1// Copyright 2013 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5package sort_test
6
7import (
8	"fmt"
9	"sort"
10)
11
12// A couple of type definitions to make the units clear.
13type earthMass float64
14type au float64
15
16// A Planet defines the properties of a solar system object.
17type Planet struct {
18	name     string
19	mass     earthMass
20	distance au
21}
22
23// By is the type of a "less" function that defines the ordering of its Planet arguments.
24type By func(p1, p2 *Planet) bool
25
26// Sort is a method on the function type, By, that sorts the argument slice according to the function.
27func (by By) Sort(planets []Planet) {
28	ps := &planetSorter{
29		planets: planets,
30		by:      by, // The Sort method's receiver is the function (closure) that defines the sort order.
31	}
32	sort.Sort(ps)
33}
34
35// planetSorter joins a By function and a slice of Planets to be sorted.
36type planetSorter struct {
37	planets []Planet
38	by      func(p1, p2 *Planet) bool // Closure used in the Less method.
39}
40
41// Len is part of sort.Interface.
42func (s *planetSorter) Len() int {
43	return len(s.planets)
44}
45
46// Swap is part of sort.Interface.
47func (s *planetSorter) Swap(i, j int) {
48	s.planets[i], s.planets[j] = s.planets[j], s.planets[i]
49}
50
51// Less is part of sort.Interface. It is implemented by calling the "by" closure in the sorter.
52func (s *planetSorter) Less(i, j int) bool {
53	return s.by(&s.planets[i], &s.planets[j])
54}
55
56var planets = []Planet{
57	{"Mercury", 0.055, 0.4},
58	{"Venus", 0.815, 0.7},
59	{"Earth", 1.0, 1.0},
60	{"Mars", 0.107, 1.5},
61}
62
63// ExampleSortKeys demonstrates a technique for sorting a struct type using programmable sort criteria.
64func Example_sortKeys() {
65	// Closures that order the Planet structure.
66	name := func(p1, p2 *Planet) bool {
67		return p1.name < p2.name
68	}
69	mass := func(p1, p2 *Planet) bool {
70		return p1.mass < p2.mass
71	}
72	distance := func(p1, p2 *Planet) bool {
73		return p1.distance < p2.distance
74	}
75	decreasingDistance := func(p1, p2 *Planet) bool {
76		return distance(p2, p1)
77	}
78
79	// Sort the planets by the various criteria.
80	By(name).Sort(planets)
81	fmt.Println("By name:", planets)
82
83	By(mass).Sort(planets)
84	fmt.Println("By mass:", planets)
85
86	By(distance).Sort(planets)
87	fmt.Println("By distance:", planets)
88
89	By(decreasingDistance).Sort(planets)
90	fmt.Println("By decreasing distance:", planets)
91
92	// Output: By name: [{Earth 1 1} {Mars 0.107 1.5} {Mercury 0.055 0.4} {Venus 0.815 0.7}]
93	// By mass: [{Mercury 0.055 0.4} {Mars 0.107 1.5} {Venus 0.815 0.7} {Earth 1 1}]
94	// By distance: [{Mercury 0.055 0.4} {Venus 0.815 0.7} {Earth 1 1} {Mars 0.107 1.5}]
95	// By decreasing distance: [{Mars 0.107 1.5} {Earth 1 1} {Venus 0.815 0.7} {Mercury 0.055 0.4}]
96}
97