queue.h

#ifndef KAHN_QUEUE_H
#define KAHN_QUEUE_H

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

// A structure to represent a queue
struct Queue {
	int front, rear, size;
	unsigned capacity;
	int *array;
};

// Function to create a queue of given capacity.
// It initializes size of queue as 0
struct Queue *createQueue(unsigned capacity) {
	struct Queue *queue = (struct Queue *)malloc(sizeof(struct Queue));
	queue->capacity = capacity;
	queue->front = queue->size = 0;
	queue->rear = capacity - 1; // This is important, see the enqueue
	queue->array = (int *)malloc(queue->capacity * sizeof(int));
	return queue;
}

// Queue is full when size becomes equal to the capacity
int isFull(struct Queue *queue) { return (queue->size == queue->capacity); }

// Queue is empty when size is 0
int isEmpty(struct Queue *queue) { return (queue->size == 0); }

// Function to add an item to the queue.
// It changes rear and size
void enqueue(struct Queue *queue, int item) {
	if (isFull(queue)) { return; }
	queue->rear = (queue->rear + 1) % queue->capacity;
	queue->array[queue->rear] = item;
	queue->size = queue->size + 1;
	printf("%d enqueued to queue\n", item);
}

// Function to remove an item from queue.
// It changes front and size
int dequeue(struct Queue *queue) {
	if (isEmpty(queue)) { return INT_MIN; }
	int item = queue->array[queue->front];
	queue->front = (queue->front + 1) % queue->capacity;
	queue->size = queue->size - 1;
	return item;
}

// Function to get front of queue
int front(struct Queue *queue) {
	if (isEmpty(queue)) { return INT_MIN; }
	return queue->array[queue->front];
}

// Function to get rear of queue
int rear(struct Queue *queue) {
	if (isEmpty(queue)) { return INT_MIN; }
	return queue->array[queue->rear];
}

// Usage
void usage() {
	struct Queue *queue = createQueue(1000);
	enqueue(queue, 10);
	enqueue(queue, 20);
	enqueue(queue, 30);
	enqueue(queue, 40);
	printf("%d dequeued from queue\n\n", dequeue(queue));
	printf("Front item is %d\n", front(queue));
	printf("Rear item is %d\n", rear(queue));
}

#endif