libtommath/bn_mp_export.c

#include <tommath.h>
#ifdef BN_MP_EXPORT_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
 *
 * LibTomMath is a library that provides multiple-precision
 * integer arithmetic as well as number theoretic functionality.
 *
 * The library was designed directly after the MPI library by
 * Michael Fromberger but has been written from scratch with
 * additional optimizations in place.
 *
 * The library is free for all purposes without any express
 * guarantee it works.
 *
 * Tom St Denis, tstdenis82@gmail.com, http://libtom.org
 */

/* based on gmp's mpz_export.
 * see http://gmplib.org/manual/Integer-Import-and-Export.html
 */
int mp_export(void* rop, size_t* countp, int order, size_t size, 
                                int endian, size_t nails, mp_int* op) {
	int result;
	size_t odd_nails, nail_bytes, i, j, bits, count;
	unsigned char odd_nail_mask;

	mp_int t;

	if ((result = mp_init_copy(&t, op)) != MP_OKAY) {
		return result;
	}

	if (endian == 0) {
		union {
			unsigned int i;
			char c[4];
		} lint = {0x01020304};

		endian = (lint.c[0] == 4 ? -1 : 1);
	}

	odd_nails = (nails % 8);
	odd_nail_mask = 0xff;
	for (i = 0; i < odd_nails; ++i) {
		odd_nail_mask ^= (1 << (7 - i));
	}
	nail_bytes = nails / 8;

	bits = mp_count_bits(&t);
	count = bits / (size * 8 - nails) + (bits % (size * 8 - nails) ? 1 : 0);

	for (i = 0; i < count; ++i) {
		for (j = 0; j < size; ++j) {
			unsigned char* byte = (
				(unsigned char*)rop + 
				(order == -1 ? i : count - 1 - i) * size + 
				(endian == -1 ? j : size - 1 - j)
			);

			if (j >= (size - nail_bytes)) {
				*byte = 0;
				continue;
			}

			*byte = (unsigned char)(j == size - nail_bytes - 1 ? (t.dp[0] & odd_nail_mask) : t.dp[0] & 0xFF);

			if ((result = mp_div_2d(&t, (j == size - nail_bytes - 1 ? 8 - odd_nails : 8), &t, NULL)) != MP_OKAY) {
				mp_clear(&t);
				return result;
			}
		}
	}

	mp_clear(&t);

	if (countp) {
		*countp = count;
	}

	return MP_OKAY;
}

#endif

/* $Source$ */
/* $Revision$ */
/* $Date$ */