Optimize cross.

Author: mbostock

.eslintrc.json

@@ -10,6 +10,7 @@
  "browser": true
  },
  "rules": {
- "no-cond-assign": 0
+ "no-cond-assign": 0,
+ "no-constant-condition": 0
  }
 }

src/cross.js

@@ -1,23 +1,28 @@
-function* product(head, ...rest) {
- if (rest.length) {
- const tail = rest.pop();
- for (const p of product(head, ...rest)) {
- for (const t of tail) {
- yield [...p, t];
- }
- }
- } else {
- for (const h of head) {
- yield [h];
- }
- }
+function length(array) {
+ return array.length;
+}
+
+function arrayify(values) {
+ return typeof values !== "object" || "length" in values ? values : Array.from(values);
 }
 
 function reducer(reduce) {
  return values => reduce(...values);
 }
 
 export default function cross(...values) {
- const reduce = typeof values[values.length - 1] === "function" ? reducer(values.pop()) : undefined;
- return Array.from(product(...values), reduce);
+ const reduce = typeof values[values.length - 1] === "function" && reducer(values.pop());
+ values = values.map(arrayify);
+ const lengths = values.map(length);
+ const j = values.length - 1;
+ const index = new Array(j + 1).fill(0);
+ const product = [];
+ while (true) {
+ product.push(index.map((j, i) => values[i][j]));
+ let i = j;
+ while (++index[i] === lengths[i]) {
+ if (i === 0) return reduce ? product.map(reduce) : product;
+ index[i--] = 0;
+ }
+ }
 }

test/cross-test.js

@@ -33,3 +33,12 @@ tape("cross(a, b, c, f) invokes the specified function for each triple", functio
  test.deepEqual(arrays.cross([1, 2], [3, 4], [5, 6, 7], (a, b, c) => a + b + c), [9, 10, 11, 10, 11, 12, 10, 11, 12, 11, 12, 13]);
  test.end();
 });
+
+tape("cross(a, b) returns Cartesian product a×b of generators", function(test) {
+ test.deepEqual(arrays.cross(generate(1, 2), generate("x", "y")), [[1, "x"], [1, "y"], [2, "x"], [2, "y"]]);
+ test.end();
+});
+
+function* generate(...values) {
+ yield* values;
+}