#include "doubly-linked-list.h"

using namespace seq;

// return the size (number of items in the doubly linked list)
size_t DoublyLinkedList::size() const
{
   size_t count = 0;
   for(DoublyLinkedListNode* n = this->head; n != nullptr; n = n->get_next()) count++;
   return count;
}

// return pointer to the node at position i, counting from 0
// for negative numbers, count from the tail (-1) backward (-2, -3, ...)
DoublyLinkedListNode* DoublyLinkedList::index(int i) const
{
   DoublyLinkedListNode* n = this->head;
   if(i > 0)
   {      
      for(int k = 0; k < i; k++)
      {
         assert(n != nullptr);
         n = n->get_next();
      }
   }
   else  /** in the doubly linked list, we can walk backward, so let us allow it for negative i **/
   {
      DoublyLinkedListNode* n = this->tail;
      for(int k = -1; k > i; k--)
      {
         assert(n != nullptr);
         n = n->get_prev();
      }
   }
   return n;
}

// add an item at the end of the list
void DoublyLinkedList::push_back(const int& pushed_item)
{
   DoublyLinkedListNode* new_node = new DoublyLinkedListNode;
   new_node->set_item(pushed_item);
   
   if(this->empty()) this->head = new_node;
   else
   {
      this->tail->set_next(new_node);
      new_node->set_prev(this->tail);  /** attachment in the doubly linked list goes both ways **/
   }
   this->tail = new_node;
}

// add an item at the beginning of the list
void DoublyLinkedList::push_front(const int& pushed_item)
{
   DoublyLinkedListNode* new_node = new DoublyLinkedListNode;
   new_node->set_item(pushed_item);
   
   if(this->empty()) this->tail = new_node;
   else
   {
      new_node->set_next(this->head);  // FIX BUG from the previous version, which was "new_node->set_next(this->head->get_next());"
      this->head->set_prev(new_node);  // FIX BUG from the previous version, which was "this->head->get_next()->set_prev(new_node);"
   }
   this->head = new_node;
}

// remove the head node and item
void DoublyLinkedList::pop_front()
{
   if(this->empty()) return;  // nothing there to remove
   DoublyLinkedListNode* successor = this->head->get_next();

   if(successor) successor->set_prev(nullptr);  /** we must detach the previous head node from its successor **/   
   delete this->head;
   this->head = successor;  // successor of the previous head is the new head
   if(this->head == nullptr) this->tail = nullptr;  // catch special case: the list is now empty
}

// remove the tail node and item
/** note how this is much easier for the doubly than for the singly linked list;        **
 ** we can simply take the "pop_front()" implementation and do everything symmetrically **/
void DoublyLinkedList::pop_back()
{
   if(this->empty()) return;  // nothing there to remove
   DoublyLinkedListNode* predecessor = this->tail->get_prev();
   
   if(predecessor) predecessor->set_next(nullptr);
   delete this->tail;
   this->tail = predecessor;  /** predecessor of the previous tail is the new tail **/
   if(this->tail == nullptr) this->head = nullptr;  /** catch special case: the list is now empty **/
}

// insert an item at index i
// for negative numbers, count from the tail (-1) backward (-2, -3, ...)
void DoublyLinkedList::insert_at(int i, const int& inserted_item)
{
   if(i == 0)
   {
      this->push_front(inserted_item);
      return;
   }
   else if(i == -1)
   {
      this->push_back(inserted_item);
      return;
   }
   DoublyLinkedListNode* predecessor = this->index(i-1);
   this->insert_successor_to(predecessor, inserted_item);
}

// remove the item at index i
// for negative numbers, count from the tail (-1) backward (-2, -3, ...)
void DoublyLinkedList::erase_at(int i)
{
   if(i == 0)
   {
      this->pop_front();
      return;
   }
   else if(i == -1)
   {
      this->pop_back();
      return;
   }
   DoublyLinkedListNode* erased_node = this->index(i);
   this->erase_node(erased_node);
}

// insert an item after given node
void DoublyLinkedList::insert_successor_to(DoublyLinkedListNode* predecessor, const int& inserted_item)
{
   DoublyLinkedListNode* new_node =  new DoublyLinkedListNode;
   new_node->set_item(inserted_item);
   
   DoublyLinkedListNode* successor = predecessor->get_next();
   predecessor->set_next(new_node);
   new_node->set_prev(predecessor); /** attach both ways **/
   new_node->set_next(successor);
   
   if(!successor) this->tail = new_node;
   else successor->set_prev(new_node);  /** attach both ways **/
}

/** remove "erased_node" from the doubly linked list **/
void DoublyLinkedList::erase_node(DoublyLinkedListNode* erased_node)
{
   if(erased_node->get_prev() == nullptr)  /** erase the head **/
   {
      this->pop_front();
      return;
   }
   if(erased_node->get_next() == nullptr)  /** erase the tail **/
   {
      this->pop_back();
      return;
   }
   
   /** now attach predecessor and successor to each other **/
   erased_node->get_prev()->set_next(erased_node->get_next());
   erased_node->get_next()->set_prev(erased_node->get_prev());

   delete erased_node;  /** "erased_node" detached now, we can delete it **/
}