#include <tommath.h>
#ifdef BN_S_MP_MUL_DIGS_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
 */

/* multiplies |a| * |b| and only computes upto digs digits of result
 * HAC pp. 595, Algorithm 14.12  Modified so you can control how 
 * many digits of output are created.
 */
int s_mp_mul_digs(mp_int * a, mp_int * b, mp_int * c, int digs)
{
	mp_int t;
	int res, pa, pb, ix, iy;
	mp_digit u;
	mp_word r;
	mp_digit tmpx, *tmpt, *tmpy;

	/* can we use the fast multiplier? */
	if (((digs) < MP_WARRAY) &&
	    MIN(a->used, b->used) <
	    (1 << ((CHAR_BIT * sizeof(mp_word)) - (2 * DIGIT_BIT)))) {
		return fast_s_mp_mul_digs(a, b, c, digs);
	}

	if ((res = mp_init_size(&t, digs)) != MP_OKAY) {
		return res;
	}
	t.used = digs;

	/* compute the digits of the product directly */
	pa = a->used;
	for (ix = 0; ix < pa; ix++) {
		/* set the carry to zero */
		u = 0;

		/* limit ourselves to making digs digits of output */
		pb = MIN(b->used, digs - ix);

		/* setup some aliases */
		/* copy of the digit from a used within the nested loop */
		tmpx = a->dp[ix];

		/* an alias for the destination shifted ix places */
		tmpt = t.dp + ix;

		/* an alias for the digits of b */
		tmpy = b->dp;

		/* compute the columns of the output and propagate the carry */
		for (iy = 0; iy < pb; iy++) {
			/* compute the column as a mp_word */
			r = ((mp_word) * tmpt) +
			    ((mp_word) tmpx) * ((mp_word) * tmpy++) +
			    ((mp_word) u);

			/* the new column is the lower part of the result */
			*tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));

			/* get the carry word from the result */
			u = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
		}
		/* set carry if it is placed below digs */
		if (ix + iy < digs) {
			*tmpt = u;
		}
	}

	mp_clamp(&t);
	mp_exch(&t, c);

	mp_clear(&t);
	return MP_OKAY;
}
#endif

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