#include <stdio.h>

#define QUEUE_SIZE 10

/*
 * Simple queue algorithm implementation
 *
 * tail will always point to the next available slot
 */
struct queue {
	int data[QUEUE_SIZE];
	int head;
	int tail;
};

int queue_empty(struct queue *Q)
{
	return Q->head == Q->tail;
}

/*
 * Queue is full when:
 * tail + 1 == HEAD or
 * TAIL == QUEUE_SIZE - 1 and HEAD == 0
 */
int queue_full(struct queue *Q)
{
	if (((Q->tail + 1) == Q->head) ||
	    ((Q->tail == (QUEUE_SIZE - 1)) && (Q->head == 0)))
		return 1;
	else
		return 0;
}

int queue_push(struct queue *Q, int value)
{
	if (queue_full(Q)) {
	    printf("Queue is full, avoiding overflow\n");
	    return 0;
	}

	Q->data[Q->tail] = value;

	if (Q->tail == (QUEUE_SIZE - 1))
		Q->tail = 0;
	else
		Q->tail++;

	return value;

}

/* FIXME: function return 0 in case of failure, should return something
 *	  else, once 0 is a valid value to the queue
 */
int queue_pop(struct queue *Q)
{
	int value = 0;

	if (queue_empty(Q)) {
		printf("Queue is empty, avoiding underflow\n");
		return value;
	}

	value = Q->data[Q->head];

	if (Q->head == (QUEUE_SIZE - 1))
		Q->head = 0;
	else
		Q->head++;

	return value;
}

void print_queue(struct queue *Q)
{
	int i = Q->head;
	int pos = 1;

	if (queue_empty(Q)) {
		printf("Nothing to print, queue is empty\n");
		return;
	}

	printf(" Pos  | Tail \n");
	for (;;) {
		printf(" %d | %d \n", pos, Q->data[i]);

		if (i == (QUEUE_SIZE - 1))
			i = 0;
		else
			i++;
		pos++;

		if (i == Q->tail)
			break;
	}
}

void debug_queue(struct queue *Q)
{
	int i;

	for (i = 0; i < QUEUE_SIZE; i++) {
		printf("| %d | ", Q->data[i]);

		if (i == Q->head)
			printf("<-- HEAD ");
		if (i == Q->tail)
			printf("<-- TAIL");
		printf("\n");
	}
}

int main(void)
{
	struct queue my_queue;
	int i, val;
	char cmd;

	/* Init queue */
	my_queue.head = 0;
	my_queue.tail = 0;
	for (i = 0; i < QUEUE_SIZE; i++)
		my_queue.data[i] = 0;

	while (1) {
		printf("Select an option: ");
		scanf(" %c", &cmd);

		switch (cmd) {
			case 'a':
				printf("Value to add: ");
				scanf(" %d", &val);
				queue_push(&my_queue, val);
				break;
			case 'p':
				queue_pop(&my_queue);
				break;
			case 'd':
				debug_queue(&my_queue);
				break;
			case 'l':
				print_queue(&my_queue);
				break;
			default:
				printf("Invalid option\n");
				printf("a = add, p = pop, d = debug, l = print\n");
		}
	}

	return 0;
}