add flatmap

Signed-off-by: Ruidy <rnemausat@newstore.com>

docs/content/collections/flatmap.md

@@ -0,0 +1,23 @@
+---
+title: "Flatmap"
+date: 2022-08-10T16:49:56+02:00
+draft: false
+---
+
+Flatmap flattens the input slice element into the new slice. FlatMap maps every element with the help of a mapper function, then flattens the input slice element into the new slice.
+
+```go
+package main
+
+import (
+	"fmt"
+	u "github.com/rjNemo/underscore"
+)
+
+func main() {
+  nums := []int{1, 2, 3, 4}
+  mapper := func(n int) []int { return []int{(n - 1) * n, (n) * n} }
+  res := u.Flatmap(nums, mapper)
+  fmt.Println(res) // {0, 1, 2, 4, 6, 9, 12, 16}
+}
+```

flatmap.go

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+package underscore
+
+// Flatmap flatten the input slice element into the new slice. FlatMap maps every element with the help of a mapper function, then flattens the input slice element into the new slice.
+func Flatmap[T any](values []T, mapper func(n T) []T) []T {
+	res := make([]T, 0)
+	for _, v := range values {
+		vs := mapper(v)
+		res = append(res, vs...)
+	}
+	return res
+}

flatmap_test.go

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+package underscore_test
+
+import (
+	"testing"
+
+	u "github.com/rjNemo/underscore"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFlatmap(t *testing.T) {
+	nums := []int{1, 2, 3, 4}
+	transform := func(n int) []int { return []int{(n - 1) * n, (n) * n} }
+	want := []int{0, 1, 2, 4, 6, 9, 12, 16}
+
+	assert.Equal(t, want, u.Flatmap(nums, transform))
+}