STL 源代码剖析 算法 stl_algo.h -- inplace_merge

Posted 2020-09-11 blfbuaa

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inplace_merge(应用于有序区间)
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描写叙述：假设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。
源代码：

template <class BidirectionalIterator>
inline void inplace_merge(BidirectionalIterator first,
                          BidirectionalIterator middle,
                          BidirectionalIterator last) {
  if (first == middle || middle == last) return; //仅仅要有不论什么一个序列为空，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(临时缓冲区)	
  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 // 在有临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);
  }
}

演示样例：

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".
}