source/libs/gmp/gmp-src/mpq/md_2exp.c

   1 /* mpq_mul_2exp, mpq_div_2exp - multiply or divide by 2^N */
   2
   3 /*
   4 Copyright 2000, 2002, 2012 Free Software Foundation, Inc.
   5
   6 This file is part of the GNU MP Library.
   7
   8 The GNU MP Library is free software; you can redistribute it and/or modify
   9 it under the terms of either:
  10
  11   * the GNU Lesser General Public License as published by the Free
  12     Software Foundation; either version 3 of the License, or (at your
  13     option) any later version.
  14
  15 or
  16
  17   * the GNU General Public License as published by the Free Software
  18     Foundation; either version 2 of the License, or (at your option) any
  19     later version.
  20
  21 or both in parallel, as here.
  22
  23 The GNU MP Library is distributed in the hope that it will be useful, but
  24 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  25 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  26 for more details.
  27
  28 You should have received copies of the GNU General Public License and the
  29 GNU Lesser General Public License along with the GNU MP Library.  If not,
  30 see https://www.gnu.org/licenses/.  */
  31
  32 #include "gmp.h"
  33 #include "gmp-impl.h"
  34 #include "longlong.h"
  35
  36
  37 /* The multiplier/divisor "n", representing 2^n, is applied by right shifting
  38    "r" until it's odd (if it isn't already), and left shifting "l" for the
  39    rest. */
  40
  41 static void
  42 mord_2exp (mpz_ptr ldst, mpz_ptr rdst, mpz_srcptr lsrc, mpz_srcptr rsrc,
  43            mp_bitcnt_t n)
  44 {
  45   mp_size_t  rsrc_size = SIZ(rsrc);
  46   mp_size_t  len = ABS (rsrc_size);
  47   mp_ptr     rsrc_ptr = PTR(rsrc);
  48   mp_ptr     p, rdst_ptr;
  49   mp_limb_t  plow;
  50
  51   p = rsrc_ptr;
  52   plow = *p;
  53   while (n >= GMP_NUMB_BITS && plow == 0)
  54     {
  55       n -= GMP_NUMB_BITS;
  56       p++;
  57       plow = *p;
  58     }
  59
  60   /* no realloc here if rsrc==rdst, so p and rsrc_ptr remain valid */
  61   len -= (p - rsrc_ptr);
  62   rdst_ptr = MPZ_REALLOC (rdst, len);
  63
  64   if ((plow & 1) || n == 0)
  65     {
  66       /* need INCR when src==dst */
  67       if (p != rdst_ptr)
  68         MPN_COPY_INCR (rdst_ptr, p, len);
  69     }
  70   else
  71     {
  72       unsigned long  shift;
  73       if (plow == 0)
  74         shift = n;
  75       else
  76         {
  77           count_trailing_zeros (shift, plow);
  78           shift = MIN (shift, n);
  79         }
  80       mpn_rshift (rdst_ptr, p, len, shift);
  81       len -= (rdst_ptr[len-1] == 0);
  82       n -= shift;
  83     }
  84   SIZ(rdst) = (rsrc_size >= 0) ? len : -len;
  85
  86   if (n)
  87     mpz_mul_2exp (ldst, lsrc, n);
  88   else if (ldst != lsrc)
  89     mpz_set (ldst, lsrc);
  90 }
  91
  92
  93 void
  94 mpq_mul_2exp (mpq_ptr dst, mpq_srcptr src, mp_bitcnt_t n)
  95 {
  96   mord_2exp (NUM(dst), DEN(dst), NUM(src), DEN(src), n);
  97 }
  98
  99 void
 100 mpq_div_2exp (mpq_ptr dst, mpq_srcptr src, mp_bitcnt_t n)
 101 {
 102   if (SIZ(NUM(src)) == 0)
 103     {
 104       SIZ(NUM(dst)) = 0;
 105       SIZ(DEN(dst)) = 1;
 106       PTR(DEN(dst))[0] = 1;
 107       return;
 108     }
 109
 110   mord_2exp (DEN(dst), NUM(dst), DEN(src), NUM(src), n);
 111 }