//---------------------------15/04/01----------------------------
//inplace_merge(要求有序)
template<class BidirectionalIterator>
inline void inplace_merge(BidirectionalIterator first,
BidirectionalIterator middle,
BidirectionalIterator last)
{
if(first == middle || middle == last)
return;
__inplace_merge_aux(first, middle, last, value_type(first),
distance_type(first));
}
template<class BidirectionalIterator, class T, class Distance>
inline void __inplace_merge_aux(BidirectionalIterator first,
BidirectionalIterator middle,
BidirectionalIterator last,
T*, Distance*)
{
Distance len1 = 0;
distance(first, middle, len1);
Distance len2 = 0;
distance(middle, last, len2);
//用来申请暂时的缓冲区
temporary_buffer<BidirectionalIterator, T> buf(first, last);
if(buf.begin() == 0)
__merge_without_buffer(first, middle, last, len1, len2);
else
__merge_adaptive(first, middle, last, len1, len2,
buf.begin(), Distance(buf.size()));
}
//有缓冲区的情况
template<class BidirectionalIterator, class Distance, class Pointer>
void __merge_adaptive(BidirectionalIterator first,
BidirectionalIterator middle,
BidirectionalIterator last,
Distance len1, Distance len2,
Pointer buffer, Distance buffer_size)
{
if(len1 <= len2 && len1 <= buffer_size)
{
//先复制[first,middle)区间的元素到缓冲区
Pointer end_buffer = copy(first, middle, buffer);
//调用merge函数，把元素放入first开始的区间，也就是first到last
merge(buffer, end_buffer, middle, last, first);
}
else if(len2 <= buffer_size)
{
Pointer end_buffer = copy(middle, last, buffer);
//从后面开始merge。
__merge_backward(first, middle, buffer, end_buffer, last);
}
else
{
//缓冲区放不下len1 或 len2，需要进行裁剪
//搞不懂为什么前面是lower_bound，后面是upper_bound
//有一种可能是：len1 > len2时，取lower_bound可以使更少的元素换到左边的区间
//len1 < len2时，取upper_bound可以时更少的元素换到右边。
//这么做可以平衡两个区间的元素量
BidirectionalIterator first_cut =first;
BidirectionalIterator second_cut = middle;
Distance len11 = 0;
Distance len22 = 0;
if(len1 > len2)
{
len11 = len1 / 2;
advance(first_cut, len11);
second_cut = lower_bound(middle, last, *first_cut);
distance(middle, second_cut, len22);
}
else
{
len22 = len2 / 2;
advance(second_cut, len22);
first_cut = upper_bound(first, middle, *second_cut);
distance(first, first_cut, len11);
}
//到这时区间是这样的 first first_cut middle second_cut last
//为了让为了merge必须让要merge的区间并起来 所以把middle到second_cut，换到first_cut的位置就可以了
//rotate后 first first_cut(原middle) new_middle(原first_cut)
// second_cut(原second_cut) last
BidirectionalIterator new_middle =
__rotate_adaptive(first_cut, middle, second_cut, len1 - len11,
len22, buffer, buffer_size);
__merge_adaptive(first, first_cut, new_middle, len11, len22, buffer,
buffer_size);
__merge_adaptive(new_middle, second_cut, last, len1 -len11,
len2 - len22, buffer, buffer_size);
}
}
template<class BidirectionalIterator1, class BidirectionalIterator2,
class Distance>
BidirectionalIterator __rotate_adaptive(BidirectionalIterator1 first,
BidirectionalIterator1 middle,
BidirectionalIterator1 last,
Distance len1, Distance len2,
BidirectionalIterator2 buffer,
Distance buffer_size)
{
//缓冲区足够 就利用缓冲区翻转，不够就调用全局rotate
BidirectionalIterator2 buffer_end;
if(len1 > len2 && len2 <= buffer_size)
{
buffer_end = copy(middle, last, buffer);
copy_backward(first, middle, last);
return copy(buffer, buffer_end, first);
}
else if(len1 <= buffer_size)
{
buffer_end = copy(first, middle, buffer);
copy(middle, last, first);
return copy_backward(buffer, buffer_end, last);
}
else
{
rotate(first, middle, last);
advance(first, len2);
return first;
}
}
//nth_element
//使的nth的元素处在完全排序后的位置，只能保证这一个元素处在对的位置
template<class RandomAccessIterator>
inline void nth_element(RandomAccessIterator first,
RandomAccessIterator nth,
RandomAccessIterator last)
{
__nth_element(first, nth, last, value_type(first));
}
template<class RandomAccessIterator, class T>
void __nth_element(RandomAccessIterator first,
RandomAccessIterator nth,
RandomAccessIterator last, T*)
{
while (last - first > 3)
{
//划分一次
RandomAccessIterator cut = __unguarded_partition
(first, last, T(__median(*first,
*(first + (last - first) / 2),
*(last - 1))));
//nth处在哪边就继续划分那边
if(cut <= nth)
first = cut;
else
last = cut;
}
__insertion_sort(first, last);
}
//merge sort
template<class BidirectionalIterator>
void mergesort(BidirectionalIterator first, BidirectionalIterator last)
{
typename iterator_traits<BidirectionalIterator>::difference_type n
=distance(first, last);
if(n == 0 || n == 1)
return;
else
{
BidirectionalIterator mid = first + n / 2;
mergesort(first, mid);
mergesort(mid, last);
inplace_merge(first, mid, last);
}
}
原文地址：http://blog.csdn.net/boydfd/article/details/44807805
