/* mpf_mul_2exp -- Multiply a float by 2^n.

Copyright 1993, 1994, 1996, 2000-2002, 2004 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify

it under the terms of either:

  * the GNU Lesser General Public License as published by the Free

    Software Foundation; either version 3 of the License, or (at your

    option) any later version.

or

  * the GNU General Public License as published by the Free Software

    Foundation; either version 2 of the License, or (at your option) any

    later version.

or both in parallel, as here.

The GNU MP Library is distributed in the hope that it will be useful, but

WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY

or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

for more details.

You should have received copies of the GNU General Public License and the

GNU Lesser General Public License along with the GNU MP Library.  If not,

see https://www.gnu.org/licenses/.  */

#include "gmp.h"

#include "gmp-impl.h"

/* Multiples of GMP_NUMB_BITS in exp simply mean an amount added to EXP(u)

   to set EXP(r).  The remainder exp%GMP_NUMB_BITS is then a left shift for

   the limb data.

   If exp%GMP_NUMB_BITS == 0 then there's no shifting, we effectively just

   do an mpz_set with changed EXP(r).  Like mpz_set we take prec+1 limbs in

   this case.  Although just prec would suffice, it's nice to have

   mpf_mul_2exp with exp==0 come out the same as mpz_set.

   When shifting we take up to prec many limbs from the input.  Our shift is

   cy = mpn_lshift (PTR(r), PTR(u)+k, size, ...), where k is the number of

   low limbs dropped from u, and the carry out is stored to PTR(r)[size].

   It may be noted that the low limb PTR(r)[0] doesn't incorporate bits from

   PTR(u)[k-1] (when k>=1 makes that limb available).  Taking just prec

   limbs from the input (with the high non-zero) is enough bits for the

   application requested precision, there's no need for extra work.

   If r==u the shift will have overlapping operands.  When k==0 (ie. when

   usize <= prec), the overlap is supported by lshift (ie. dst == src).

   But when r==u and k>=1 (ie. usize > prec), we would have an invalid

   overlap (ie. mpn_lshift (rp, rp+k, ...)).  In this case we must instead

   use mpn_rshift (PTR(r)+1, PTR(u)+k, size, NUMB-shift) with the carry out

   stored to PTR(r)[0].  An rshift by NUMB-shift bits like this gives

   identical data, it's just its overlap restrictions which differ.

   Enhancements:

   The way mpn_lshift is used means successive mpf_mul_2exp calls on the

   same operand will accumulate low zero limbs, until prec+1 limbs is

   reached.  This is wasteful for subsequent operations.  When abs_usize <=

   prec, we should test the low exp%GMP_NUMB_BITS many bits of PTR(u)[0],

   ie. those which would be shifted out by an mpn_rshift.  If they're zero

   then use that mpn_rshift.  */

void

mpf_mul_2exp (mpf_ptr r, mpf_srcptr u, mp_bitcnt_t exp)

{

  mp_srcptr up;

  mp_ptr rp = r->_mp_d;

  mp_size_t usize;

  mp_size_t abs_usize;

  mp_size_t prec = r->_mp_prec;

  mp_exp_t uexp = u->_mp_exp;

  usize = u->_mp_size;

  if (UNLIKELY (usize == 0))

    {

      r->_mp_size = 0;

      r->_mp_exp = 0;

      return;

    }

  abs_usize = ABS (usize);

  up = u->_mp_d;

  if (exp % GMP_NUMB_BITS == 0)

    {

      prec++;			/* retain more precision here as we don't need

				   to account for carry-out here */

      if (abs_usize > prec)

	{

	  up += abs_usize - prec;

	  abs_usize = prec;

	}

      if (rp != up)

	MPN_COPY_INCR (rp, up, abs_usize);

      r->_mp_exp = uexp + exp / GMP_NUMB_BITS;

    }

  else

    {

      mp_limb_t cy_limb;

      mp_size_t adj;

      if (abs_usize > prec)

	{

	  up += abs_usize - prec;

	  abs_usize = prec;

	  /* Use mpn_rshift since mpn_lshift operates downwards, and we

	     therefore would clobber part of U before using that part, in case

	     R is the same variable as U.  */

	  cy_limb = mpn_rshift (rp + 1, up, abs_usize,

				GMP_NUMB_BITS - exp % GMP_NUMB_BITS);

	  rp[0] = cy_limb;

	  adj = rp[abs_usize] != 0;

	}

      else

	{

	  cy_limb = mpn_lshift (rp, up, abs_usize, exp % GMP_NUMB_BITS);

	  rp[abs_usize] = cy_limb;

	  adj = cy_limb != 0;

	}

      abs_usize += adj;

      r->_mp_exp = uexp + exp / GMP_NUMB_BITS + adj;

    }

  r->_mp_size = usize >= 0 ? abs_usize : -abs_usize;

}