/*
 * double-buffering - an example of double-buffering with pthreads
 *
 * Copyright (C) 2013  Antonio Ospite <ospite@studenti.unina.it>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

/*
 * NOTE: the synchronization scheme used in this program assumes that ALL the
 * buffers produced will be consumed, each exactly ONCE.
 *
 * In other cases when buffer skipping or buffer duplication is wanted the
 * synchronization scheme can change.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#include <pthread.h>
#include <time.h>

#define BUFFER_SIZE 10
#define MAXLEN 1024

/* 
 * The time spent by the producer and the consumer thread is quite predictable
 * in this program, this may not be the case when some communication with
 * actual hardware influences the production or consumption phase.
 */
#define PRODUCER_DELAY 50000
#define CONSUMER_DELAY 20000

#define NUM_ITERATIONS 23

struct buffer {
	int *data;
	unsigned int size;
};

struct double_buffer_context {
	struct buffer buffer[2];
	int back_buffer_index;
	int front_buffer_is_consumable;
	pthread_cond_t front_buffer_cond;
	pthread_mutex_t mutex;
};

#define BACK_BUFFER(ctx) ((ctx)->buffer[(ctx)->back_buffer_index])
#define FRONT_BUFFER(ctx) ((ctx)->buffer[(ctx)->back_buffer_index ^ 1])
#define SWAP_BUFFERS(ctx) ((ctx)->back_buffer_index ^= 1)

static void do_log(char *message)
{
	struct timespec now;
	double timestamp;

	clock_gettime(CLOCK_MONOTONIC, &now);
	timestamp = now.tv_sec + now.tv_nsec / 1000000000.0;
	fprintf(stderr, "[%f] %s\n", timestamp, message);
}

/* the consumer thread will only read from the front buffer */
static void *consumer_cb(void *arg)
{
	struct double_buffer_context *ctx = (struct double_buffer_context *) arg;
	unsigned int i;

	char message[MAXLEN];
	int len;

	while(1) {
		pthread_mutex_lock(&ctx->mutex);
		while (!ctx->front_buffer_is_consumable) {
			pthread_cond_wait(&ctx->front_buffer_cond, &ctx->mutex);
		}
		pthread_mutex_unlock(&ctx->mutex);

		/* consume front buffer */
		len = snprintf(message, MAXLEN, "Consuming value: ");
		for (i = 0; i < FRONT_BUFFER(ctx).size; i++) {
			len += snprintf(message + len, MAXLEN - len, "%d ", FRONT_BUFFER(ctx).data[i]);
		}
		usleep(CONSUMER_DELAY);
		do_log(message);

		pthread_mutex_lock(&ctx->mutex);
		ctx->front_buffer_is_consumable = 0;
		pthread_cond_signal(&ctx->front_buffer_cond);
		pthread_mutex_unlock(&ctx->mutex);
	}

	pthread_exit(0);
	return (void *) 0;
}

int main(void)
{
	int ret;
	pthread_t consumer_tid;
	struct double_buffer_context *ctx;

	unsigned int iterations;
	unsigned int i;

	char message[MAXLEN];
	int len;

	ctx = malloc(sizeof(*ctx));
	if (ctx == NULL) {
		perror("malloc");
		exit(EXIT_FAILURE);
	}
	memset(ctx, 0, sizeof(*ctx));

	ctx->buffer[0].data = malloc(BUFFER_SIZE * sizeof(*ctx->buffer[0].data));
	ctx->buffer[0].size = BUFFER_SIZE;
	ctx->buffer[1].data = malloc(BUFFER_SIZE * sizeof(*ctx->buffer[1].data));
	ctx->buffer[1].size = BUFFER_SIZE;

	ctx->back_buffer_index = 0;
	ctx->front_buffer_is_consumable = 0;
	pthread_mutex_init(&ctx->mutex, NULL);
	pthread_cond_init(&ctx->front_buffer_cond, NULL);

	ret = pthread_create(&consumer_tid, NULL, consumer_cb, (void *)ctx);
	if (ret != 0)
	{
		fprintf(stderr, "can't create thread: %s\n", strerror(ret));
		exit(1);
	}

	/* fixed seed, always the same sequence */
	srand(0xa02);
	iterations = NUM_ITERATIONS;
	while (iterations--) {
		/* produce: in the back buffer */
		len = snprintf(message, MAXLEN, "Producing value: ");
		for (i = 0; i < BACK_BUFFER(ctx).size; i++) {
			BACK_BUFFER(ctx).data[i] = rand();
			len += snprintf(message + len, MAXLEN - len, "%d ", BACK_BUFFER(ctx).data[i]);
		}
		usleep(PRODUCER_DELAY);
		do_log(message);

		pthread_mutex_lock(&ctx->mutex);
		while (ctx->front_buffer_is_consumable) {
			pthread_cond_wait(&ctx->front_buffer_cond, &ctx->mutex);
		}

		SWAP_BUFFERS(ctx);

		/* signal that the front buffer is ready to be consumed */
		ctx->front_buffer_is_consumable = 1;
		pthread_cond_signal(&ctx->front_buffer_cond);

		pthread_mutex_unlock(&ctx->mutex);
	}

	/* wait for the last consumer run before terminating the thread */
	pthread_mutex_lock(&ctx->mutex);
	while (ctx->front_buffer_is_consumable) {
		pthread_cond_wait(&ctx->front_buffer_cond, &ctx->mutex);
	}
	pthread_mutex_unlock(&ctx->mutex);

	pthread_cancel(consumer_tid);
	pthread_join(consumer_tid, NULL);

	free(ctx->buffer[1].data);
	free(ctx->buffer[0].data);
	free(ctx);

	exit(0);
}