STL 源码剖析 算法 stl_algo.h -- inplace_merge

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inplace_merge(应用于有序区间)

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描述：如果两个连接在一起的序列[first, middle)和 [middle, last]都已排序，

那么 inplace_merge 可将它们结合成单一一个序列，并仍有序。

源码：

template <class BidirectionalIterator>
inline void inplace_merge(BidirectionalIterator first,
BidirectionalIterator middle,
BidirectionalIterator last) {
if (first == middle || middle == last) return; //只要有任何一个序列为空，就什么都不必做
__inplace_merge_aux(first, middle, last, value_type(first),
distance_type(first));
}

//辅助函数
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);
//使用了额外的内存空间(暂时缓冲区)
temporary_buffer<BidirectionalIterator, T> buf(first, last);
if (buf.begin() == 0) // 内存分配失败
__merge_without_buffer(first, middle, last, len1, len2);
else // 在有暂时缓冲区的情况下进行
__merge_adaptive(first, middle, last, len1, len2,
buf.begin(), Distance(buf.size()));
}

//辅助函数。有缓冲区的情况下
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) {
Pointer end_buffer = copy(first, middle, buffer);
merge(buffer, end_buffer, middle, last, first);
}
else if (len2 <= buffer_size) {
Pointer end_buffer = copy(middle, last, buffer);
__merge_backward(first, middle, buffer, end_buffer, last);
}
else {
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);
}
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);
}
}

示例：

int main()
{
int A[] = { 1, 3, 5, 7, 2, 4, 6, 8 };

inplace_merge(A, A + 4, A + 8);
copy(A, A + 8, ostream_iterator<int>(cout, " "));
// The output is "1 2 3 4 5 6 7 8".
}