#ifndef LIST_H
#define LIST_H

#include <iostream>                                                      

using namespace std;

struct element{                //declaration of a new structure
    element* nextel;      //a pointer to the element structure called nextel. It will hold the address of the next element in the list
    element* prevel;      //a pointer to the element structure called prevel. It will hold the address of the previous element in the list
    int data;                  //just the data that each element will hold
};

class list{                        //declaration of a new class
public:
    static element* NewElement();                             //A function used to create a new element in the list with a return type element*
    static element* SearchElement(int searchitem);     //A function used to search for elements in the list with a return type element*
    static element* DeleteElement(int searchitem);      //A function used to delete elements in the list with a return type element*

private:
    static element* FirstEl;    //a pointer to the element structure called FirstEl. It will hold the address of the first element in the list
    static element* LastEl;    //a pointer to the element structure called LastEl. It will hold the address of the last element in the list
};

#endif

#include <iostream>
#include "list.h"

using namespace std;

element* list::FirstEl = NULL;      //This initialises the static element* 'FirstEl' to NULL
element* list::LastEl = NULL;      //This initialises the static element* 'LastEl' to NULL
int data = 0;

element* list::NewElement(){    
    element* currentel;                      //This creates a new pointer to element. This will hold the address of the new element
    if (FirstEl == NULL){                     //This checks whether FirstEl is equal to NULL
        currentel = new element;     //This creates the new element and makes currentel equal to it
        FirstEl = currentel;               //This makes FirstEl equal to the new element
        LastEl = currentel;               //This makes LastEl equal to the new element
        currentel->nextel =  NULL;   //This makes the new elements, next element equal to NULL
        currentel->prevel = NULL;    //This makes the new elements, previous element equal to NULL
        currentel->data = data;  
        data++;
        return currentel;
    }
    if (FirstEl != NULL){                     //This checks whether FirstEl is not equal to NULL
        currentel = new element;    //This creates the new element and makes currentel equal to it
        currentel->nextel = NULL;   //This makes the new elements, next element equal to NULL
        LastEl->nextel = currentel;  //This makes the last elements (in the list), next element equal to the new element
        currentel->prevel = LastEl;  //This makes the new elements, previous element equal to the last element in the list
        LastEl = currentel;              //This makes LastEl equal to the new element
        currentel->data = data;
        data++;
        return currentel;
    }
    return NULL;
}

element* list::SearchElement(int searchitem){  //This traverses through the list, and is realy very simple so doesn't need explaining
    element* currentel;
    if (FirstEl == NULL){
        system("CLS");
        cout << "There are no elements in the list\n";
        return NULL;
    }
    if (FirstEl != NULL){
        currentel = FirstEl;
        while (currentel != NULL){
            if (currentel->data == searchitem){
                return currentel;
            }
            else
                currentel = currentel->nextel;
        }
    }
    system("CLS");
    cout << searchitem << " was not found in the list\n";
    return NULL;
}

element* list::DeleteElement(int searchitem){                    //This function deletes an element anywhere within the list
    element* deleteptr;
    deleteptr = SearchElement(searchitem);

    if (deleteptr == FirstEl && deleteptr == LastEl){         //This checks the element to be deleted is equal to the fist and last element
        FirstEl = NULL;                                                //This makes FirstEl equal to NULL
        LastEl = NULL;                                                //This makes LastEl equal to NULL
        delete deleteptr;                                             //This deletes the element from the list
        return NULL;
    }
    if (deleteptr == FirstEl && deleteptr != LastEl){          //This checks the element to be deleted is equal to the fist but notlast element
        FirstEl = deleteptr->nextel;                              //This makes FirstEl equal to the element to be deleted, next element
        deleteptr->nextel->prevel = NULL;                   //This makes t next element, previous element equal to NULL
        delete deleteptr;                                             //This deletes the element from the list
        return NULL;
    }
    if (deleteptr != FirstEl && deleteptr == LastEl){         //This checks the element to be deleted is equal to the last but notfist element
        LastEl = deleteptr->prevel;                             //This makes LastEl equal to the element to be deleted, previous element
        deleteptr->prevel->nextel = NULL;                  //This makes t previous element, next element equal to NULL
        delete deleteptr;                                            //This deletes the element from the list
        return NULL;
    }
    if (deleteptr != FirstEl && deleteptr != LastEl){           //This checks the element to be deleted is equal to the fist and last element
        deleteptr->prevel->nextel = deleteptr->nextel; //This statement links the previous element to the next element in the list
        deleteptr->nextel->prevel = deleteptr->prevel; //This statement links the next element to the previous element in the list
        delete deleteptr;                                             //This deletes the element from the list
        return NULL;
    }
    
    if (deleteptr == NULL){
        system("CLS");
        cout << "No such item found within the list to delete\n";
    }
    return NULL;
}