refactor(transducers): extract common logic for add/mul/sub reducers

packages/transducers/src/rfn/add.ts

@@ -1,17 +1,12 @@
 import { Reducer } from "../api";
-import { $$reduce, reducer } from "../reduce";
+import { __mathop } from "./mathop";
 
 /**
- * Reducer to compute sum of values with given `init` value.
+ * Reducer to compute sum of values with given `init` value. Default: 0
  */
 export function add(init?: number): Reducer<number, number>;
 export function add(xs: Iterable<number>): number;
 export function add(init: number, xs: Iterable<number>): number;
 export function add(...args: any[]): any {
-    const res = $$reduce(add, args);
-    if (res !== undefined) {
-        return res;
-    }
-    const init = args[0] || 0;
-    return reducer(() => init, (acc, x: number) => acc + x);
+    return __mathop(add, (acc, x: number) => acc + x, 0, args);
 }

packages/transducers/src/rfn/mathop.ts

@@ -0,0 +1,17 @@
+import { FnAny } from "@thi.ng/api";
+import { Reducer, ReductionFn } from "../api";
+import { $$reduce, reducer } from "../reduce";
+
+export const __mathop = (
+    rfn: FnAny<Reducer<number, number>>,
+    fn: ReductionFn<number, number>,
+    initDefault: number,
+    args: any[]
+) => {
+    const res = $$reduce(rfn, args);
+    if (res !== undefined) {
+        return res;
+    }
+    const init = args[0] || initDefault;
+    return reducer(() => init, fn);
+};

packages/transducers/src/rfn/mul.ts

@@ -1,5 +1,5 @@
 import { Reducer } from "../api";
-import { $$reduce, reducer } from "../reduce";
+import { __mathop } from "./mathop";
 
 /**
  * Reducer to compute product of values with optional `init` value
@@ -9,10 +9,5 @@ export function mul(init?: number): Reducer<number, number>;
 export function mul(xs: Iterable<number>): number;
 export function mul(init: number, xs: Iterable<number>): number;
 export function mul(...args: any[]): any {
-    const res = $$reduce(mul, args);
-    if (res !== undefined) {
-        return res;
-    }
-    const init = args[0] || 1;
-    return reducer(() => init, (acc, x: number) => acc * x);
+    return __mathop(mul, (acc, x: number) => acc * x, 1, args);
 }

packages/transducers/src/rfn/sub.ts

@@ -1,5 +1,5 @@
 import { Reducer } from "../api";
-import { $$reduce, reducer } from "../reduce";
+import { __mathop } from "./mathop";
 
 /**
  * Reducer to successively subtract values from optional `init` value
@@ -9,10 +9,5 @@ export function sub(init?: number): Reducer<number, number>;
 export function sub(xs: Iterable<number>): number;
 export function sub(init: number, xs: Iterable<number>): number;
 export function sub(...args: any[]): any {
-    const res = $$reduce(sub, args);
-    if (res !== undefined) {
-        return res;
-    }
-    const init = args[0] || 0;
-    return reducer(() => init, (acc, x: number) => acc - x);
+    return __mathop(sub, (acc, x: number) => acc - x, 0, args);
 }