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

struct list {
   int head;
   struct list *tail;
};

typedef struct list list;

typedef struct {
   list *front;
   list *back;
} queue;

void list_free(list *lst) {
   list *p;

   while (lst != NULL) {
      p = lst;
      lst = lst->tail;
      free(p);
   }
}

list * cons(int val, list *lst) {
   list *new = malloc(sizeof(list));
   if (new == NULL) abort();

   new->head = val;
   new->tail = lst;

   return new;
}

void list_print(list *lst) {
   list *p = lst;

   while (p != NULL) {
      printf("%d ", p->head);
      p = p->tail;
   }
}

queue * make_queue() {
   queue *new = malloc(sizeof(queue));
   if (new == NULL) abort();

   new->front = NULL;
   new->back = NULL;

   return new;
}

void push(int val, queue *q) {
   if (q->front == NULL) { // empty queue
      q->front = cons(val, NULL);
      q->back = q->front;
   }

   else {
      list *new = cons(val, NULL);
      (q->back)->tail = new;
      q->back = new;
   }
}

int pop(queue *q) {
   int val = (q->front)->head;
   list *newhead = (q->front)->tail;
   (q->front)->tail = NULL;
   list_free(q->front);
   q->front = newhead;

   return val;
}

void queue_print(queue *q) {
   printf("back ");
   list_print(q->front);
   printf("front\n");
}

void queue_free(queue *q) {
   list_free(q->front);
   free(q);
}

int main() {
   queue *q = make_queue();
   push(5, q);
   push(3, q);
   push(1, q);
   queue_print(q);
   printf("popped %d!\n", pop(q));
   queue_print(q);
   queue_free(q);

   return 0;
}