JavaScript中Array.sort()的底层实现及应用

1. V8 引擎的 array.js

  js中的sort()方法用于对数组元素进行排序，具体是如何实现的？查阅资料发现，V8 引擎 sort 函数只给出了两种排序 InsertionSort 和 QuickSort，数组长度小于等于 22 的用插入排序 InsertionSort，比22大的数组则使用快速排序 QuickSort。源码中这样写道：

// In-place QuickSort algorithm.
// For short (length <= 22) arrays, insertion sort is used for efficiency.

此外，附上其他引擎的sort实现方式

Mozilla/Firefox : 归并排序（jsarray.c 源码）
Webkit ：底层实现用了 C++ 库中的 qsort() 方法（JSArray.cpp 源码）

V8的array.js源码关于sort的部分 https://github.com/v8/v8.git

function InnerArraySort(array, length, comparefn) {  // In-place QuickSort algorithm.  // For short (length <= 22) arrays, insertion sort is used for efficiency.  //……  var InsertionSort = function InsertionSort(a, from, to) {    for (var i = from + 1; i < to; i++) {      var element = a[i];      for (var j = i - 1; j >= from; j--) {        var tmp = a[j];        var order = comparefn(tmp, element);        if (order > 0) {          a[j + 1] = tmp;        } else {          break;        }      }      a[j + 1] = element;    }  };  var QuickSort = function QuickSort(a, from, to) {    var third_index = 0;    while (true) {      // Insertion sort is faster for short arrays.      if (to - from <= 10) {        InsertionSort(a, from, to);        return;      }      if (to - from > 1000) {        third_index = GetThirdIndex(a, from, to);      } else {        third_index = from + ((to - from) >> 1);      }      // Find a pivot as the median of first, last and middle element.      var v0 = a[from];      var v1 = a[to - 1];      var v2 = a[third_index];      var c01 = comparefn(v0, v1);      if (c01 > 0) {        // v1 < v0, so swap them.        var tmp = v0;        v0 = v1;        v1 = tmp;      } // v0 <= v1.      var c02 = comparefn(v0, v2);      if (c02 >= 0) {        // v2 <= v0 <= v1.        var tmp = v0;        v0 = v2;        v2 = v1;        v1 = tmp;      } else {        // v0 <= v1 && v0 < v2        var c12 = comparefn(v1, v2);        if (c12 > 0) {          // v0 <= v2 < v1          var tmp = v1;          v1 = v2;          v2 = tmp;        }      }      // v0 <= v1 <= v2      a[from] = v0;      a[to - 1] = v2;      var pivot = v1;      var low_end = from + 1;   // Upper bound of elements lower than pivot.      var high_start = to - 1;  // Lower bound of elements greater than pivot.      a[third_index] = a[low_end];      a[low_end] = pivot;      // From low_end to i are elements equal to pivot.      // From i to high_start are elements that haven't been compared yet.      partition: for (var i = low_end + 1; i < high_start; i++) {        var element = a[i];        var order = comparefn(element, pivot);        if (order < 0) {          a[i] = a[low_end];          a[low_end] = element;          low_end++;        } else if (order > 0) {          do {            high_start--;            if (high_start == i) break partition;            var top_elem = a[high_start];            order = comparefn(top_elem, pivot);          } while (order > 0);          a[i] = a[high_start];          a[high_start] = element;          if (order < 0) {            element = a[i];            a[i] = a[low_end];            a[low_end] = element;            low_end++;          }        }      }      if (to - high_start < low_end - from) {        QuickSort(a, high_start, to);        to = low_end;      } else {        QuickSort(a, from, low_end);        from = high_start;      }    }  };  //……}

2. sort方法的使用

2.1 api

  语法：arrayObject.sort(sortby)；参数sortby可选，用于规定排序规则，必须是函数。

注：如果调用该方法时没有使用参数，将按字母顺序对数组中的元素进行排序(按照字符编码的顺序)

  如果想按照其他标准进行排序，就需要提供比较函数，该函数要比较两个值，然后返回一个用于说明这两个值的相对顺序的数字。比较函数应该具有两个参数 a 和 b，其返回值如下：

a < b，排序后的数组中 a 在 b 前，返回一个小于 0 的值
a == b，返回 0
a > b，返回一个大于 0 的值

快速记忆秘诀

csxiaoyao的个人记忆方法为：return一个值，表示是否调换这两个数的顺序，a-b –> 小到大，b-a –> 大到小

2.2 例1：无参sort()对字符串数组排序

var arr = new Array(5);arr = ["csxiaoyao","sunshine","studio","sunshine studio","sun"];console.log(arr.sort().toString());

输出：

csxiaoyao,studio,sun,sunshine,sunshine studio

2.3 例2：无参sort()对Number数组排序

var arr = new Array(5)arr = [80,70,700,7,8];console.log(arr.sort().toString());

输出：

7,70,700,8,80

注意 : 结果没有按照数值大小对数字进行排序，而是按字符串排序，要实现按数值排序，必须使用排序函数。

2.4 例3：带参sort()对字符串类型的数值数组排序

var arr = new Array(5);arr = ["80","70","700","7","8"];console.log(arr.sort(sortNumber).toString());function sortNumber(a,b){    return a - b;}

输出：

7,8,70,80,700

2.5 例4：带参sort()对简单对象List的自定义属性排序

var objectList = new Array();function Person(name,age){    this.name=name;    this.age=age;}objectList.push(new Person('csxiaoyao',20));objectList.push(new Person('sunshine',25));objectList.push(new Person('sunjianfeng',26));//按年龄从小到大排序objectList.sort(function(a,b){    return a.age-b.age;});for(var i=0;i<objectList.length;i++){    console.log(objectList[i].name+" "+objectList[i].age);}

输出：

csxiaoyao 20sunshine 25sunjianfeng 26