#include "stdstuff.h"
#include "List.h"

// the destructor destroys the list one node at a time, starting from the head
List::~List () {

  Node *t;
  while (head != NULL) {
    t = head; head = head -> next; delete t;
  }

}


// prints the list to cout one node at a time, starting with the head
void List::outputList () const {
  
  Node *c = head;
  int i = 0;

  cout << "The list contains:" << endl;

  while (c != NULL) {
    cout << "  " << c -> data;
    if (++i == 10) {
      cout << endl; i = 0;
    }
    c = c -> next;
  }
  if (i != 0) cout << endl;

}


// inserts a node into the list at the head of the list
void List::insert (int data) {

  head = new Node (data, head);

}


// Method "sumEvenValues" returns the sum of the nodes in the list containing even values.
// Note that "containing" refers to the *contents* of the node, not its position in the list.
// If the list has no even values, it returns 0.
int List::sumEvenValues() const {

   int sum = 0;

   for (Node* c = head; c!=NULL; c=c->next) { // check every node
      if (c->data%2==0)  // if it's even, add to the sum
         sum += c->data;
   }

   return sum;

}


// Method "deleteEveryThirdNode" deletes the 3rd, 6th, 9th, etc. node of the list.
// Note that the list starts with the first (not "zeroth") node.
// If the list has fewer than three nodes, this method has no effect.
void List::deleteEveryThirdNode() {

   Node *c = head, *p, *temp;

   while (c!=NULL && c->next!=NULL && c->next->next!=NULL) { // while there are at least 3 nodes left to check

      p = c->next;
      c = c->next->next; // c now points to the node to delete, and p to the previous node (and p is never NULL)

      p->next = c->next; // make previous node point to next node in preparation for deletion
      
      temp = c; 
      c = c->next; // move on to next node
      delete temp; // and delete the one that was in the 3rd, 6th, etc. position in the list

   }

}