STL sort

 

   1: template<class _RanIt> inline

   2:     void sort(_RanIt _First, _RanIt _Last)

   3: {    // order [_First, _Last), using operator<

   4:     _DEBUG_RANGE(_First, _Last);

   5:     _Sort(_CHECKED_BASE(_First), _CHECKED_BASE(_Last), _Last - _First);

   6: }

   1: template<class _RanIt,

   2:     class _Diff,

   3:     class _Pr> inline

   4:     void _Sort(_RanIt _First, _RanIt _Last, _Diff _Ideal, _Pr _Pred)

   5:     {    

   6:     // order [_First, _Last), using _Pred

   7:     _Diff _Count;

   8:     for (; _ISORT_MAX < (_Count = _Last - _First) && 0 < _Ideal; )

   9:     {    

  10:         // divide and conquer by quicksort

  11:  

  12:         // First, we get the middle position, divide the set to two part

  13:         pair<_RanIt, _RanIt> _Mid =

  14:             _Unguarded_partition(_First, _Last, _Pred);

  15:  

  16:         

  17:         _Ideal /= 2, _Ideal += _Ideal / 2;    // allow 1.5 log2(N) divisions

  18:  

  19:         if (_Mid.first - _First < _Last - _Mid.second)    // loop on larger half

  20:             _Sort(_First, _Mid.first, _Ideal, _Pred), _First = _Mid.second;

  21:         else

  22:             _Sort(_Mid.second, _Last, _Ideal, _Pred), _Last = _Mid.first;

  23:     }

  24:  

  25:     if (_ISORT_MAX < _Count)

  26:     {    

  27:         // heap sort if too many divisions

  28:         std::make_heap(_First, _Last, _Pred);

  29:         std::sort_heap(_First, _Last, _Pred);

  30:     }

  31:     else if (1 < _Count)

  32:         _Insertion_sort(_First, _Last, _Pred);    // small, insertion sort

  33:     }

其中

   1: // COMMON SORT PARAMETERS

   2: const int _ISORT_MAX = 32;    // maximum size for insertion sort

 

1. _ISORT_MAX用来控制能够conquer的集合的大小，如果集合已经足够小了，比如这里集合中元素的个数小于32个，则可以使用Insertion_Sort来进行排序操作了；

2. _Ideal用来控制(Divide-and-Conquer)递归的层次，

相当 (N * 1.5^_Ideal) >> (_Ideal) 次就达到了0，即2^_Ideal == N*1.5^_Ideal,

即_Ideal = log_4/3N, 但是实际上由于在“除2”操作过程中的低位损耗，_Ideal的值会比公式计算出来的偏小，不过原comment给出的1.5 log2(N)是不准确的，以下做了一些对比

   1: int x = 1024;

   2:     int y = 0;

   3:     while (x > 0)

   4:     {

   5:         x /= 2, x += x / 2;

   6:         printf("0x%08X\n", x);

   7:         y++;

   8:     }

N	1.5 log2(N)	log4/3N	程序计算出的值y
1024	15	24	21

 

言归正传，当_Ideal的层次太深的时候，for循环结束

3. 如果不是因为分块已经足够小了而结束的，就调用Heap_Sort进行堆排序

4. 如果分块已经都足够小了，就进行Insertion_Sort排序

转载于:https://www.cnblogs.com/long123king/p/3487753.html