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

#define MAXITER   256 * 100
#define MAX(x,y)  ((x) > (y) ? (x) : (y))
#define MIN(x,y)  ((x) < (y) ? (x) : (y))

int
main(int argc, const char **argv)
{
	const char *hashstr;
	char c, hexbuf[3] = { 0 }, *end, *buf;
	int i, k, v, maxlen, sum, *hash, sublen, aftersum;

	if (argc < 2) {
		fprintf(stderr, "USAGE: revhash <hash>\n");
		return EXIT_FAILURE;
	}

	hashstr = argv[1];
	if (strlen(hashstr) % 2 != 0)
		goto invalid;

	/* alloc */
	maxlen = strlen(hashstr) / 2;
	hash = calloc(maxlen, sizeof(int));
	buf = malloc(strlen(hashstr));
	if (!hash) return EXIT_FAILURE;

	/* convert hex to int array */
	for (i = 0; i < maxlen; i++) {
		memcpy(hexbuf, hashstr + 2 * i, 2);
		hash[i] = strtol(hexbuf, &end, 16);
		if (end && *end) goto invalid;
	}

	/* bruteforce srand seed */
	for (i = 0; i < MAXITER; i++) {
		srand(i);

		/* reverse chars for given sum */
		for (sum = i, k = 0; k < maxlen && sum > 0; k++) {
			buf[k] = (char) hash[k] ^ (rand() % 256);
			if (buf[k] < 0) break;
			sum -= buf[k];
		}

		/* repeat if too short */
		if (k && sum == 0) {
			sublen = k;
			for (k = sublen; k < maxlen; k++) {
				buf[k] = (char) hash[k] ^ (rand() % 256);
				if (buf[k] < 0 || buf[k] != buf[k % sublen]) break;
			}
		}

		if (k < maxlen) continue;

		/* output first part we know */
		printf("%.*s", sublen, buf);

		/* add rest of chars */
		while (sum > 0) {
			c = MIN(127, sum);
			printf("%c", c);
			sum -= c;
		}

		printf("\n");
		return EXIT_SUCCESS;
	}

	return EXIT_FAILURE;

invalid:
	fprintf(stderr, "Invalid hash string!\n");
	return EXIT_FAILURE;
}