feat: adding FilterReject (#472)

README.md

@@ -105,6 +105,7 @@ Supported helpers for slices:
 - [DropWhile](#dropwhile)
 - [DropRightWhile](#droprightwhile)
 - [Reject](#reject)
+- [FilterReject](#filterreject)
 - [Count](#count)
 - [CountBy](#countby)
 - [CountValues](#countvalues)
@@ -749,6 +750,18 @@ odd := lo.Reject([]int{1, 2, 3, 4}, func(x int, _ int) bool {
 
 [[play](https://go.dev/play/p/YkLMODy1WEL)]
 
+### FilterReject
+
+FilterReject mixes Filter and Reject, this method returns two slices, one for the elements of collection that predicate returns truthy for and one for the elements that predicate does not return truthy for.
+
+```go
+kept, rejected := lo.FilterReject([]int{1, 2, 3, 4}, func(x int, _ int) bool {
+    return x%2 == 0
+})
+// []int{2, 4}
+// []int{1, 3}
+```
+
 ### Count
 
 Counts the number of elements in the collection that compare equal to value.

slice.go

@@ -431,6 +431,23 @@ func Reject[V any](collection []V, predicate func(item V, index int) bool) []V {
 	return result
 }
 
+// FilterReject mixes Filter and Reject, this method returns two slices, one for the elements of collection that
+// predicate returns truthy for and one for the elements that predicate does not return truthy for.
+func FilterReject[V any](collection []V, predicate func(V, int) bool) (kept []V, rejected []V) {
+	kept = make([]V, 0, len(collection))
+	rejected = make([]V, 0, len(collection))
+
+	for i, item := range collection {
+		if predicate(item, i) {
+			kept = append(kept, item)
+		} else {
+			rejected = append(rejected, item)
+		}
+	}
+
+	return kept, rejected
+}
+
 // Count counts the number of elements in the collection that compare equal to value.
 // Play: https://go.dev/play/p/Y3FlK54yveC
 func Count[T comparable](collection []T, value T) (count int) {

slice_test.go

@@ -388,7 +388,7 @@ func TestAssociate(t *testing.T) {
 
 func TestSliceToMap(t *testing.T) {
 	t.Parallel()
-	
+
 	type foo struct {
 		baz string
 		bar int
@@ -500,6 +500,25 @@ func TestReject(t *testing.T) {
 	is.Equal(r2, []string{"foo", "bar"})
 }
 
+func TestFilterReject(t *testing.T) {
+	t.Parallel()
+	is := assert.New(t)
+
+	left1, right1 := FilterReject([]int{1, 2, 3, 4}, func(x int, _ int) bool {
+		return x%2 == 0
+	})
+
+	is.Equal(left1, []int{2, 4})
+	is.Equal(right1, []int{1, 3})
+
+	left2, right2 := FilterReject([]string{"Smith", "foo", "Domin", "bar", "Olivia"}, func(x string, _ int) bool {
+		return len(x) > 3
+	})
+
+	is.Equal(left2, []string{"Smith", "Domin", "Olivia"})
+	is.Equal(right2, []string{"foo", "bar"})
+}
+
 func TestCount(t *testing.T) {
 	t.Parallel()
 	is := assert.New(t)
@@ -626,7 +645,7 @@ func TestSlice(t *testing.T) {
 	out16 := Slice(in, -10, 1)
 	out17 := Slice(in, -1, 3)
 	out18 := Slice(in, -10, 7)
-	
+
 	is.Equal([]int{}, out1)
 	is.Equal([]int{0}, out2)
 	is.Equal([]int{0, 1, 2, 3, 4}, out3)