deque.h

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        destroy(start.current);
        ++start.current;
        --length; 
        if (empty() || begin().current == begin().last)
            deallocate_at_begin();
    }
    void pop_back() {
        --finish.current;
        destroy(finish.current);
        --length; 
        if (empty() || end().current == end().first)
            deallocate_at_end();
    }
    void swap(deque<T>& x) {
        ::swap(start, x.start);
        ::swap(finish, x.finish);
        ::swap(length, x.length);
        ::swap(map, x.map);
        ::swap(map_size, x.map_size);
    }
    iterator insert(iterator position, const T& x);
    void insert(iterator position, size_type n, const T& x);
//  template <class Iterator> void insert(iterator position,
//                                        Iterator first, Iterator last);
    void insert(iterator position, const T* first, const T* last);
    void erase(iterator position);
    void erase(iterator first, iterator last);    
    deque(size_type n, const T& value = T())
        : start(), finish(), length(0), map(0), map_size(0) {
        buffer_size = data_allocator.init_page_size();  
        insert(begin(), n, value);
    }
//  template <class Iterator> deque(Iterator first, Iterator last);
    deque(const T* first, const T* last)
        : start(), finish(), length(0), map(0), map_size(0) {
        buffer_size = data_allocator.init_page_size();  
        copy(first, last, back_inserter(*this));
    }
    deque(const deque<T>& x)
        : start(), finish(), length(0), map(0), map_size(0) {
        buffer_size = data_allocator.init_page_size();  
        copy(x.begin(), x.end(), back_inserter(*this));
    }
    deque<T>& operator=(const deque<T>& x) {
        if (this != &x)
            if (size() >= x.size()) 
                erase(copy(x.begin(), x.end(), begin()), end());
            else 
                copy(x.begin() + size(), x.end(),
                     inserter(*this, copy(x.begin(), x.begin() + size(),
                                          begin())));
        return *this;
    }
    ~deque();
};

template <class T>
deque<T>::data_allocator_type deque<T>::data_allocator;

template <class T>
deque<T>::map_allocator_type deque<T>::map_allocator;

template <class T>
deque<T>::size_type deque<T>::buffer_size = 0; 
// should be data_allocator.init_page_size(); // Borland bug

template <class T>
bool operator==(const deque<T>& x, const deque<T>& y) {
    return x.size() == y.size() && equal(x.begin(), x.end(), y.begin());
}

template <class T>
bool operator<(const deque<T>& x, const deque<T>& y) {
    return lexicographical_compare(x.begin(), x.end(), y.begin(), y.end());
}

template <class T>
deque<T>::~deque() { while (!empty()) pop_front(); }     

template <class T>
void deque<T>::allocate_at_begin() {
    pointer p = data_allocator.allocate(buffer_size);
    if (!empty()) {
        if (start.node == map) {
            difference_type i = finish.node - start.node;
            map_size = (i + 1) * 2;
            map_pointer tmp = map_allocator.allocate(map_size);
            copy(start.node, finish.node + 1, tmp + map_size / 4 + 1);
            map_allocator.deallocate(map);
            map = tmp;
            map[map_size / 4] = p;
            start = iterator(p + buffer_size, map + map_size / 4);
            finish = iterator(finish.current, map + map_size / 4 + i + 1);
        } else {
            *--start.node = p;
            start = iterator(p + buffer_size, start.node);
        }
    } else {
        map_size = map_allocator.init_page_size();
        map = map_allocator.allocate(map_size);
        map[map_size / 2] = p;
        start = iterator(p + buffer_size / 2 + 1, map + map_size / 2);
        finish = start;
    }
}

template <class T>
void deque<T>::allocate_at_end() {
    pointer p = data_allocator.allocate(buffer_size);
    if (!empty()) {
        if (finish.node == map + map_size - 1) {
            difference_type i = finish.node - start.node;
                 map_size = (i + 1) * 2;
            map_pointer tmp = map_allocator.allocate(map_size);
            copy(start.node, finish.node + 1, tmp + map_size / 4);
            map_allocator.deallocate(map);
            map = tmp;
                 map[map_size / 4 + i + 1] = p;
            start = iterator(start.current, map + map_size / 4);
            finish = iterator(p, map + map_size / 4 + i + 1);
        } else {
            *++finish.node = p;
            finish = iterator(p, finish.node);
        }
    } else {
        map_size = map_allocator.init_page_size();
        map = map_allocator.allocate(map_size);
        map[map_size / 2] = p;
        start = iterator(p + buffer_size / 2, map + map_size / 2);
        finish = start;
    }
}

template <class T>
void deque<T>::deallocate_at_begin() {
    data_allocator.deallocate(*start.node++);
    if (empty()) {
        start = iterator();
        finish = start;
        map_allocator.deallocate(map);
    } else
        start = iterator(*start.node, start.node);
}

template <class T>
void deque<T>::deallocate_at_end() {
    data_allocator.deallocate(*finish.node--);
    if (empty()) {
        start = iterator();
        finish = start;
        map_allocator.deallocate(map);
    } else
        finish = iterator(*finish.node + buffer_size, finish.node);
}

template <class T>
deque<T>::iterator deque<T>::insert(iterator position, const T& x) {
    if (position == begin()) {
        push_front(x);
        return begin();
    } else if (position == end()) {
        push_back(x);
        return end() - 1;
    } else if (end() - position > position - begin()) {
        push_front(*begin());
        copy(begin() + 2, position, begin() + 1); 
        *(position - 1) = x;
        return position - 1;
    } else {
        push_back(*(end() - 1));
        copy_backward(position, end() - 2, end() - 1); 
        *position = x;
        return position;
    }
}

template <class T>
void deque<T>::insert(iterator position, size_type n, const T& x) {
    if (end() - position > position - begin()) {
        iterator old_begin = begin();
        if (n > position - old_begin) {
                 size_type m = n - (position - old_begin);
            while (m-- > 0) push_front(x);
            iterator i = position;
            while (i != old_begin) push_front(*--i);
            fill(old_begin, position, x);
        } else {
            iterator i = old_begin + n;
            while (i != old_begin) push_front(*--i);
            copy(old_begin + n, position, old_begin);
            fill(position - n, position, x);
        }
    } else {
        iterator old_end = end();
        if (n > old_end - position) {
                 size_type m = n - (old_end - position);
            while (m-- > 0) push_back(x);
            iterator i = position;
            while (i != old_end) push_back(*i++);
            fill(position, old_end, x);
        } else {
            iterator i = old_end - n;
            while (i != old_end) push_back(*i++);
            copy_backward(position, old_end - n, old_end);
            fill(position, position + n, x);
        }
    }
}

template <class T>
void deque<T>::insert(iterator position, const T* first, const T* last) {
    size_type n = 0;
    distance(first, last, n);
    if (end() - position > position - begin()) {
        iterator old_begin = begin();
        if (n > position - old_begin) {
                 const T* m = last - (position - old_begin);
            while (m != first) push_front(*--m);
            iterator i = position;
            while (i != old_begin) push_front(*--i);
            copy(last - (position - old_begin), last, old_begin);
        } else {
            iterator i = old_begin + n;
            while (i != old_begin) push_front(*--i);
            copy(old_begin + n, position, old_begin);
            copy(first, last, position - n);
        }
    } else {
        iterator old_end = end();
        if (n > old_end - position) {
                 const T* m = first + (old_end - position);
            while (m != last) push_back(*m++);
            iterator i = position;
            while (i != old_end) push_back(*i++);
                 copy(first, first + (old_end - position), position);
        } else {
            iterator i = old_end - n;
            while (i != old_end) push_back(*i++);
            copy_backward(position, old_end - n, old_end);
            copy(first, last, position);
        }
    }
}

template <class T>
void deque<T>::erase(iterator position) {
    if (end() - position > position - begin()) {
        copy_backward(begin(), position, position + 1);
        pop_front();
    } else {
        copy(position + 1, end(), position);
        pop_back();
    }
}
    
template <class T>
void deque<T>::erase(iterator first, iterator last) {
         difference_type n = last - first;
    if (end() - last > first - begin()) {
        copy_backward(begin(), first, last);
        while(n-- > 0) pop_front();
    } else   {
        copy(last, end(), first);
        while(n-- > 0) pop_back();
    }
}

#undef Allocator
#undef deque

#endif