#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>

struct node {
  int data;
  int key;

  struct node *next;
  struct node *prev;
};

//this link always point to first Link
struct node *head = NULL;

//this link always point to last Link
struct node *last = NULL;

struct node *current = NULL;

//is list empty
bool isEmpty() {
  return head == NULL;
}

int length() {
  int length = 0;
  struct node *current;

  for(current = head; current != NULL; current = current->next){
    length++;
  }

  return length;
}

//display the list in from first to last
void displayForward() {

  //start from the beginning
  struct node *ptr = head;

  //navigate till the end of the list
  printf("\n[ ");

  while(ptr != NULL) {
    printf("(%d,%d) ",ptr->key,ptr->data);
    ptr = ptr->next;
  }

  printf(" ]\n");
}

//display the list from last to first
void displayBackward() {

  //start from the last
  struct node *ptr = last;

  //navigate till the start of the list
  printf("\n[ ");

  while(ptr != NULL) {

    //print data
    printf("(%d,%d) ",ptr->key,ptr->data);

    //move to next item
    ptr = ptr ->prev;

  }
}

//insert link at the first location
void insertFirst(int key, int data) {

  //create a link
  struct node *link = (struct node*) malloc(sizeof(struct node));
  link->key = key;
  link->data = data;

  if(isEmpty()) {
    //make it the last link
    last = link;
  } else {
    //update first prev link
    head->prev = link;
  }

  //point it to old first link
  link->next = head;

  //point first to new first link
  head = link;
}

//insert link at the last location
void insertLast(int key, int data) {

  //create a link
  struct node *link = (struct node*) malloc(sizeof(struct node));
  link->key = key;
  link->data = data;

  if(isEmpty()) {
    //make it the last link
    last = link;
  } else {
    //make link a new last link
    last->next = link;

    //mark old last node as prev of new link
    link->prev = last;
  }

  //point last to new last node
  last = link;
}

//delete first item
struct node* deleteFirst() {

  //save reference to first link
  struct node *tempLink = head;

  //if only one link
  if(head->next == NULL){
    last = NULL;
  } else {
    head->next->prev = NULL;
  }

  head = head->next;
  //return the deleted link
  return tempLink;
}

//delete link at the last location

struct node* deletelast() {
  //save reference to last link
  struct node *tempLink = last;

  //if only one link
  if(head->next == NULL) {
    head = NULL;
  } else {
    last->prev->next = NULL;
  }

  last = last->prev;

  //return the deleted link
  return tempLink;
};

//delete a link with given key

struct node* deleten(int key) {

  //start from the first link
  struct node* current = head;
  struct node* previous = NULL;

  //if list is empty
  if(head == NULL) {
    return NULL;
  }

  //navigate through list
  while(current->key != key) {
    //if it is last node

    if(current->next == NULL) {
      return NULL;
    } else {
      //store reference to current link
      previous = current;

      //move to next link
      current = current->next;
    }
  }

  //found a match, update the link
  if(current == head) {
    //change first to point to next link
    head = head->next;
  } else {
    //bypass the current link
    current->prev->next = current->next;
  }

  if(current == last) {
    //change last to point to prev link
    last = current->prev;
  } else {
    current->next->prev = current->prev;
  }

  return current;
};

bool insertAfter(int key, int newKey, int data) {
  //start from the first link
  struct node *current = head;

  //if list is empty
  if(head == NULL) {
    return false;
  }

  //navigate through list
  while(current->key != key) {

    //if it is last node
    if(current->next == NULL) {
      return false;
    } else {
      //move to next link
      current = current->next;
    }
  }

  //create a link
  struct node *newLink = (struct node*) malloc(sizeof(struct node));
  newLink->key = newKey;
  newLink->data = data;

  if(current == last) {
    newLink->next = NULL;
    last = newLink;
  } else {
    newLink->next = current->next;
    current->next->prev = newLink;
  }

  newLink->prev = current;
  current->next = newLink;
  return true;
}

int main(int argc, char** argv) {
  insertFirst(1,10);
  insertFirst(2,20);
  insertFirst(3,30);
  insertFirst(4,1);
  insertFirst(5,40);
  insertFirst(6,56);

  printf("\nList (First to Last): ");
  displayForward();

  printf("\n");
  printf("\nList (Last to first): ");
  displayBackward();

  printf("\nList , after deleting first record: ");
  deleteFirst();
  displayForward();

  printf("\nList , after deleting last record: ");
  deletenast();
  displayForward();

  printf("\nList , insert after key(4) : ");
  insertAfter(4,7, 13);
  displayForward();

  printf("\nList  , after delete key(4) : ");
  deleten(4);
  displayForward();
}