94 lines
2.5 KiB
C
94 lines
2.5 KiB
C
/* mpn_sb_bdiv_q -- schoolbook Hensel division with precomputed inverse,
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returning quotient only.
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Contributed to the GNU project by Niels Möller.
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THE FUNCTIONS IN THIS FILE ARE INTERNAL FUNCTIONS WITH MUTABLE INTERFACES.
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IT IS ONLY SAFE TO REACH THEM THROUGH DOCUMENTED INTERFACES. IN FACT, IT IS
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ALMOST GUARANTEED THAT THEY'LL CHANGE OR DISAPPEAR IN A FUTURE GMP RELEASE.
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Copyright 2005, 2006, 2009 Free Software Foundation, Inc.
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Copyright 2009 David Havey
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Copyright 2010 William Hart
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This file is part of the GNU MP Library.
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The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 3 of the License, or (at your
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option) any later version.
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
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#include "mpir.h"
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#include "gmp-impl.h"
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/* Computes Q = N / D mod B^nn, with an overflow W. Destroys N.
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D must be odd. dinv is D^-1 mod B.
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Note Q will be nn limbs.
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To compute Q we cancel one limb at a time, using
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qp[i] = D^{-1} * np[i] (mod B)
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N -= B^i * qp[i] * D
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The overflow is defined as follows:
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Let X = sum(dp[i]*qp[j]*B^(i+j), 0 <= i < dn, 0 <= i+j < nn).
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Then X is nn + 2 limbs. The low nn limbs by definition agree with N.
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The overflow is the remaining high two limbs.
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*/
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void
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mpn_sb_bdiv_q (mp_ptr qp, mp_ptr wp,
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mp_ptr np, mp_size_t nn,
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mp_srcptr dp, mp_size_t dn,
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mp_limb_t dinv)
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{
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mp_size_t i;
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mp_limb_t cy, q, hi, w0, w1;
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ASSERT (dn > 0);
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ASSERT ((dp[0] & 1) != 0);
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ASSERT (((dinv * dp[0]) & GMP_NUMB_MASK) == 1);
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ASSERT (nn >= dn);
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w0 = 0;
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for (i = nn - dn; i > 0; i--)
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{
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q = dinv * np[0];
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cy = mpn_submul_1 (np, dp, dn, q);
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w0 += mpn_sub_1 (np + dn, np + dn, i, cy);
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ASSERT (np[0] == 0);
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qp[0] = q;
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qp++;
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np++;
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}
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w1 = 0;
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for (i = dn; i > 0; i--)
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{
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q = dinv * np[0];
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hi = mpn_submul_1 (np, dp, i, q);
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ADDC_LIMB(hi, w0, w0, hi);
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w1 += hi;
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ASSERT (np[0] == 0);
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qp[0] = q;
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qp++;
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np++;
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}
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wp[0] = w0;
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wp[1] = w1;
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}
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