encode.c

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdint.h>
#include <string.h>
#include <errno.h>

int main(int argc, char *argv[])
{
	int infile = STDIN_FILENO, outfile = STDOUT_FILENO;

	switch (argc) {
	case 3:
		outfile = open(argv[1], O_WRONLY | O_CREAT);
		if (outfile == -1) {
			fprintf(stderr, "could not open file %s: %s\n",
					argv[1], strerror(errno));
			return 1;
		}
	case 2:
		infile = open(argv[2], O_RDONLY);
		if (infile == -1) {
			fprintf(stderr, "could not open file %s: %s\n",
					argv[2], strerror(errno));
			return 1;
		}
		break;
	case 1:
		break;
	default:
		fprintf(stderr, "incorrect number of arguments (max 2)\n");
		return 2;
	}

	char c;
	int mode = 0;
	uint8_t cc = 0;

	/* write header bytes */
	write(outfile, "OW", 2);

	/* compress */
	while (read(infile, &c, 1) == 1) {
		for (int i = 0; i < 8; i++) {
			if (mode == (c & 1)) {
				cc++;
				if (cc == 255) {
					write(outfile, &cc, 1);
					cc = 0;
					mode = !mode;
				}
			} else {
				write(outfile, &cc, 1);
				cc = 1;
				mode = !mode;
			}

			c >>= 1;
		}
	}

	/* cleanup */
	if (infile != STDIN_FILENO)
		close(infile);

	if (outfile != STDOUT_FILENO)
		close(outfile);
}