<A, B>(...args: any[]): A {
-    let acc: A, xs: Iterable<B>;
+    let acc: A, xs: Iterable<B> | IReducible<A, B>;
     switch (args.length) {
         case 3:
             xs = args[2];
@@ -20,11 +24,23 @@ export function reduce<A, B>(...args: any[]): A {
     }
     const [init, complete, reduce] = args[0];
     acc = acc == null ? init() : acc;
-    for (let x of xs) {
-        acc = <any>reduce(acc, x);
-        if (isReduced(acc)) {
-            acc = (<any>acc).deref();
-            break;
+    if (implementsFunction(xs, "$reduce")) {
+        acc = <any>(<IReducible<A, B>>xs).$reduce(reduce, acc);
+    } else if (isArrayLike(xs)) {
+        for (let i = 0, n = xs.length; i < n; i++) {
+            acc = <any>reduce(acc, xs[i]);
+            if (isReduced(acc)) {
+                acc = (<any>acc).deref();
+                break;
+            }
+        }
+    } else {
+        for (let x of <Iterable<B>>xs) {
+            acc = <any>reduce(acc, x);
+            if (isReduced(acc)) {
+                acc = (<any>acc).deref();
+                break;
+            }
         }
     }
     return unreduced(complete(acc));