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view libtommath/bn_mp_div_3.c @ 293:9d110777f345 contrib-blacklist
propagate from branch 'au.asn.ucc.matt.dropbear' (head 7ad1775ed65e75dbece27fe6b65bf1a234db386a)
to branch 'au.asn.ucc.matt.dropbear.contrib.blacklist' (head 1d86a4f0a401cc68c2670d821a2f6366c37af143)
author | Matt Johnston <matt@ucc.asn.au> |
---|---|
date | Fri, 10 Mar 2006 06:31:29 +0000 |
parents | eed26cff980b |
children | 5ff8218bcee9 |
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#include <tommath.h> #ifdef BN_MP_DIV_3_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, [email protected], http://math.libtomcrypt.org */ /* divide by three (based on routine from MPI and the GMP manual) */ int mp_div_3 (mp_int * a, mp_int *c, mp_digit * d) { mp_int q; mp_word w, t; mp_digit b; int res, ix; /* b = 2**DIGIT_BIT / 3 */ b = (((mp_word)1) << ((mp_word)DIGIT_BIT)) / ((mp_word)3); if ((res = mp_init_size(&q, a->used)) != MP_OKAY) { return res; } q.used = a->used; q.sign = a->sign; w = 0; for (ix = a->used - 1; ix >= 0; ix--) { w = (w << ((mp_word)DIGIT_BIT)) | ((mp_word)a->dp[ix]); if (w >= 3) { /* multiply w by [1/3] */ t = (w * ((mp_word)b)) >> ((mp_word)DIGIT_BIT); /* now subtract 3 * [w/3] from w, to get the remainder */ w -= t+t+t; /* fixup the remainder as required since * the optimization is not exact. */ while (w >= 3) { t += 1; w -= 3; } } else { t = 0; } q.dp[ix] = (mp_digit)t; } /* [optional] store the remainder */ if (d != NULL) { *d = (mp_digit)w; } /* [optional] store the quotient */ if (c != NULL) { mp_clamp(&q); mp_exch(&q, c); } mp_clear(&q); return res; } #endif