/*
 * Convert a file containing ASCII integers into packed BCD.
 */
#define _POSIX_C_SOURCE 200809L
#include <ctype.h>
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

static int	worker(char*, size_t, char*, size_t);
static int	nextchar(char*, size_t, size_t*, ssize_t*);

extern int
main()
{
	struct stat st;
	size_t inbuflen, outbuflen;
	char *inbuf, *outbuf;

	if (fstat(STDIN_FILENO, &st) == -1) {
		perror("Cannot stat stdin");
		return EXIT_FAILURE;
	}

	inbuflen = (size_t)st.st_blksize;

	if (fstat(STDOUT_FILENO, &st) == -1) {
		perror("Cannot stat stdout");
		return EXIT_FAILURE;
	}

	outbuflen = (size_t)st.st_blksize;

	inbuf = malloc(inbuflen);
	outbuf = malloc(outbuflen);

	if (inbuf == NULL || outbuf == NULL) {
		perror("Cannot malloc");
		return EXIT_FAILURE;
	}

	return worker(inbuf, inbuflen, outbuf, outbuflen);
}

static int
worker(char *inbuf, size_t inbuflen, char *outbuf, size_t outbuflen)
{
	size_t inidx = 0, outidx = 0;
	ssize_t incount = 0, outcount;
	int high, low;

	errno = 0;

	for (;;) {
		high = nextchar(inbuf, inbuflen, &inidx, &incount);
		if (high == EOF)
			break;

		low = nextchar(inbuf, inbuflen, &inidx, &incount);
		if (low == EOF)
			break;

		outbuf[outidx++] = high << 4 | low;
		if (outidx < outbuflen)
			continue;

		outidx = 0;
		outcount = write(STDOUT_FILENO, outbuf, outbuflen);
		if (outcount < 0) {
			perror("Error writing to stdout");
			break;
		} else if (outcount < outbuflen) {
			fprintf(stderr, "Short write on stdout");
			return 1;
		}
	}

	return errno != 0;
}

static int
nextchar(char *inbuf, size_t inbuflen, size_t *inidx, ssize_t *incount)
{

	do {
		if (*inidx < *incount)
			continue;

		*inidx = 0;
		*incount = read(STDIN_FILENO, inbuf, inbuflen);
		if (*incount < 0)
			perror("Error reading stdin");

		if (*incount <= 0)
			return EOF;
	} while (!isdigit(inbuf[(*inidx)++]));

	return (unsigned char)inbuf[*inidx - 1] - '0';
}