#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/stat.h>

#define RAND_SOURCE "/dev/urandom"

unsigned int f_flag;
unsigned int u_flag;
unsigned int z_flag;
unsigned int n_loops;

int shred(int rand_fd, int fd) {
	/* Get size */
	off_t size = lseek(fd, 0, SEEK_END);
	if (size <= 0)
		return 1;

	void *buf = mmap(NULL, size, PROT_WRITE, MAP_SHARED, fd, 0);
	if (buf == MAP_FAILED) {
		fprintf(stderr, "shred: mmap: %s\n", strerror(errno));
		return 1;
	}

	for (unsigned int n = 0; n < n_loops; n++) {
		read(rand_fd, buf, size);
		fsync(fd);
	}

	if (z_flag)
		memset(buf, 0, size);

	if (munmap(buf, size)) {
		fprintf(stderr, "shred: munmap: %s\n", strerror(errno));
		return 1;
	}

	return 0;
}

int main(const int argc, const char **argv) {
	n_loops = 3;
	int i;

	for (i = 1; i < argc; i++) {
		if (argv[i][0] != '-')
			break;

		else if (!strcmp(argv[i], "-f"))
			f_flag = 1;

		else if (!strcmp(argv[i], "-u"))
			u_flag = 1;

		else if (!strcmp(argv[i], "-z"))
			z_flag = 1;

		else if (!strncmp(argv[i], "-n=", 3)) {
			char *val = strchr(argv[i], '=');
			if (!val)
				break;

			val[0] = '\0';
			n_loops = atoi(val + 1);
		}

		else if (!strcmp(argv[i], "--help")) {
			printf("shred [-n=N Overwrite N times (default 3)] [-z Final overwrite with zeros] [-u Remove file] [-f Chmod to ensure writability] [file file2...]\n");
			return 1;
		}
	}


	int rand_fd = open(RAND_SOURCE, O_RDONLY);
	if (rand_fd < 0) {
		fprintf(stderr, "shred: %s is %s\n", RAND_SOURCE, strerror(errno));
		return 1;
	}

	for (; i < argc; i++) {
		int fd = open(argv[i], O_RDWR);
		if (fd < 0) {
			fprintf(stderr, "shred: %s is %s\n", argv[i], strerror(errno));
			continue;
		}

		shred(rand_fd, fd);
		close(fd);

		if (u_flag)
			if (unlink(argv[i]) < 0)
				fprintf(stderr, "shred: %s is %s\n", argv[i], strerror(errno));

		if (f_flag)
			if (chmod(argv[i], 0) < 0)
				fprintf(stderr, "shred: %s is %s\n", argv[i], strerror(errno));
	}

	close(rand_fd);
	return 0;
}