sort: add interface examples

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/5677060

src/pkg/sort/example_interface_test.go

+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sort_test
+
+import (
+	"fmt"
+	"sort"
+)
+
+type Grams int
+
+func (g Grams) String() string { return fmt.Sprintf("%dg", int(g)) }
+
+type Organ struct {
+	Name   string
+	Weight Grams
+}
+
+type Organs []*Organ
+
+func (s Organs) Len() int      { return len(s) }
+func (s Organs) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
+
+// ByName implements sort.Interface by providing Less and using the Len and
+// Swap methods of the embedded Organs value.
+type ByName struct{ Organs }
+
+func (s ByName) Less(i, j int) bool { return s.Organs[i].Name < s.Organs[j].Name }
+
+// ByWeight implements sort.Interface by providing Less and using the Len and
+// Swap methods of the embedded Organs value.
+type ByWeight struct{ Organs }
+
+func (s ByWeight) Less(i, j int) bool { return s.Organs[i].Weight < s.Organs[j].Weight }
+
+func ExampleInterface() {
+	s := []*Organ{
+		{"brain", 1340},
+		{"heart", 290},
+		{"liver", 1494},
+		{"pancreas", 131},
+		{"prostate", 62},
+		{"spleen", 162},
+	}
+
+	sort.Sort(ByWeight{s})
+	fmt.Println("Organs by weight:")
+	printOrgans(s)
+
+	sort.Sort(ByName{s})
+	fmt.Println("Organs by name:")
+	printOrgans(s)
+
+	// Output:
+	// Organs by weight:
+	// prostate (62g)
+	// pancreas (131g)
+	// spleen   (162g)
+	// heart    (290g)
+	// brain    (1340g)
+	// liver    (1494g)
+	// Organs by name:
+	// brain    (1340g)
+	// heart    (290g)
+	// liver    (1494g)
+	// pancreas (131g)
+	// prostate (62g)
+	// spleen   (162g)
+}
+
+func printOrgans(s []*Organ) {
+	for _, o := range s {
+		fmt.Printf("%-8s (%v)\n", o.Name, o.Weight)
+	}
+}

src/pkg/sort/example_reverse_test.go

+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sort_test
+
+import (
+	"fmt"
+	"sort"
+)
+
+// Reverse embeds a sort.Interface value and implements a reverse sort over
+// that value.
+type Reverse struct {
+	// This embedded Interface permits Reverse to use the methods of
+	// another Interface implementation.
+	sort.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)
+}
+
+func ExampleInterface_reverse() {
+	s := []int{5, 2, 6, 3, 1, 4} // unsorted
+	sort.Sort(Reverse{sort.IntSlice(s)})
+	fmt.Println(s)
+	// Output: [6 5 4 3 2 1]
+}