From 5acf4a46716b8d7074d25a692059d48f4bccaf29 Mon Sep 17 00:00:00 2001
From: Ruidy <rnemausat@newstore.com>
Date: Wed, 20 Apr 2022 15:10:40 -0400
Subject: [PATCH] add groupBy function

---
 groupby.go      | 15 +++++++++++++++
 groupby_test.go | 22 ++++++++++++++++++++++
 2 files changed, 37 insertions(+)
 create mode 100644 groupby.go
 create mode 100644 groupby_test.go

diff --git a/groupby.go b/groupby.go
new file mode 100644
index 0000000..e4a554b
--- /dev/null
+++ b/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
+}
diff --git a/groupby_test.go b/groupby_test.go
new file mode 100644
index 0000000..ef4d4e0
--- /dev/null
+++ b/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))
+}