/* mpihelp-mul_3.c  -  MPI helper functions
   * Copyright (C) 1994, 1996, 1997, 1998, 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"
  #include "longlong.h"
  
  mpi_limb_t
  mpihelp_submul_1(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
  		 mpi_size_t s1_size, mpi_limb_t s2_limb)
  {
  	mpi_limb_t cy_limb;
  	mpi_size_t j;
  	mpi_limb_t prod_high, prod_low;
  	mpi_limb_t x;
  
  	/* The loop counter and index J goes from -SIZE to -1.  This way
  	 * the loop becomes faster.  */
  	j = -s1_size;
  	res_ptr -= j;
  	s1_ptr -= j;
  
  	cy_limb = 0;
  	do {
  		umul_ppmm(prod_high, prod_low, s1_ptr[j], s2_limb);
  
  		prod_low += cy_limb;
  		cy_limb = (prod_low < cy_limb ? 1 : 0) + prod_high;
  
  		x = res_ptr[j];
  		prod_low = x - prod_low;
  		cy_limb += prod_low > x ? 1 : 0;
  		res_ptr[j] = prod_low;
  	} while (++j);
  
  	return cy_limb;
  }