diff --git a/setutil_generic.go b/setutil_generic.go
index 9edcc902..9fcdd0b6 100644
--- a/setutil_generic.go
+++ b/setutil_generic.go
@@ -2,10 +2,13 @@
 
 package roaring
 
+
 func union2by2(set1 []uint16, set2 []uint16, buffer []uint16) int {
-	pos := 0
-	k1 := 0
-	k2 := 0
+	pos := uint(0)
+	k1 := uint(0)
+	k2 := uint(0)
+	len1 := uint(len(set1))
+	len2 := uint(len(set2))
 	if 0 == len(set2) {
 		buffer = buffer[:len(set1)]
 		copy(buffer, set1[:])
@@ -18,16 +21,16 @@ func union2by2(set1 []uint16, set2 []uint16, buffer []uint16) int {
 	}
 	s1 := set1[k1]
 	s2 := set2[k2]
-	buffer = buffer[:cap(buffer)]
-	for {
+	buffer = buffer[:len1 + len2]
+	for pos < uint(len(buffer)) {
 		if s1 < s2 {
 			buffer[pos] = s1
 			pos++
 			k1++
-			if k1 >= len(set1) {
+			if k1 >= len1 {
 				copy(buffer[pos:], set2[k2:])
-				pos += len(set2) - k2
-				break
+				pos += len2 - k2
+				return int(pos)
 			}
 			s1 = set1[k1]
 		} else if s1 == s2 {
@@ -35,14 +38,14 @@ func union2by2(set1 []uint16, set2 []uint16, buffer []uint16) int {
 			pos++
 			k1++
 			k2++
-			if k1 >= len(set1) {
+			if k1 >= len1 {
 				copy(buffer[pos:], set2[k2:])
-				pos += len(set2) - k2
+				pos += len2 - k2
 				break
 			}
-			if k2 >= len(set2) {
+			if k2 >= len2 {
 				copy(buffer[pos:], set1[k1:])
-				pos += len(set1) - k1
+				pos += len1 - k1
 				break
 			}
 			s1 = set1[k1]
@@ -51,13 +54,13 @@ func union2by2(set1 []uint16, set2 []uint16, buffer []uint16) int {
 			buffer[pos] = s2
 			pos++
 			k2++
-			if k2 >= len(set2) {
+			if k2 >= len2 {
 				copy(buffer[pos:], set1[k1:])
-				pos += len(set1) - k1
+				pos += len1 - k1
 				break
 			}
 			s2 = set2[k2]
 		}
 	}
-	return pos
+	return int(pos)
 }