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STL源码剖析——STL算法之merge合并算法
前言
由于在前文的《STL算法剖析》中，源码剖析非常多，不方便学习，也不方便以后复习，这里把这些算法进行归类，对他们单独的源码剖析进行讲解。本文介绍的STL算法中的merge合并算法。源码中介绍了函数merge、inplace_merge。并对这些函数的源码进行详细的剖析，并适当给出使用例子，具体详见下面源码剖析。
merge合并算法源码剖析
// merge, with and without an explicitly supplied comparison function.
//将两个已排序的区间[first1,last1)和区间[first2,last2)合并
/*
函数功能：Combines the elements in the sorted ranges [first1,last1) and [first2,last2),
into a new range beginning at result with all its elements sorted.
函数原型：
default (1) ：版本一
template <class InputIterator1, class InputIterator2, class OutputIterator>
OutputIterator merge (InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result);
custom (2) ：版本二
template <class InputIterator1, class InputIterator2,
class OutputIterator, class Compare>
OutputIterator merge (InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result, Compare comp);
*/
//版本一：
template <class _InputIter1, class _InputIter2, class _OutputIter>
_OutputIter merge(_InputIter1 __first1, _InputIter1 __last1,
_InputIter2 __first2, _InputIter2 __last2,
_OutputIter __result) {
__STL_REQUIRES(_InputIter1, _InputIterator);
__STL_REQUIRES(_InputIter2, _InputIterator);
__STL_REQUIRES(_OutputIter, _OutputIterator);
__STL_REQUIRES_SAME_TYPE(
typename iterator_traits<_InputIter1>::value_type,
typename iterator_traits<_InputIter2>::value_type);
__STL_REQUIRES(typename iterator_traits<_InputIter1>::value_type,
_LessThanComparable);
//两个序列都尚未到达尾端，则执行while循环
/*
情况1：若序列二元素较小,则记录到目标区，且移动序列二的迭代器，但是序列一的迭代器不变.
情况2：若序列一元素较小或相等,则记录到目标区，且移动序列一的迭代器，但是序列二的迭代器不变.
最后：把剩余元素的序列复制到目标区
*/
while (__first1 != __last1 && __first2 != __last2) {
//情况1
if (*__first2 < *__first1) {//若序列二元素较小
*__result = *__first2;//将元素记录到目标区
++__first2;//移动迭代器
}
//情况2
else {//若序列一元素较小或相等
*__result = *__first1;//将元素记录到目标区
++__first1;//移动迭代器
}
++__result;//更新目标区位置，以便下次记录数据
}
//若有序列到达尾端，则把没到达尾端的序列剩余元素复制到目标区
//此时，区间[first1,last1)和区间[first2,last2)至少一个必定为空
return copy(__first2, __last2, copy(__first1, __last1, __result));
}
//版本二
template <class _InputIter1, class _InputIter2, class _OutputIter,
class _Compare>
_OutputIter merge(_InputIter1 __first1, _InputIter1 __last1,
_InputIter2 __first2, _InputIter2 __last2,
_OutputIter __result, _Compare __comp) {
__STL_REQUIRES(_InputIter1, _InputIterator);
__STL_REQUIRES(_InputIter2, _InputIterator);
__STL_REQUIRES_SAME_TYPE(
typename iterator_traits<_InputIter1>::value_type,
typename iterator_traits<_InputIter2>::value_type);
__STL_REQUIRES(_OutputIter, _OutputIterator);
__STL_BINARY_FUNCTION_CHECK(_Compare, bool,
typename iterator_traits<_InputIter1>::value_type,
typename iterator_traits<_InputIter1>::value_type);
while (__first1 != __last1 && __first2 != __last2) {
if (__comp(*__first2, *__first1)) {
*__result = *__first2;
++__first2;
}
else {
*__result = *__first1;
++__first1;
}
++__result;
}
return copy(__first2, __last2, copy(__first1, __last1, __result));
}
//merge函数举例：
/*
#include <iostream> // std::cout
#include <algorithm> // std::merge, std::sort
#include <vector> // std::vector
int main () {
int first[] = {5,10,15,20,25};
int second[] = {50,40,30,20,10};
std::vector<int> v(10);
std::sort (first,first+5);
std::sort (second,second+5);
std::merge (first,first+5,second,second+5,v.begin());
std::cout << "The resulting vector contains:";
for (std::vector<int>::iterator it=v.begin(); it!=v.end(); ++it)
std::cout << ' ' << *it;
std::cout << '\n';
return 0;
}
Output:
The resulting vector contains: 5 10 10 15 20 20 25 30 40 50
*/
// inplace_merge and its auxiliary functions.
//版本一的辅助函数，无缓冲区的操作
template <class _BidirectionalIter, class _Distance>
void __merge_without_buffer(_BidirectionalIter __first,
_BidirectionalIter __middle,
_BidirectionalIter __last,
_Distance __len1, _Distance __len2) {
if (__len1 == 0 || __len2 == 0)
return;
if (__len1 + __len2 == 2) {
if (*__middle < *__first)
iter_swap(__first, __middle);
return;
}
_BidirectionalIter __first_cut = __first;
_BidirectionalIter __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);
}
_BidirectionalIter __new_middle
= rotate(__first_cut, __middle, __second_cut);
__merge_without_buffer(__first, __first_cut, __new_middle,
__len11, __len22);
__merge_without_buffer(__new_middle, __second_cut, __last, __len1 - __len11,
__len2 - __len22);
}
template <class _BidirectionalIter, class _Distance, class _Compare>
void __merge_without_buffer(_BidirectionalIter __first,
_BidirectionalIter __middle,
_BidirectionalIter __last,
_Distance __len1, _Distance __len2,
_Compare __comp) {
if (__len1 == 0 || __len2 == 0)
return;
if (__len1 + __len2 == 2) {
if (__comp(*__middle, *__first))
iter_swap(__first, __middle);
return;
}
_BidirectionalIter __first_cut = __first;
_BidirectionalIter __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, __comp);
distance(__middle, __second_cut, __len22);
}
else {
__len22 = __len2 / 2;
advance(__second_cut, __len22);
__first_cut = upper_bound(__first, __middle, *__second_cut, __comp);
distance(__first, __first_cut, __len11);
}
_BidirectionalIter __new_middle
= rotate(__first_cut, __middle, __second_cut);
__merge_without_buffer(__first, __first_cut, __new_middle, __len11, __len22,
__comp);
__merge_without_buffer(__new_middle, __second_cut, __last, __len1 - __len11,
__len2 - __len22, __comp);
}
//版本一的辅助函数，有缓冲区的操作
template <class _BidirectionalIter1, class _BidirectionalIter2,
class _Distance>
_BidirectionalIter1 __rotate_adaptive(_BidirectionalIter1 __first,
_BidirectionalIter1 __middle,
_BidirectionalIter1 __last,
_Distance __len1, _Distance __len2,
_BidirectionalIter2 __buffer,
_Distance __buffer_size) {
_BidirectionalIter2 __buffer_end;
if (__len1 > __len2 && __len2 <= __buffer_size) {//缓冲区足够放置序列二
__buffer_end = copy(__middle, __last, __buffer);
copy_backward(__first, __middle, __last);
return copy(__buffer, __buffer_end, __first);
}
else if (__len1 <= __buffer_size) {//缓冲区足够放置序列一
__buffer_end = copy(__first, __middle, __buffer);
copy(__middle, __last, __first);
return copy_backward(__buffer, __buffer_end, __last);
}
else//若缓冲区仍然不够，则调用STL算法rotate，不使用缓冲区
return rotate(__first, __middle, __last);
}
template <class _BidirectionalIter1, class _BidirectionalIter2,
class _BidirectionalIter3>
_BidirectionalIter3 __merge_backward(_BidirectionalIter1 __first1,
_BidirectionalIter1 __last1,
_BidirectionalIter2 __first2,
_BidirectionalIter2 __last2,
_BidirectionalIter3 __result) {
if (__first1 == __last1)
return copy_backward(__first2, __last2, __result);
if (__first2 == __last2)
return copy_backward(__first1, __last1, __result);
--__last1;
--__last2;
while (true) {
if (*__last2 < *__last1) {
*--__result = *__last1;
if (__first1 == __last1)
return copy_backward(__first2, ++__last2, __result);
--__last1;
}
else {
*--__result = *__last2;
if (__first2 == __last2)
return copy_backward(__first1, ++__last1, __result);
--__last2;
}
}
}
template <class _BidirectionalIter1, class _BidirectionalIter2,
class _BidirectionalIter3, class _Compare>
_BidirectionalIter3 __merge_backward(_BidirectionalIter1 __first1,
_BidirectionalIter1 __last1,
_BidirectionalIter2 __first2,
_BidirectionalIter2 __last2,
_BidirectionalIter3 __result,
_Compare __comp) {
if (__first1 == __last1)
return copy_backward(__first2, __last2, __result);
if (__first2 == __last2)
return copy_backward(__first1, __last1, __result);
--__last1;
--__last2;
while (true) {
if (__comp(*__last2, *__last1)) {
*--__result = *__last1;
if (__first1 == __last1)
return copy_backward(__first2, ++__last2, __result);
--__last1;
}
else {
*--__result = *__last2;
if (__first2 == __last2)
return copy_backward(__first1, ++__last1, __result);
--__last2;
}
}
}
//版本一的辅助函数，有缓冲区的操作
template <class _BidirectionalIter, class _Distance, class _Pointer>
void __merge_adaptive(_BidirectionalIter __first,
_BidirectionalIter __middle,
_BidirectionalIter __last,
_Distance __len1, _Distance __len2,
_Pointer __buffer, _Distance __buffer_size) {
if (__len1 <= __len2 && __len1 <= __buffer_size) {
//case1：把序列一放在缓冲区
_Pointer __buffer_end = copy(__first, __middle, __buffer);
//直接调用归并函数merge
merge(__buffer, __buffer_end, __middle, __last, __first);
}
else if (__len2 <= __buffer_size) {
//case2：把序列二放在缓冲区
_Pointer __buffer_end = copy(__middle, __last, __buffer);
__merge_backward(__first, __middle, __buffer, __buffer_end, __last);
}
else {//case3：缓冲区不足放置任何一个序列
_BidirectionalIter __first_cut = __first;
_BidirectionalIter __second_cut = __middle;
_Distance __len11 = 0;
_Distance __len22 = 0;
if (__len1 > __len2) {//若序列一比较长
__len11 = __len1 / 2;//计算序列一的一半
advance(__first_cut, __len11);//让first_cut指向序列一的中间位置
//找出*__first_cut在[middle,last)区间中的第一个不小于*__first_cut的元素位置
__second_cut = lower_bound(__middle, __last, *__first_cut);
//计算middle到__second_cut之间的距离，保存在__len22
distance(__middle, __second_cut, __len22);
}
else {//若序列二比较长
__len22 = __len2 / 2;//计算序列二的一半
advance(__second_cut, __len22);//让__second_cut指向序列二的中间位置
//找出*__second_cut在[first,middle)区间中的第一个大于*__second_cut的元素位置
__first_cut = upper_bound(__first, __middle, *__second_cut);
//计算__first到__first_cut之间的距离，保存在__len11
distance(__first, __first_cut, __len11);
}
_BidirectionalIter __new_middle =
__rotate_adaptive(__first_cut, __middle, __second_cut, __len1 - __len11,
__len22, __buffer, __buffer_size);
//对左半段递归调用
__merge_adaptive(__first, __first_cut, __new_middle, __len11,
__len22, __buffer, __buffer_size);
//对右半段递归调用
__merge_adaptive(__new_middle, __second_cut, __last, __len1 - __len11,
__len2 - __len22, __buffer, __buffer_size);
}
}
template <class _BidirectionalIter, class _Distance, class _Pointer,
class _Compare>
void __merge_adaptive(_BidirectionalIter __first,
_BidirectionalIter __middle,
_BidirectionalIter __last,
_Distance __len1, _Distance __len2,
_Pointer __buffer, _Distance __buffer_size,
_Compare __comp) {
if (__len1 <= __len2 && __len1 <= __buffer_size) {
_Pointer __buffer_end = copy(__first, __middle, __buffer);
merge(__buffer, __buffer_end, __middle, __last, __first, __comp);
}
else if (__len2 <= __buffer_size) {
_Pointer __buffer_end = copy(__middle, __last, __buffer);
__merge_backward(__first, __middle, __buffer, __buffer_end, __last,
__comp);
}
else {
_BidirectionalIter __first_cut = __first;
_BidirectionalIter __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, __comp);
distance(__middle, __second_cut, __len22);
}
else {
__len22 = __len2 / 2;
advance(__second_cut, __len22);
__first_cut = upper_bound(__first, __middle, *__second_cut, __comp);
distance(__first, __first_cut, __len11);
}
_BidirectionalIter __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, __comp);
__merge_adaptive(__new_middle, __second_cut, __last, __len1 - __len11,
__len2 - __len22, __buffer, __buffer_size, __comp);
}
}
//版本一的辅助函数
template <class _BidirectionalIter, class _Tp, class _Distance>
inline void __inplace_merge_aux(_BidirectionalIter __first,
_BidirectionalIter __middle,
_BidirectionalIter __last, _Tp*, _Distance*) {
_Distance __len1 = 0;
distance(__first, __middle, __len1);//计算序列一的长度
_Distance __len2 = 0;
distance(__middle, __last, __len2);//计算序列二的长度
//使用暂时缓冲区
_Temporary_buffer<_BidirectionalIter, _Tp> __buf(__first, __last);
if (__buf.begin() == 0)//若缓冲区配置失败
//则调用不使用缓冲区的合并操作
__merge_without_buffer(__first, __middle, __last, __len1, __len2);
else//若分配成功
//则调用具有缓冲区的合并操作
__merge_adaptive(__first, __middle, __last, __len1, __len2,
__buf.begin(), _Distance(__buf.size()));
}
template <class _BidirectionalIter, class _Tp,
class _Distance, class _Compare>
inline void __inplace_merge_aux(_BidirectionalIter __first,
_BidirectionalIter __middle,
_BidirectionalIter __last, _Tp*, _Distance*,
_Compare __comp) {
_Distance __len1 = 0;
distance(__first, __middle, __len1);
_Distance __len2 = 0;
distance(__middle, __last, __len2);
_Temporary_buffer<_BidirectionalIter, _Tp> __buf(__first, __last);
if (__buf.begin() == 0)
__merge_without_buffer(__first, __middle, __last, __len1, __len2, __comp);
else
__merge_adaptive(__first, __middle, __last, __len1, __len2,
__buf.begin(), _Distance(__buf.size()),
__comp);
}
//将两个已排序的序列[first,middle)和[middle,last)合并成单一有序序列.
//若原来是增序，现在也是递增排序，若原来是递减排序，现在也是递减排序
/*
函数功能：Merges two consecutive sorted ranges: [first,middle) and [middle,last),
putting the result into the combined sorted range [first,last).
函数原型：
default (1) ：版本一
template <class BidirectionalIterator>
void inplace_merge (BidirectionalIterator first, BidirectionalIterator middle,
BidirectionalIterator last);
custom (2) ：版本二
template <class BidirectionalIterator, class Compare>
void inplace_merge (BidirectionalIterator first, BidirectionalIterator middle,
BidirectionalIterator last, Compare comp);
*/
//版本一
template <class _BidirectionalIter>
inline void inplace_merge(_BidirectionalIter __first,
_BidirectionalIter __middle,
_BidirectionalIter __last) {
__STL_REQUIRES(_BidirectionalIter, _Mutable_BidirectionalIterator);
__STL_REQUIRES(typename iterator_traits<_BidirectionalIter>::value_type,
_LessThanComparable);
if (__first == __middle || __middle == __last)//若有空序列，则之间返回
return;
__inplace_merge_aux(__first, __middle, __last,
__VALUE_TYPE(__first), __DISTANCE_TYPE(__first));
}
//版本二
template <class _BidirectionalIter, class _Compare>
inline void inplace_merge(_BidirectionalIter __first,
_BidirectionalIter __middle,
_BidirectionalIter __last, _Compare __comp) {
__STL_REQUIRES(_BidirectionalIter, _Mutable_BidirectionalIterator);
__STL_BINARY_FUNCTION_CHECK(_Compare, bool,
typename iterator_traits<_BidirectionalIter>::value_type,
typename iterator_traits<_BidirectionalIter>::value_type);
if (__first == __middle || __middle == __last)
return;
__inplace_merge_aux(__first, __middle, __last,
__VALUE_TYPE(__first), __DISTANCE_TYPE(__first),
__comp);
}
//inplace_merge函数举例：
/*
#include <iostream> // std::cout
#include <algorithm> // std::inplace_merge, std::sort, std::copy
#include <vector> // std::vector
int main () {
int first[] = {5,10,15,20,25};
int second[] = {50,40,30,20,10};
std::vector<int> v(10);
std::vector<int>::iterator it;
std::sort (first,first+5);
std::sort (second,second+5);
it=std::copy (first, first+5, v.begin());
std::copy (second,second+5,it);
std::inplace_merge (v.begin(),v.begin()+5,v.end());
std::cout << "The resulting vector contains:";
for (it=v.begin(); it!=v.end(); ++it)
std::cout << ' ' << *it;
std::cout << '\n';
return 0;
}
Output：
The resulting vector contains: 5 10 10 15 20 20 25 30 40 50
*/参考资料：
《STL源码剖析》侯捷
STL源码剖析——STL算法之merge合并算法
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