add groupBy function (#28)

groupby.go

@@ -0,0 +1,15 @@
+package underscore
+
+// GroupBy splits a slice into a map[K][]V grouped by the result of the iterator function.
+func GroupBy[K comparable, V any](values []V, f func(V) K) map[K][]V {
+	res := make(map[K][]V, 0)
+	for _, v := range values {
+		k := f(v)
+		if r, ok := res[k]; ok {
+			res[k] = append(r, v)
+		} else {
+			res[k] = []V{v}
+		}
+	}
+	return res
+}

groupby_test.go

@@ -0,0 +1,22 @@
+package underscore_test
+
+import (
+	"math"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	u "github.com/rjNemo/underscore"
+)
+
+func TestGroupBy(t *testing.T) {
+	nums := []float64{1.3, 2.1, 2.4}
+	want := map[int][]float64{
+		1: {1.3},
+		2: {2.1, 2.4},
+	}
+	f := func(n float64) int {
+		return int(math.Floor(n))
+	}
+	assert.Equal(t, want, u.GroupBy(nums, f))
+}