#include <ctype.h> /* isdigit(3) */
#include <stdio.h> /* fprintf(3), getc(3), putc(3) */
#include <stdlib.h> /* stdin, stderr, stdout */
#include <sysexits.h> /* EX_DATAERR, EX_OK */

/* Unicode compatibility coming someday */
#define GETC getc
#define PUTC putc

#define NUMBER_TERMINATOR '*'

/* syntax error if a decoding file ends with [char][char] */
/* if(STRICT) just print it out */
#define STRICT 0

static int decode(FILE *input, FILE *output){
	int c[2];
	unsigned int num;
	enum{NUMBER_PARSING = 1} state;

	c[0] = EOF;

	/* reading -> (current char == prev char)> number parsing -> reading */
	while((c[1] = GETC(input)) != EOF)
		if(state == NUMBER_PARSING){
			if(isdigit(c[1]))
				num = num * 10 + c[1] - '0';
			else if(c[1] == NUMBER_TERMINATOR){
				for(; num > 0; --num)
					PUTC(c[0], output);
				c[0] = EOF; c[1] = EOF;
				state = 0;
			}else
				return EX_DATAERR;
		}else if(c[1] == c[0]){
			num = 0;
			state = NUMBER_PARSING;
		}else{
			if(c[0] != EOF)
				PUTC(c[0], output);
			c[0] = c[1];
		}

	if(state == NUMBER_PARSING && !STRICT)
		/* it doesn't make sense to put this in a loop */
		{ PUTC(c[0], output); PUTC(c[0], output); }
	else if(state == NUMBER_PARSING)
		return EX_DATAERR;
	else if(c[0] != EOF)
		PUTC(c[0], output);

	return EX_OK;
}

static int encode(FILE *input, FILE *output){
	int c[2];
	unsigned int num;
	enum{COUNTING = 1} state;

	/* It was more fun to use gotos than to use sane structure. */
	for(c[0] = EOF, num = 2, state = 0; ;)
		if((c[1] = GETC(input)) == EOF){
			if(state == COUNTING)
				goto dump;
			else
				goto place;
		}else if(state == COUNTING){
			if(c[1] != c[0]){
dump:				PUTC(c[0], output);
				fprintf(output,
					"%d%c",
					num, NUMBER_TERMINATOR
				);
				num = 2;
				state = 0;
				goto next;
			}else
				++num;
		}else if(c[1] == c[0])
			state = COUNTING;
		else{ /* c[1] != c[0] */
			if(c[0] != EOF) /* c[0] will be EOF at first */
place:				PUTC(c[0], output);
next:			if(c[1] == EOF)
				return EX_OK;
			c[0] = c[1];
		}
}

int main(int argc, char *argv[]){
	int r;

	if(argc != 1){
		fprintf(stderr,
			"Usage: %s\n",
			argv[0] == NULL ?
				(f == decode ? "decode" : "encode")
			: argv[0]
		);
		return EX_USAGE;
	}

	if((r = f(stdin, stdout)) == EX_DATAERR)
		fprintf(stderr, "%s: syntax error.\n", argv[0]);

	return r;
}