Update group function

README.md

@@ -194,6 +194,18 @@ valid := Some(list, func(item int) bool { item % 2 == 0 })
 valid := Some(list, func(item string) bool { len(item) < 20 }) 
 ```
 
+## Group
+
+![goterators-Group](https://user-images.githubusercontent.com/1828895/147878206-bef39880-96db-4269-b54e-2dcbb06f6bac.png)
+
+ - Group function group elements into nested list by group condition.
+
+```
+groups := Group(list, func(item Product) groupKey {
+   return item.ComposeGroupKey()
+}) // Group contains [ [ Product1, Product2, Product3 ], [ Product4, Product5 ] ]
+```
+
 ## Flat
 
 ![goterators-Flat](https://user-images.githubusercontent.com/1828895/147876403-25e84044-d761-45b7-b126-6ad8a7c5a4d1.png)

flat_test.go

@@ -32,7 +32,7 @@ func TestFlatNested(t *testing.T) {
 
 		for index := range expectedItems[groupIndex] {
 			if expectedItems[groupIndex][index] != actualItems[groupIndex][index] {
-				t.Errorf("Index %v, expected = %v, actual = %v", index, expectedItems[groupIndex][index], actualItems[groupIndex][index])
+				t.Errorf("Group index %v, Index %v, expected = %v, actual = %v", groupIndex, index, expectedItems[groupIndex][index], actualItems[groupIndex][index])
 			}
 		}
 	}

group.go

@@ -0,0 +1,10 @@
+package goterators
+
+func Group[T any, G comparable](source []T, buildGroup func(item T) G) [][]T {
+	output := map[G][]T{}
+	for index, item := range source {
+		group := buildGroup(item)
+		output[group] = append(output[group], source[index])
+	}
+	return output
+}

group_test.go

@@ -0,0 +1,60 @@
+package goterators
+
+import "testing"
+
+func TestGroup(t *testing.T) {
+	type Product struct {
+		productType string
+		productName string
+	}
+	testSource := []Product{{
+		productType: "food",
+		productName: "Coca-laco",
+	}, {
+		productType: "sport",
+		productName: "Old Balance",
+	}, {
+		productType: "sport",
+		productName: "Noke",
+	}, {
+		productType: "food",
+		productName: "Chip chip",
+	}}
+	expectedItems := [][]Product{
+		{
+			{
+				productType: "food",
+				productName: "Coca-laco",
+			}, {
+				productType: "food",
+				productName: "Chip chip",
+			},
+		},
+		{
+			{
+				productType: "sport",
+				productName: "Old Balance",
+			}, {
+				productType: "sport",
+				productName: "Noke",
+			},
+		},
+	}
+
+	actualItems := Group(testSource, func(item Product) string {
+		return item.productType
+	})
+	if len(expectedItems) != len(actualItems) {
+		t.Fatalf("Expected length = %v, actual length = %v", len(expectedItems), len(actualItems))
+	}
+	for groupIndex := range expectedItems {
+		if len(expectedItems[groupIndex]) != len(actualItems[groupIndex]) {
+			t.Fatalf("Group index %v, expected = %v, actual = %v", groupIndex, expectedItems[key], actualItems[key])
+		}
+		for index := range expectedItems[groupIndex] {
+			if expectedItems[groupIndex][index] != actualItems[groupIndex][index] {
+				t.Errorf("Group index %v, Index %v, expected = %v, actual = %v", groupIndex, index, expectedItems[key][index], actualItems[key][index])
+			}
+		}
+	}
+}