feat(Go-Tool)：2020/11/12:新增linq包使用示例(未完成)

vendor/github.com/ahmetalpbalkan/go-linq/groupby.go

@@ -0,0 +1,48 @@
+package linq
+
+// Group is a type that is used to store the result of GroupBy method.
+type Group struct {
+	Key   interface{}
+	Group []interface{}
+}
+
+// GroupBy method groups the elements of a collection according
+// to a specified key selector function and projects the elements for each group
+// by using a specified function.
+func (q Query) GroupBy(
+	keySelector func(interface{}) interface{},
+	elementSelector func(interface{}) interface{},
+) Query {
+
+	return Query{
+		func() Iterator {
+			next := q.Iterate()
+			set := make(map[interface{}][]interface{})
+
+			for item, ok := next(); ok; item, ok = next() {
+				key := keySelector(item)
+				set[key] = append(set[key], elementSelector(item))
+			}
+
+			len := len(set)
+			idx := 0
+			groups := make([]Group, len)
+			for k, v := range set {
+				groups[idx] = Group{k, v}
+				idx++
+			}
+
+			index := 0
+
+			return func() (item interface{}, ok bool) {
+				ok = index < len
+				if ok {
+					item = groups[index]
+					index++
+				}
+
+				return
+			}
+		},
+	}
+}