STL 源代码剖析 算法 stl_algo.h -- rotate

描写叙述：将[first, middle) 内的元素和[middle, last) 内的元素互换。

/*------------------------------------------------------------
*分派函数(dispatch function)
*/
template <class ForwardIterator>
inline void rotate(ForwardIterator first, ForwardIterator middle, ForwardIterator last){
  if(first == middle || middle == last) return ;
  __rotate(first, middle, last, distance_type(first), iterator_category(first));
}

以下依据不同的迭代器类型完毕三个旋转操作

rotate 的 forward iterator 版

思路：

1.以短段为根据。相应元素逐一对调

2.调整，对新的前、后段再作交换

3.当两段都结束时，循环结束

复杂度：O(n)

template<class ForwardIterator, class Distance>
void __rotate(ForwardIterator first, ForwardIterator middle, 
      ForwardIterator last, Distance*, forward_iterator_tag){
  for(ForwardIterator i = middle ; ; ){
    iter_swap(first, i); //前段，后段的元素一一交换
    ++first; ++i;  // 双双前进 1
    // 下面推断是前段[first, middle)先结束还是后段 [middle, last) 后结束
    if(first == middle){ //前段结束了
      if(i == last) return; //假设后段同一时候也结束，整个就结束了
      middle = i;
    }
    else if(i == last){ //后段先结束
      i = middle; // 调整。准备对新的的前、后段再作交换
    }
  }
}

rotate 的 bidirectional iterator 版

思路：

1.对前段逆转

2.对后段逆转

3.对总体逆转

复杂度：O(n)

template<class BidirectionalIterator, class Distance>
void __rotate(BidirectionalIterator first, BidirectionalIterator middle, 
    BidirectionalIterator last, Distance *,
    bidirectional_iterator_tag){
  reverse(first, middle);
  reverse(middle,last);
  reverse(first, last);
}

rotate 的 random access iterator 版 --> ?? 看不太懂。。。

template<class RandomAccessIterator, class Distance>
void __rotate(RandomAccessIterator first, RandomAccessIterator middle，
    RandomAccessIterator last, Distance *,
    random_access_iterator_tag){
  //取全长和前段长度的最大公因子
  Distance n = __gcd(last - first, middle - first);
  while(n--){
    __rotate_cycle(first, last, first + n, middle - first, value_type(first));
  }
}

template <class EuclideanRingElement>
EuclideanRingElement __gcd(EuclideanRingElement m, EuclideanRingElement n)
{
  while (n != 0) {
    EuclideanRingElement t = m % n;
    m = n;
    n = t;
  }
  return m;
}

template <class RandomAccessIterator, class Distance, class T>
void __rotate_cycle(RandomAccessIterator first, RandomAccessIterator last,
                    RandomAccessIterator initial, Distance shift, T*) {
  T value = *initial;
  RandomAccessIterator ptr1 = initial;
  RandomAccessIterator ptr2 = ptr1 + shift;
  while (ptr2 != initial) {
    *ptr1 = *ptr2;
    ptr1 = ptr2;
    if (last - ptr2 > shift)
      ptr2 += shift;
    else
      ptr2 = first + (shift - (last - ptr2));
  }
  *ptr1 = value;
}

演示样例：

char alpha[] = "abcdefghijklmnopqrstuvwxyz";
rotate(alpha, alpha + 13, alpha + 26);
printf("%s\n", alpha);
// The output is nopqrstuvwxyzabcdefghijklm