stat/combin: allow user allocation for Cartesian product

stat/combin/combin.go

@@ -341,8 +341,8 @@ func (g *CartesianGenerator) Next() bool {
 
 // Product generates one product of the cartesian set according to the current index which is increased by Next().
 // Next needs to be called at least one time before this method, otherwise it will panic.
-func (g *CartesianGenerator) Product() []int {
-	return SubFor(nil, g.idx, g.lens)
+func (g *CartesianGenerator) Product(dst []int) []int {
+	return SubFor(dst, g.idx, g.lens)
 }
 
 // IdxFor converts a multi-dimensional index into a linear index for a

stat/combin/combin_test.go

@@ -318,7 +318,7 @@ func TestCartesianGenerator(t *testing.T) {
 	gen := NewCartesianGenerator([]int{1, 2, 3})
 	iterations := 0
 	for gen.Next() {
-		got := gen.Product()
+		got := gen.Product(nil)
 		if !reflect.DeepEqual(got, want[iterations]) {
 			t.Errorf("Cartesian product does not match. want: %v got: %v", want[iterations], got)
 		}

stat/combin/combinations_example_test.go

@@ -39,7 +39,7 @@ func ExampleCartesianGenerator() {
 	// Now loop over all products.
 	var i int
 	for gen.Next() {
-		fmt.Println(i, gen.Product())
+		fmt.Println(i, gen.Product(nil))
 		i++
 	}