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209 lines
5.7 KiB
C
209 lines
5.7 KiB
C
/* mpn_divmod_1(quot_ptr, dividend_ptr, dividend_size, divisor_limb) --
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Divide (DIVIDEND_PTR,,DIVIDEND_SIZE) by DIVISOR_LIMB.
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Write DIVIDEND_SIZE limbs of quotient at QUOT_PTR.
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Return the single-limb remainder.
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There are no constraints on the value of the divisor.
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QUOT_PTR and DIVIDEND_PTR might point to the same limb.
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Copyright (C) 1991, 1993, 1994, 1996 Free Software Foundation, Inc.
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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 Library General Public License as published by
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the Free Software Foundation; either version 2 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 Library General Public
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License for more details.
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You should have received a copy of the GNU Library General Public License
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along with the GNU MP Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
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MA 02111-1307, USA. */
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#include "gmp.h"
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#include "gmp-impl.h"
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#include "longlong.h"
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#ifndef UMUL_TIME
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#define UMUL_TIME 1
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#endif
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#ifndef UDIV_TIME
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#define UDIV_TIME UMUL_TIME
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#endif
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/* FIXME: We should be using invert_limb (or invert_normalized_limb)
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here (not udiv_qrnnd). */
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mp_limb_t
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#if __STDC__
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mpn_divmod_1 (mp_ptr quot_ptr,
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mp_srcptr dividend_ptr, mp_size_t dividend_size,
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mp_limb_t divisor_limb)
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#else
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mpn_divmod_1 (quot_ptr, dividend_ptr, dividend_size, divisor_limb)
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mp_ptr quot_ptr;
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mp_srcptr dividend_ptr;
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mp_size_t dividend_size;
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mp_limb_t divisor_limb;
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#endif
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{
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mp_size_t i;
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mp_limb_t n1, n0, r;
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int dummy;
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/* ??? Should this be handled at all? Rely on callers? */
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if (dividend_size == 0)
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return 0;
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/* If multiplication is much faster than division, and the
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dividend is large, pre-invert the divisor, and use
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only multiplications in the inner loop. */
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/* This test should be read:
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Does it ever help to use udiv_qrnnd_preinv?
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&& Does what we save compensate for the inversion overhead? */
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if (UDIV_TIME > (2 * UMUL_TIME + 6)
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&& (UDIV_TIME - (2 * UMUL_TIME + 6)) * dividend_size > UDIV_TIME)
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{
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int normalization_steps;
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count_leading_zeros (normalization_steps, divisor_limb);
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if (normalization_steps != 0)
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{
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mp_limb_t divisor_limb_inverted;
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divisor_limb <<= normalization_steps;
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/* Compute (2**2N - 2**N * DIVISOR_LIMB) / DIVISOR_LIMB. The
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result is a (N+1)-bit approximation to 1/DIVISOR_LIMB, with the
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most significant bit (with weight 2**N) implicit. */
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/* Special case for DIVISOR_LIMB == 100...000. */
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if (divisor_limb << 1 == 0)
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divisor_limb_inverted = ~(mp_limb_t) 0;
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else
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udiv_qrnnd (divisor_limb_inverted, dummy,
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-divisor_limb, 0, divisor_limb);
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n1 = dividend_ptr[dividend_size - 1];
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r = n1 >> (BITS_PER_MP_LIMB - normalization_steps);
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/* Possible optimization:
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if (r == 0
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&& divisor_limb > ((n1 << normalization_steps)
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| (dividend_ptr[dividend_size - 2] >> ...)))
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...one division less... */
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for (i = dividend_size - 2; i >= 0; i--)
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{
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n0 = dividend_ptr[i];
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udiv_qrnnd_preinv (quot_ptr[i + 1], r, r,
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((n1 << normalization_steps)
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| (n0 >> (BITS_PER_MP_LIMB - normalization_steps))),
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divisor_limb, divisor_limb_inverted);
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n1 = n0;
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}
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udiv_qrnnd_preinv (quot_ptr[0], r, r,
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n1 << normalization_steps,
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divisor_limb, divisor_limb_inverted);
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return r >> normalization_steps;
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}
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else
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{
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mp_limb_t divisor_limb_inverted;
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/* Compute (2**2N - 2**N * DIVISOR_LIMB) / DIVISOR_LIMB. The
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result is a (N+1)-bit approximation to 1/DIVISOR_LIMB, with the
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most significant bit (with weight 2**N) implicit. */
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/* Special case for DIVISOR_LIMB == 100...000. */
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if (divisor_limb << 1 == 0)
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divisor_limb_inverted = ~(mp_limb_t) 0;
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else
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udiv_qrnnd (divisor_limb_inverted, dummy,
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-divisor_limb, 0, divisor_limb);
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i = dividend_size - 1;
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r = dividend_ptr[i];
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if (r >= divisor_limb)
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r = 0;
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else
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{
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quot_ptr[i] = 0;
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i--;
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}
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for (; i >= 0; i--)
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{
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n0 = dividend_ptr[i];
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udiv_qrnnd_preinv (quot_ptr[i], r, r,
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n0, divisor_limb, divisor_limb_inverted);
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}
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return r;
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}
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}
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else
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{
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if (UDIV_NEEDS_NORMALIZATION)
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{
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int normalization_steps;
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count_leading_zeros (normalization_steps, divisor_limb);
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if (normalization_steps != 0)
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{
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divisor_limb <<= normalization_steps;
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n1 = dividend_ptr[dividend_size - 1];
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r = n1 >> (BITS_PER_MP_LIMB - normalization_steps);
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/* Possible optimization:
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if (r == 0
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&& divisor_limb > ((n1 << normalization_steps)
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| (dividend_ptr[dividend_size - 2] >> ...)))
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...one division less... */
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for (i = dividend_size - 2; i >= 0; i--)
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{
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n0 = dividend_ptr[i];
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udiv_qrnnd (quot_ptr[i + 1], r, r,
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((n1 << normalization_steps)
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| (n0 >> (BITS_PER_MP_LIMB - normalization_steps))),
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divisor_limb);
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n1 = n0;
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}
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udiv_qrnnd (quot_ptr[0], r, r,
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n1 << normalization_steps,
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divisor_limb);
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return r >> normalization_steps;
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}
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}
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/* No normalization needed, either because udiv_qrnnd doesn't require
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it, or because DIVISOR_LIMB is already normalized. */
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i = dividend_size - 1;
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r = dividend_ptr[i];
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if (r >= divisor_limb)
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r = 0;
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else
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{
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quot_ptr[i] = 0;
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i--;
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}
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for (; i >= 0; i--)
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{
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n0 = dividend_ptr[i];
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udiv_qrnnd (quot_ptr[i], r, r, n0, divisor_limb);
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}
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return r;
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}
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}
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