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

void byteswap_4(char *b, int size) {
	while (size >= 4) {
		char b0 = b[0];
		char b1 = b[1];
		b[0] = b[3];
		b[1] = b[2];
		b[2] = b1;
		b[3] = b0;
		size -= 4;
		b += 4;
	}
}

void byteswap_2(char *b, int size) {
	while (size >= 2) {
		char b0 = b[0];
		b[0] = b[1];
		b[1] = b0;
		size -= 2;
		b += 2;
	}
}

#define BUFSIZE 4096 /* should be multiple of 4 */

int main(int argc, char **argv) {
	FILE *in = stdin;
	FILE *out = stdout;
	char buf[BUFSIZE];
	int mode = 4;

	if (argc >= 2) {
		if (!strcmp(argv[1], "--help")) {
			printf(
				"Usage: byteswap [-2] <infile >outfile\n"
				"\n"
				"-2 means 16 bit word byteswap.\n"
				"Otherwise it does 32 bit word byteswap.\n"
				"Stdin and Stdout are used.\n"
				);
			exit(0);
		}
		else if (!strcmp(argv[1], "-2"))
			mode = 2;
	}

	while (!feof(in)) {
		int n= fread(buf, 1, BUFSIZE, in);
		if (mode == 2)
			byteswap_2(buf, n);
		else
			byteswap_4(buf, n);
		fwrite(buf, 1, n, out);
	}

	return 0;
}