1  /* mpq_mul_2exp, mpq_div_2exp - multiply or divide by 2^N */
       2  
       3  /*
       4  Copyright 2000, 2002, 2012, 2018 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-impl.h"
      33  #include "longlong.h"
      34  
      35  
      36  /* The multiplier/divisor "n", representing 2^n, is applied by right shifting
      37     "r" until it's odd (if it isn't already), and left shifting "l" for the
      38     rest. */
      39  
      40  static void
      41  mord_2exp (mpz_ptr ldst, mpz_ptr rdst, mpz_srcptr lsrc, mpz_srcptr rsrc,
      42             mp_bitcnt_t n)
      43  {
      44    mp_size_t  rsrc_size = SIZ(rsrc);
      45    mp_size_t  len = ABS (rsrc_size);
      46    mp_ptr     rsrc_ptr = PTR(rsrc);
      47    mp_ptr     p, rdst_ptr;
      48    mp_limb_t  plow;
      49  
      50    p = rsrc_ptr;
      51    plow = *p;
      52    while (n >= GMP_NUMB_BITS && plow == 0)
      53      {
      54        n -= GMP_NUMB_BITS;
      55        p++;
      56        plow = *p;
      57      }
      58  
      59    /* no realloc here if rsrc==rdst, so p and rsrc_ptr remain valid */
      60    len -= (p - rsrc_ptr);
      61    rdst_ptr = MPZ_REALLOC (rdst, len);
      62  
      63    if ((plow & 1) || n == 0)
      64      {
      65        /* need INCR when src==dst */
      66        if (p != rdst_ptr)
      67          MPN_COPY_INCR (rdst_ptr, p, len);
      68      }
      69    else
      70      {
      71        unsigned long  shift;
      72        if (plow == 0)
      73          shift = n;
      74        else
      75          {
      76            count_trailing_zeros (shift, plow);
      77            shift = MIN (shift, n);
      78          }
      79        mpn_rshift (rdst_ptr, p, len, shift);
      80        len -= (rdst_ptr[len-1] == 0);
      81        n -= shift;
      82      }
      83    SIZ(rdst) = (rsrc_size >= 0) ? len : -len;
      84  
      85    if (n)
      86      mpz_mul_2exp (ldst, lsrc, n);
      87    else if (ldst != lsrc)
      88      mpz_set (ldst, lsrc);
      89  }
      90  
      91  
      92  void
      93  mpq_mul_2exp (mpq_ptr dst, mpq_srcptr src, mp_bitcnt_t n)
      94  {
      95    mord_2exp (NUM(dst), DEN(dst), NUM(src), DEN(src), n);
      96  }
      97  
      98  void
      99  mpq_div_2exp (mpq_ptr dst, mpq_srcptr src, mp_bitcnt_t n)
     100  {
     101    if (SIZ(NUM(src)) == 0)
     102      {
     103        SIZ(NUM(dst)) = 0;
     104        SIZ(DEN(dst)) = 1;
     105        MPZ_NEWALLOC (DEN(dst), 1)[0] = 1;
     106        return;
     107      }
     108  
     109    mord_2exp (DEN(dst), NUM(dst), DEN(src), NUM(src), n);
     110  }