Check if a simple transform before applying zstd reduce file size

simple_transform_before_zstd.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <time.h>
#include <unistd.h>

char buf[1024*1024];
volatile uint64_t sink;

#define USEC_IN_SEC 1000000ULL
#define NSEC_IN_USEC 1000ULL

int read_all(char* buf, size_t buf_size, char *file_name);

int main(int argc, char** argv) {
	int narg;
	if (argc < 2) {
		fputs("too little argumetns\n", stdout);
		return 1;
	}
	uint64_t total_usec = 0;
	uint64_t total_bytes = 0;
	for (narg = 1; narg < argc; narg++) {
		uint64_t i;
		int file_size_bytes = read_all(buf, sizeof(buf), argv[narg]);
		uint64_t* ptr = (uint64_t*)&buf[0];
		struct timespec before, after;

		printf("running %d iterations\n", file_size_bytes);
		clock_gettime(CLOCK_MONOTONIC, &before);
		__sync_synchronize();
		for (i=1; i<file_size_bytes/sizeof(*ptr); i++) {
			ptr[i] = ptr[i-1]-ptr[i];
		}
		__sync_synchronize();
		clock_gettime(CLOCK_MONOTONIC, &after);

		total_bytes += file_size_bytes;
		total_usec += ((uint64_t)after.tv_sec - before.tv_sec) * USEC_IN_SEC +
			((uint64_t)after.tv_nsec - before.tv_nsec) / 1000;

		// make sure compile is obligated to do the above change
		for (i=0; i<file_size_bytes/sizeof(*ptr); i++) {
			sink = ptr[i];
		}
	}
	const double bytes_in_MB = 1024 * 1024;
	printf("%lu bytes (%.03f MB) took %lu usecs\n", total_bytes, total_bytes / bytes_in_MB, total_usec);
	double bytes_in_sec = ((double)total_bytes / (double)total_usec) * USEC_IN_SEC;
	printf("%.03f MB in sec\n", bytes_in_sec / bytes_in_MB);
}

int read_all(char* buf, size_t buf_size, char *file_name) {
	int fd = open(file_name, O_RDONLY);
	if (fd < 0) {
		perror("error opening file");
		exit(2);
	}
	size_t bytes_read_so_far = 0;
	ssize_t bytes_read;
	while ((bytes_read = read(fd, buf+bytes_read_so_far, buf_size-bytes_read_so_far)) != 0) {
		if (bytes_read < 0) {
			perror("error reading file");
			close(fd);
			exit(1);
		}
		bytes_read_so_far += bytes_read;
	}
	close(fd);
	return bytes_read_so_far;
}