Refactor 4 unexported set-operation functions

combine.go

@@ -13,8 +13,8 @@ func combine[E Elem[E]](
 	x = sets[0]
 	xIsSets0 := true
 
-	for _, set := range sets[1:] {
-		y = op(x, set, y)
+	for _, s := range sets[1:] {
+		y = op(y, x, s)
 
 		if xIsSets0 {
 			x = nil

difference.go

@@ -4,17 +4,19 @@ import "sort"
 
 // Difference returns the set of elements that are in x, but not in y.
 func (x Set[E]) Difference(y Set[E]) Set[E] {
-	return difference(x, y, nil)
+	return differenceInto(nil, x, y)
 }
 
 // Difference returns the set of elements that are in sets[0], but not in
 // any of sets[1:]. If sets is empty, Difference returns an empty set.
 func Difference[E Elem[E]](sets ...Set[E]) Set[E] {
-	return combine(difference, sets...)
+	return combine(differenceInto, sets...)
 }
 
-func difference[E Elem[E]](x, y, out Set[E]) Set[E] {
-	z := out[:0]
+// differenceInto returns the set of elements that are in x, but not in y,
+// overwriting z.
+func differenceInto[E Elem[E]](z, x, y Set[E]) Set[E] {
+	z = z[:0]
 
 	inv := false
 

intersection.go

@@ -4,16 +4,18 @@ import "sort"
 
 // Intersection returns the set of elements that are in both x and y.
 func (x Set[E]) Intersection(y Set[E]) Set[E] {
-	return intersection(x, y, nil)
+	return intersectionInto(nil, x, y)
 }
 
 // Intersection returns the set of elements that are in each of sets.
 func Intersection[E Elem[E]](sets ...Set[E]) Set[E] {
-	return combine(intersection, sets...)
+	return combine(intersectionInto, sets...)
 }
 
-func intersection[E Elem[E]](x, y, out Set[E]) Set[E] {
-	z := out[:0]
+// intersectionInto returns the set of elements that are in both x and y,
+// overwriting z.
+func intersectionInto[E Elem[E]](z, x, y Set[E]) Set[E] {
+	z = z[:0]
 
 	for {
 		if len(x) < len(y) {

symdiff.go

@@ -5,17 +5,19 @@ import "sort"
 // SymmetricDifference returns the set of elements that are in one of x and y,
 // but not in both.
 func (x Set[E]) SymmetricDifference(y Set[E]) Set[E] {
-	return symmetricDifference(x, y, nil)
+	return symmetricDifferenceInto(nil, x, y)
 }
 
 // SymmetricDifference returns the set of elements that are in an odd number
 // of sets.
 func SymmetricDifference[E Elem[E]](sets ...Set[E]) Set[E] {
-	return combine(symmetricDifference, sets...)
+	return combine(symmetricDifferenceInto, sets...)
 }
 
-func symmetricDifference[E Elem[E]](x, y, out Set[E]) Set[E] {
-	z := out[:0]
+// symmetricDifferenceInto returns the set of elements that are in one of x and
+// y, but not in both, overwriting z.
+func symmetricDifferenceInto[E Elem[E]](z, x, y Set[E]) Set[E] {
+	z = z[:0]
 
 	for {
 		if len(x) < len(y) {

union.go

@@ -4,16 +4,18 @@ import "sort"
 
 // Union returns the set of elements that are in either x, or y, or both.
 func (x Set[E]) Union(y Set[E]) Set[E] {
-	return union(x, y, nil)
+	return unionInto(nil, x, y)
 }
 
 // Union returns the set of elements that are in any of sets.
 func Union[E Elem[E]](sets ...Set[E]) Set[E] {
-	return combine(union, sets...)
+	return combine(unionInto, sets...)
 }
 
-func union[E Elem[E]](x, y, out Set[E]) Set[E] {
-	z := out[:0]
+// unionInto returns the set of elements that are in either x, or y, or both,
+// overwriting z.
+func unionInto[E Elem[E]](z, x, y Set[E]) Set[E] {
+	z = z[:0]
 
 	for {
 		if len(x) < len(y) {