#include <tommath.h>
#ifdef BN_MP_REDUCE_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, tomstdenis@gmail.com, http://libtom.org
 */

/* reduces x mod m, assumes 0 < x < m**2, mu is 
 * precomputed via mp_reduce_setup.
 * From HAC pp.604 Algorithm 14.42
 */
int mp_reduce(mp_int * x, mp_int * m, mp_int * mu)
{
	mp_int q;
	int res, um = m->used;

	/* q = x */
	if ((res = mp_init_copy(&q, x)) != MP_OKAY) {
		return res;
	}

	/* q1 = x / b**(k-1)  */
	mp_rshd(&q, um - 1);

	/* according to HAC this optimization is ok */
	if (((unsigned long)um) > (((mp_digit) 1) << (DIGIT_BIT - 1))) {
		if ((res = mp_mul(&q, mu, &q)) != MP_OKAY) {
			goto CLEANUP;
		}
	} else {
#ifdef BN_S_MP_MUL_HIGH_DIGS_C
		if ((res = s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
			goto CLEANUP;
		}
#elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C)
		if ((res = fast_s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
			goto CLEANUP;
		}
#else
		{
			res = MP_VAL;
			goto CLEANUP;
		}
#endif
	}

	/* q3 = q2 / b**(k+1) */
	mp_rshd(&q, um + 1);

	/* x = x mod b**(k+1), quick (no division) */
	if ((res = mp_mod_2d(x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
		goto CLEANUP;
	}

	/* q = q * m mod b**(k+1), quick (no division) */
	if ((res = s_mp_mul_digs(&q, m, &q, um + 1)) != MP_OKAY) {
		goto CLEANUP;
	}

	/* x = x - q */
	if ((res = mp_sub(x, &q, x)) != MP_OKAY) {
		goto CLEANUP;
	}

	/* If x < 0, add b**(k+1) to it */
	if (mp_cmp_d(x, 0) == MP_LT) {
		mp_set(&q, 1);
		if ((res = mp_lshd(&q, um + 1)) != MP_OKAY)
			goto CLEANUP;
		if ((res = mp_add(x, &q, x)) != MP_OKAY)
			goto CLEANUP;
	}

	/* Back off if it's too big */
	while (mp_cmp(x, m) != MP_LT) {
		if ((res = s_mp_sub(x, m, x)) != MP_OKAY) {
			goto CLEANUP;
		}
	}

CLEANUP:
	mp_clear(&q);

	return res;
}
#endif

/* $Source: /cvs/libtom/libtommath/bn_mp_reduce.c,v $ */
/* $Revision: 1.4 $ */
/* $Date: 2006/12/28 01:25:13 $ */