perf: preallocate result slices in Difference (#830)

Difference previously initialized left and right as empty slice
literals (Slice{}), causing repeated grow-and-copy allocations
as elements were appended.

Now preallocates with make(Slice, 0, len(listN)) to avoid
unnecessary reallocations.

benchmark/slice_benchmark_test.go

@@ -353,3 +353,25 @@ func BenchmarkFilterTakeVsFilterAndTake(b *testing.B) {
 		}
 	})
 }
+
+func BenchmarkDifference(b *testing.B) {
+	for _, n := range lengths {
+		ints1 := genSliceInt(n)
+		ints2 := genSliceInt(n)
+		b.Run(fmt.Sprintf("ints_%d", n), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				_, _ = lo.Difference(ints1, ints2)
+			}
+		})
+	}
+
+	for _, n := range lengths {
+		strs1 := genSliceString(n)
+		strs2 := genSliceString(n)
+		b.Run(fmt.Sprintf("strings_%d", n), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				_, _ = lo.Difference(strs1, strs2)
+			}
+		})
+	}
+}

intersect.go

@@ -207,8 +207,8 @@ func IntersectBy[T any, K comparable, Slice ~[]T](transform func(T) K, lists ...
 // The second value is the collection of elements absent from list1.
 // Play: https://go.dev/play/p/pKE-JgzqRpz
 func Difference[T comparable, Slice ~[]T](list1, list2 Slice) (Slice, Slice) {
-	left := Slice{}
-	right := Slice{}
+	left := make(Slice, 0, len(list1))
+	right := make(Slice, 0, len(list2))
 
 	seenLeft := Keyify(list1)
 	seenRight := Keyify(list2)