sort: add Reverse as a function

This updates: https://golang.org/cl/6909059/
Fixes #4511.

R=minux.ma, rsc
CC=golang-dev
https://golang.org/cl/6932054

src/pkg/sort/example_test.go

@@ -15,3 +15,10 @@ func ExampleInts() {
 	fmt.Println(s)
 	// Output: [1 2 3 4 5 6]
 }
+
+func ExampleReverse() {
+	s := []int{5, 2, 6, 3, 1, 4} // unsorted
+	sort.Sort(sort.Reverse(sort.IntSlice(s)))
+	fmt.Println(s)
+	// Output: [6 5 4 3 2 1]
+}

src/pkg/sort/sort.go

@@ -197,6 +197,22 @@ func Sort(data Interface) {
 	quickSort(data, 0, n, maxDepth)
 }
 
+type reverse struct {
+	// This embedded Interface permits Reverse to use the methods of
+	// another Interface implementation.
+	Interface
+}
+
+// Less returns the opposite of the embedded implementation's Less method.
+func (r reverse) Less(i, j int) bool {
+	return r.Interface.Less(j, i)
+}
+
+// Reverse returns the reverse order for data.
+func Reverse(data Interface) Interface {
+	return &reverse{data}
+}
+
 // IsSorted reports whether data is sorted.
 func IsSorted(data Interface) bool {
 	n := data.Len()

src/pkg/sort/sort_test.go

@@ -92,6 +92,23 @@ func TestSortLarge_Random(t *testing.T) {
 	}
 }
 
+func TestReverseSortIntSlice(t *testing.T) {
+	data := ints
+	data1 := ints
+	a := IntSlice(data[0:])
+	Sort(a)
+	r := IntSlice(data1[0:])
+	Sort(Reverse(r))
+	for i := 0; i < len(data); i++ {
+		if a[i] != r[len(data)-1-i] {
+			t.Errorf("reverse sort didn't sort")
+		}
+		if i > len(data)/2 {
+			break
+		}
+	}
+}
+
 func BenchmarkSortString1K(b *testing.B) {
 	b.StopTimer()
 	for i := 0; i < b.N; i++ {