/* mpih-rshift.c  -  MPI helper functions
   * Copyright (C) 1994, 1996, 1998, 1999,
   *               2000, 2001 Free Software Foundation, Inc.
   *
   * This file is part of GNUPG
   *
   * GNUPG is free software; you can redistribute it and/or modify
   * it under the terms of 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.
   *
   * GNUPG 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 a copy of the GNU General Public License
   * along with this program; if not, write to the Free Software
   * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
   *
   * Note: This code is heavily based on the GNU MP Library.
   *	 Actually it's the same code with only minor changes in the
   *	 way the data is stored; this is to support the abstraction
   *	 of an optional secure memory allocation which may be used
   *	 to avoid revealing of sensitive data due to paging etc.
   *	 The GNU MP Library itself is published under the LGPL;
   *	 however I decided to publish this code under the plain GPL.
   */
  
  #include "mpi-internal.h"
  
  /* Shift U (pointed to by UP and USIZE limbs long) CNT bits to the right
   * and store the USIZE least significant limbs of the result at WP.
   * The bits shifted out to the right are returned.
   *
   * Argument constraints:
   * 1. 0 < CNT < BITS_PER_MP_LIMB
   * 2. If the result is to be written over the input, WP must be <= UP.
   */
  
  mpi_limb_t
  mpihelp_rshift(mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned cnt)
  {
  	mpi_limb_t high_limb, low_limb;
  	unsigned sh_1, sh_2;
  	mpi_size_t i;
  	mpi_limb_t retval;
  
  	sh_1 = cnt;
  	wp -= 1;
  	sh_2 = BITS_PER_MPI_LIMB - sh_1;
  	high_limb = up[0];
  	retval = high_limb << sh_2;
  	low_limb = high_limb;
  	for (i = 1; i < usize; i++) {
  		high_limb = up[i];
  		wp[i] = (low_limb >> sh_1) | (high_limb << sh_2);
  		low_limb = high_limb;
  	}
  	wp[i] = low_limb >> sh_1;
  
  	return retval;
  }