add slice filter fn (#193)

slice/filter.go

@@ -0,0 +1,14 @@
+package slice
+
+// Filter returns a sub-slice with all the elements that satisfy the fn condition.
+func Filter[T any](s []T, fn func(T) bool) []T {
+	filtered := make([]T, 0, len(s))
+
+	for _, el := range s {
+		if fn(el) {
+			filtered = append(filtered, el)
+		}
+	}
+
+	return filtered
+}

slice/filter_test.go

@@ -0,0 +1,53 @@
+package slice
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFilter(t *testing.T) {
+	t.Run("logic", func(t *testing.T) {
+		assert.Equal(t,
+			[]string{"a", "c"},
+			Filter(
+				[]string{"a", "b", "c"},
+				func(s string) bool { return s != "b" },
+			))
+		assert.Equal(t,
+			[]int{1, 1},
+			Filter(
+				[]int{1, 10, 100, 1000, 100, 10, 1},
+				func(i int) bool { return i < 10 },
+			))
+
+		type item struct {
+			price      int
+			condition  string
+			onDiscount bool
+		}
+
+		assert.Len(t,
+			Filter(
+				[]item{
+					{
+						price:      115,
+						condition:  "new",
+						onDiscount: true,
+					},
+					{
+						price:      225,
+						condition:  "used",
+						onDiscount: false,
+					},
+					{
+						price:      335,
+						condition:  "mint",
+						onDiscount: true,
+					},
+				},
+				func(i item) bool { return i.onDiscount && i.condition != "used" && i.price < 300 },
+			), 1,
+		)
+	})
+}