feat: add UnionBy for slice

slice/slice.go

@@ -594,7 +594,7 @@ func UniqueBy[T comparable](slice []T, iteratee func(item T) T) []T {
 	return Unique(result)
 }
 
-// Union creates a slice of unique values, in order, from all given slices. using == for equality comparisons.
+// Union creates a slice of unique values, in order, from all given slices.
 func Union[T comparable](slices ...[]T) []T {
 	result := []T{}
 	contain := map[T]struct{}{}
@@ -611,6 +611,24 @@ func Union[T comparable](slices ...[]T) []T {
 	return result
 }
 
+// UnionBy is like Union, what's more it accepts iteratee which is invoked for each element of each slice
+func UnionBy[T any, V comparable](predicate func(item T) V, slices ...[]T) []T {
+	result := []T{}
+	contain := map[V]struct{}{}
+
+	for _, slice := range slices {
+		for _, item := range slice {
+			val := predicate(item)
+			if _, ok := contain[val]; !ok {
+				contain[val] = struct{}{}
+				result = append(result, item)
+			}
+		}
+	}
+
+	return result
+}
+
 // Intersection creates a slice of unique values that included by all slices.
 func Intersection[T comparable](slices ...[]T) []T {
 	if len(slices) == 0 {

slice/slice_test.go

@@ -428,6 +428,17 @@ func TestUnion(t *testing.T) {
 	assert.Equal([]int{1, 3, 4, 6}, Union(s1))
 }
 
+func TestUnionBy(t *testing.T) {
+	assert := internal.NewAssert(t, "TestUnionBy")
+
+	testFunc := func(i int) int {
+		return i / 2
+	}
+
+	result := UnionBy(testFunc, []int{0, 1, 2, 3, 4, 5}, []int{0, 2, 10})
+	assert.Equal(result, []int{0, 2, 4, 10})
+}
+
 func TestIntersection(t *testing.T) {
 	s1 := []int{1, 2, 2, 3}
 	s2 := []int{1, 2, 3, 4}