From 7519463345537fad3299bfc1e4c777beeadcbcd1 Mon Sep 17 00:00:00 2001
From: gram <git@orsinium.dev>
Date: Fri, 17 Nov 2023 10:31:41 +0100
Subject: [PATCH] Fix Product2 for 0 and 1 inputs

---
 slices/slices.go      | 19 +++++++++++--------
 slices/slices_test.go |  2 ++
 2 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/slices/slices.go b/slices/slices.go
index b101e0c..eedc3c6 100644
--- a/slices/slices.go
+++ b/slices/slices.go
@@ -63,6 +63,10 @@ func Intersect[S1 ~[]T, S2 ~[]T, T comparable](items1 S1, items2 S2) []T {
 // Product2 returns the cartesian product of elements in the given slices.
 func Product2[T any](items ...[]T) chan []T {
 	c := make(chan []T, 1)
+	if len(items) == 0 {
+		close(c)
+		return c
+	}
 	go product2(items, c, []T{}, 0)
 	return c
 }
@@ -77,14 +81,13 @@ func product2[T any](items [][]T, c chan []T, left []T, pos int) {
 			result = append(result, el)
 			c <- result
 		}
-		return
-	}
-
-	for _, el := range items[pos] {
-		result := make([]T, 0, len(left)+1)
-		result = append(result, left...)
-		result = append(result, el)
-		product2(items, c, result, pos+1)
+	} else {
+		for _, el := range items[pos] {
+			result := make([]T, 0, len(left)+1)
+			result = append(result, left...)
+			result = append(result, el)
+			product2(items, c, result, pos+1)
+		}
 	}
 
 	if pos == 0 {
diff --git a/slices/slices_test.go b/slices/slices_test.go
index ca1c341..12e316c 100644
--- a/slices/slices_test.go
+++ b/slices/slices_test.go
@@ -71,6 +71,8 @@ func TestProduct2(t *testing.T) {
 	}
 	f([][]int{{1, 2}, {3, 4}}, [][]int{{1, 3}, {1, 4}, {2, 3}, {2, 4}})
 	f([][]int{{1, 2}, {3}, {4, 5}}, [][]int{{1, 3, 4}, {1, 3, 5}, {2, 3, 4}, {2, 3, 5}})
+	f([][]int{{1, 2}}, [][]int{{1}, {2}})
+	f([][]int{}, [][]int{})
 }
 
 func TestUnion(t *testing.T) {