STL源碼剖析之算法：upper_bound

upper_bound()函數返回插入value的最後一個位置，該函數要求目標序列必須是有序。另外，如果value已經存在於目標序列，upper_bound將返回value的下一位置，而lower_bound則會返回value的位置。

template <class ForwardIterator, class T>
inline ForwardIterator upper_bound(ForwardIterator first,
                                   ForwardIterator last,
                                   const T &value) {
    return __upper_bound(first, last, value,
                         distance_type(first), iterator_category(first));
}

// forward iterator 版本
template <class ForwardIterator, class T, class Distance>
ForwardIterator __upper_bound(ForwardIterator first,
                              ForwardIterator last,
                              const T &value,
                              Distance*,
                              forward_iterator_tag) {
    Distance len = 0;
    distance(first, last, len);
    Distance half;
    ForwardIterator middle;

    while(len > 0) {
        half = len >> 1;
        advance(middle, half);

        if(value < *middle)
            len = half;
        else {
            first = middle;
            ++first;
            len = len - half - 1;
        }
    }

    return first;
}

// random access iterator 版本
template <class RandomAccessIterator, class T, class Distance>
RandomAccessIterator __upper_bound(RandomAccessIterator first,
                                   RandomAccessIterator last,
                                   const T &value,
                                   Distance*,
                                   random_access_iterator_tag) {
    Distance len = last - first;
    Distance half;
    RandomAccessIterator middle;

    while(len > 0) {
        half = len >> 1;
        middle = first + half;

        if(value < *middle)
            len = half;
        else {
            first = middle + 1;
            len = len - half - 1;
        }
    }

    return first;
}