/*
 * The *.m0 format in which the Closedmoko firmware images have been
 * distributed is a form of SREC, but further examination quickly
 * reveals that the data bytes are swapped: the byte order is neither
 * the natural little-endian ARM one of the Calypso, nor a word-wise
 * ARM BE; instead the upper and lower bytes of each 16-bit shortword
 * are swapped relative to the natural byte/word order of the Calypso
 * ARM processor.
 *
 * Before this Closedmoko firmware can be examined with any of the
 * standard reverse eng tools (ARM disassemblers or even the good old
 * strings), it needs to be converted from the weird byte-swapped SREC
 * format to straight binary in the natural byte order.  The present
 * utility accomplishes that conversion.
 *
 * Written by Spacefalcon the Outlaw.
 */

#include <sys/types.h>
#include <stdio.h>
#include <ctype.h>
#include <strings.h>

char *infname;
FILE *inf, *outf;
u_char fillbyte;
char srecbuf[80];
u_char srecbin[40];
int lineno, state;
u_long lastaddr;

u_char header[6] = {0x06, 0x00, 0x00, 'H', 'D', 'R'};

decode_hex_byte(s)
	char *s;
{
	register int u, l;

	if (!isxdigit(s[0]) || !isxdigit(s[1]))
		return(-1);
	if (isdigit(s[0]))
		u = s[0] - '0';
	else if (isupper(s[0]))
		u = s[0] - 'A' + 10;
	else
		u = s[0] - 'a' + 10;
	if (isdigit(s[1]))
		l = s[1] - '0';
	else if (isupper(s[1]))
		l = s[1] - 'A' + 10;
	else
		l = s[1] - 'a' + 10;
	return((u << 4) | l);
}

srec2bin()
{
	register int i, l, b;

	l = decode_hex_byte(srecbuf + 2);
	if (l < 1) {
		fprintf(stderr, "%s line %d: S-record length octet is bad\n",
			infname, lineno);
		exit(1);
	}
	srecbin[0] = l;
	if (l > 35) {
		fprintf(stderr,
			"%s line %d: S-record is longer than expected\n",
			infname, lineno);
		exit(1);
	}
	for (i = 1; i <= l; i++) {
		b = decode_hex_byte(srecbuf + i*2 + 2);
		if (b < 0) {
			fprintf(stderr, "%s line %d: hex decode error\n",
				infname, lineno);
			exit(1);
		}
		srecbin[i] = b;
	}
	return(0);
}

srec_cksum()
{
	u_char accum;
	register int i, len;

	len = srecbin[0] + 1;
	accum = 0;
	for (i = 0; i < len; i++)
		accum += srecbin[i];
	if (accum != 0xFF) {
		fprintf(stderr, "%s line %d: bad checksum\n", infname, lineno);
		exit(1);
	}
	return(0);
}

main(argc, argv)
	char **argv;
{
	register int i;
	u_long curaddr;
	int datalen;

	if (argc < 3 || argc > 4) {
usage:		fprintf(stderr, "usage: %s input.m0 output.bin [fill-byte]\n",
			argv[0]);
		exit(1);
	}
	infname = argv[1];
	inf = fopen(infname, "r");
	if (!inf) {
		perror(infname);
		exit(1);
	}
	if (argc > 3) {
		i = decode_hex_byte(argv[3]);
		if (i >= 0)
			fillbyte = i;
		else
			goto usage;
	} else
		fillbyte = 0;

	state = 0;
	for (lineno = 1; ; lineno++) {
		if (!fgets(srecbuf, sizeof srecbuf, inf)) {
			fprintf(stderr, "%s: premature EOF\n", infname);
			exit(1);
		}
		if (srecbuf[0] != 'S') {
			fprintf(stderr, "%s line %d: not an S-record\n",
				infname, lineno);
			exit(1);
		}
		switch (srecbuf[1]) {
		case '0':
			if (state == 0)
				break;
			else
				goto badtype;
		case '3':
			if (state == 0)
				goto badtype;
			else
				break;
		case '7':
			if (state == 2)
				break;
			else
				goto badtype;
		default:
		badtype:
			fprintf(stderr,
				"%s line %d: S-record type unexpected\n",
				infname, lineno);
			exit(1);
		}
		srec2bin();
		srec_cksum();
		if (state == 0) {
			if (bcmp(srecbin, header, 6)) {
				fprintf(stderr, "%s: expected header missing\n",
					infname);
				exit(1);
			}
			state = 1;
			continue;
		}
		switch (srecbuf[1]) {
		case '3':
			if (srecbin[0] < 6) {
				fprintf(stderr,
					"%s line %d: S3 record is too short\n",
					infname, lineno);
				exit(1);
			}
			curaddr = (srecbin[1] << 24) | (srecbin[2] << 16) |
				  (srecbin[3] << 8) | srecbin[4];
			if (curaddr & 1) {
				fprintf(stderr, "%s line %d: odd address\n",
					infname, lineno);
				exit(1);
			}
			datalen = srecbin[0] - 5;
			if (datalen & 1) {
				fprintf(stderr, "%s line %d: odd data length\n",
					infname, lineno);
				exit(1);
			}
			if (state < 2) {
				outf = fopen(argv[2], "w");
				if (!outf) {
					perror(argv[2]);
					exit(1);
				}
				state = 2;
				lastaddr = 0;
			}
			if (curaddr < lastaddr) {
				fprintf(stderr,
					"%s line %d: address going backwards\n",
					infname, lineno);
				exit(1);
			}
			while (lastaddr < curaddr) {
				putc(fillbyte, outf);
				lastaddr++;
			}
			for (i = 0; i < datalen; i += 2) {
				putc(srecbin[i + 6], outf);
				putc(srecbin[i + 5], outf);
			}
			lastaddr = curaddr + datalen;
			continue;
		case '7':
			fclose(outf);
			exit(0);
		default:
			abort();
		}
	}
}