package main

import (
    "fmt"
    "sort"
)

// A couple of type definitions to make the units clear.
type earthMass float64
type au float64

// A Planet defines the properties of a solar system object.
type Planet struct {
    name     string
    mass     earthMass
    distance au
}

// By is the type of a "less" function that defines the ordering of its Planet arguments.
 

type By func(p1, p2 *Planet) bool

// Sort is a method on the function type, By, that sorts the argument slice according to the function.
func (by By) Sort(planets []Planet) {
    ps := &planetSorter{
        planets: planets,
        by:      by, // The Sort method's receiver is the function (closure) that defines the sort order.
 

    }
    sort.Sort(ps)
}

// planetSorter joins a By function and a slice of Planets to be sorted.
type planetSorter struct {
    planets []Planet
    by      func(p1, p2 *Planet) bool // Closure used in the Less method.
}

// Len is part of sort.Interface.
func (s *planetSorter) Len() int {
    return len(s.planets)
}

 
// Swap is part of sort.Interface.
func (s *planetSorter) Swap(i, j int) {
    s.planets[i], s.planets[j] = s.planets[j], s.planets[i]
}

// Less is part of sort.Interface. It is implemented by calling the "by" closure in the sorter.
func (s *planetSorter) Less(i, j int) bool {
    return s.by(&s.planets[i], &s.planets[j])
}

var planets = []Planet{
    {"Mercury", 0.055, 0.4},
    {"Venus", 0.815, 0.7},
    {"Earth", 1.0, 1.0},
    {"Mars", 0.107, 1.5},
}

// ExampleSortKeys demonstrates a technique for sorting a struct type using programmable sort criteria.
func main() {
    // Closures that order the Planet structure.
    name := func(p1, p2 *Planet) bool {
        return p1.name < p2.name
    }
    mass := func(p1, p2 *Planet) bool {
        return p1.mass < p2.mass
    }
    distance := func(p1, p2 *Planet) bool {
        return p1.distance < p2.distance
    }
    decreasingDistance := func(p1, p2 *Planet) bool {
        return distance(p2, p1)
    }

    // Sort the planets by the various criteria.
    By(name).Sort(planets)
    fmt.Println("By name:", planets)

    By(mass).Sort(planets)
    fmt.Println("By mass:", planets)

    By(distance).Sort(planets)
    fmt.Println("By distance:", planets)

    By(decreasingDistance).Sort(planets)
    fmt.Println("By decreasing distance:", planets)

}

這段程式碼的功能無所謂，這段程式碼揭示瞭如何巧妙地設計幾個排序函式，讓它和系統的sort函式結合起來使用。

首先，sort.Sort(sortFunc)，這裡使用了interface{}，只需要sortFunc來定義自己的實現；

sortFunc，需要定義Len, Swap, Less這三個函式

其中，Swap這個函式可以複用，Len這個函式也可以複用，但是Less這個函式需要自定義

不同的sortFunc，對應不同的Less。

其次，Less函式定義，不同的Less函式，對應不同的功能；

nameLess, massLess, distanceLes, decreasingDistanceLess，這表示是四個不同的Less函式；

LessContainer()可以將這些Less族函式包含進去，可以想象到LessContainer的用法：LessContainer(massLess).Sort(planet)

至於LessContainer具體的實現可以自己去實現。