2021-01-22

[转载或参考] https://blog.youkuaiyun.com/github_39319000/article/details/89554079

[转载或参考]https://blog.youkuaiyun.com/weixin_33862188/article/details/88679236

【仅供自己学习查询， 侵删】

关于 sort () 比较 遇到的坑，浅析 【附带：源码】

1、 不传值（默认）

示例： 

var arr = [1, 5, 11, 10]

arr.sort() //  [1, 10, 11, 5]

这里并没有得到我们想要的 [1, 5 ,10, 11]

原因： 

1. 默认情况，sort 是升序
2. sort 默认会将 元素转化成 string 类型 ， 因此这儿的 Number 类型比较的其实是 String 类型。

回顾下： 字符串的比较：

abc

abde 

"c".charCodeAt() // 99
"d".charCodeAt() // 100

a-z	97-122
A-Z	65-90
0-9	45-57

字符串大小比较是 比较相同index字符的 ASII码， c （99） < d (100)

因此 abc < abd

回顾问题： "5" 和 "11", 比较第一位， "1" 为46， "5"为 50， 故 "1"<"5", 比较结束， "11"<"5",

得出： [1, 10, 11, 5]

 

2、 函数返回值 bool 类型 （坑）

var arr = [1, 10, 5, 11] //  目标值： [1, 5, 10, 11]

// 返回 bool 值
arr.sort((x,y)=> x>y ) // [1, 10, 5, 11] 未生效


// 返回 Number 值
arr.sort((x,y)=> x-y ) // [1, 5, 10, 11] 生效

原因： 当传入回调函数时候，

 返回 bool 值，不生效

返回 Bumber 值时候， 生效。 转化成的 true 和 false 并不会转化成数值，因此不生效。

原因是因为，sort ( fn ) 会调用 fn ,最后根据 数字 进行区分，

• 当等于0时， 不移动
• 等大于0时， 交换
• 当小于0时，交换

 

3、总结： 

1.  不传值，默认升序，会转化成 字符串进行比较
2. 有fn传值，根据 return 的 Number 值进行比较，注意 retrun 布尔类型值。函数不生效。

 

    3、数据元素少的时候，采用的是插入排序， 元素多的时候，采用的是快排。

4、V8引擎源码

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

  var custom_compare = IS_FUNCTION(comparefn);

  function Compare(x,y) {
    // Assume the comparefn, if any, is a consistent comparison function.
    // If it isn't, we are allowed arbitrary behavior by ECMA 15.4.4.11.
    if (x === y) return 0;
    if (custom_compare) {
      // Don't call directly to avoid exposing the builtin's global object.
      return comparefn.call(null, x, y);
    }
    if (%_IsSmi(x) && %_IsSmi(y)) {
      return %SmiLexicographicCompare(x, y);
    }
    x = ToString(x);
    y = ToString(y);
    if (x == y) return 0;
    else return x < y ? -1 : 1;
  };

  function InsertionSort(a, from, to) {
    for (var i = from + 1; i < to; i++) {
      var element = a[i];
      // Pre-convert the element to a string for comparison if we know
      // it will happen on each compare anyway.
      var key =
          (custom_compare || %_IsSmi(element)) ? element : ToString(element);
      // place element in a[from..i[
      // binary search
      var min = from;
      var max = i;
      // The search interval is a[min..max[
      while (min < max) {
        var mid = min + ((max - min) >> 1);
        var order = Compare(a[mid], key);
        if (order == 0) {
          min = max = mid;
          break;
        }
        if (order < 0) {
          min = mid + 1;
        } else {
          max = mid;
        }
      }
      // place element at position min==max.
      for (var j = i; j > min; j--) {
        a[j] = a[j - 1];
      }
      a[min] = element;
    }
  }

  function QuickSort(a, from, to) {
    // Insertion sort is faster for short arrays.
    if (to - from <= 22) {
      InsertionSort(a, from, to);
      return;
    }
    var pivot_index = $floor($random() * (to - from)) + from;
    var pivot = a[pivot_index];
    // Pre-convert the element to a string for comparison if we know
    // it will happen on each compare anyway.
    var pivot_key =
      (custom_compare || %_IsSmi(pivot)) ? pivot : ToString(pivot);
    // Issue 95: Keep the pivot element out of the comparisons to avoid
    // infinite recursion if comparefn(pivot, pivot) != 0.
    a[pivot_index] = a[from];
    a[from] = pivot;
    var low_end = from;   // Upper bound of the elements lower than pivot.
    var high_start = to;  // Lower bound of the elements greater than pivot.
    // From low_end to i are elements equal to pivot.
    // From i to high_start are elements that haven't been compared yet.
    for (var i = from + 1; i < high_start; ) {
      var element = a[i];
      var order = Compare(element, pivot_key);
      if (order < 0) {
        a[i] = a[low_end];
        a[low_end] = element;
        i++;
        low_end++;
      } else if (order > 0) {
        high_start--;
        a[i] = a[high_start];
        a[high_start] = element;
      } else {  // order == 0
        i++;
      }
    }
    QuickSort(a, from, low_end);
    QuickSort(a, high_start, to);
  }

  var old_length = ToUint32(this.length);
  if (old_length < 2) return this;

  %RemoveArrayHoles(this);

  var length = ToUint32(this.length);

  // Move undefined elements to the end of the array.
  for (var i = 0; i < length; ) {
    if (IS_UNDEFINED(this[i])) {
      length--;
      this[i] = this[length];
      this[length] = void 0;
    } else {
      i++;
    }
  }

  QuickSort(this, 0, length);

  // We only changed the length of the this object (in
  // RemoveArrayHoles) if it was an array.  We are not allowed to set
  // the length of the this object if it is not an array because this
  // might introduce a new length property.
  if (IS_ARRAY(this)) {
    this.length = old_length;
  }

  return this;
}