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mpi-genesis             1 /* mpihelp_udiv_w_sdiv -- implement udiv_qrnnd on machines with only signed
mpi-genesis             2  *			  division.
mpi-genesis             3  * Copyright (C) 1992, 1994, 1996, 1998 Free Software Foundation, Inc.
mpi-genesis             4  * Contributed by Peter L. Montgomery.
mpi-genesis             5  *
mpi-genesis             6  * This file is part of GnuPG.
mpi-genesis             7  *
mpi-genesis             8  * GnuPG is free software; you can redistribute it and/or modify
mpi-genesis             9  * it under the terms of the GNU General Public License as published by
mpi-genesis            10  * the Free Software Foundation; either version 3 of the License, or
mpi-genesis            11  * (at your option) any later version.
mpi-genesis            12  *
mpi-genesis            13  * GnuPG is distributed in the hope that it will be useful,
mpi-genesis            14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
mpi-genesis            15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
mpi-genesis            16  * GNU General Public License for more details.
mpi-genesis            17  *
mpi-genesis            18  * You should have received a copy of the GNU General Public License
mpi-genesis            19  * along with this program; if not, see <http://www.gnu.org/licenses/>.
mpi-genesis            20  */
mpi-genesis            21 
mpi-genesis            22 #include <config.h>
mpi-genesis            23 #include <stdio.h>
mpi-genesis            24 #include <stdlib.h>
mpi-genesis            25 #include "mpi-internal.h"
mpi-genesis            26 #include "longlong.h"
mpi-genesis            27 
mpi-genesis            28 
mpi-genesis            29 #if 0  /* not yet ported to MPI */
mpi-genesis            30 
mpi-genesis            31 mpi_limb_t
mpi-genesis            32 mpihelp_udiv_w_sdiv( mpi_limp_t *rp,
mpi-genesis            33 		     mpi_limp_t *a1,
mpi-genesis            34 		     mpi_limp_t *a0,
mpi-genesis            35 		     mpi_limp_t *d   )
mpi-genesis            36 {
mpi-genesis            37   mp_limb_t q, r;
mpi-genesis            38   mp_limb_t c0, c1, b1;
mpi-genesis            39 
mpi-genesis            40   if ((mpi_limb_signed_t) d >= 0)
mpi-genesis            41     {
mpi-genesis            42       if (a1 < d - a1 - (a0 >> (BITS_PER_MP_LIMB - 1)))
mpi-genesis            43 	{
mpi-genesis            44 	  /* dividend, divisor, and quotient are nonnegative */
mpi-genesis            45 	  sdiv_qrnnd (q, r, a1, a0, d);
mpi-genesis            46 	}
mpi-genesis            47       else
mpi-genesis            48 	{
mpi-genesis            49 	  /* Compute c1*2^32 + c0 = a1*2^32 + a0 - 2^31*d */
mpi-genesis            50 	  sub_ddmmss (c1, c0, a1, a0, d >> 1, d << (BITS_PER_MP_LIMB - 1));
mpi-genesis            51 	  /* Divide (c1*2^32 + c0) by d */
mpi-genesis            52 	  sdiv_qrnnd (q, r, c1, c0, d);
mpi-genesis            53 	  /* Add 2^31 to quotient */
mpi-genesis            54 	  q += (mp_limb_t) 1 << (BITS_PER_MP_LIMB - 1);
mpi-genesis            55 	}
mpi-genesis            56     }
mpi-genesis            57   else
mpi-genesis            58     {
mpi-genesis            59       b1 = d >> 1;			/* d/2, between 2^30 and 2^31 - 1 */
mpi-genesis            60       c1 = a1 >> 1;			/* A/2 */
mpi-genesis            61       c0 = (a1 << (BITS_PER_MP_LIMB - 1)) + (a0 >> 1);
mpi-genesis            62 
mpi-genesis            63       if (a1 < b1)			/* A < 2^32*b1, so A/2 < 2^31*b1 */
mpi-genesis            64 	{
mpi-genesis            65 	  sdiv_qrnnd (q, r, c1, c0, b1); /* (A/2) / (d/2) */
mpi-genesis            66 
mpi-genesis            67 	  r = 2*r + (a0 & 1);		/* Remainder from A/(2*b1) */
mpi-genesis            68 	  if ((d & 1) != 0)
mpi-genesis            69 	    {
mpi-genesis            70 	      if (r >= q)
mpi-genesis            71 		r = r - q;
mpi-genesis            72 	      else if (q - r <= d)
mpi-genesis            73 		{
mpi-genesis            74 		  r = r - q + d;
mpi-genesis            75 		  q--;
mpi-genesis            76 		}
mpi-genesis            77 	      else
mpi-genesis            78 		{
mpi-genesis            79 		  r = r - q + 2*d;
mpi-genesis            80 		  q -= 2;
mpi-genesis            81 		}
mpi-genesis            82 	    }
mpi-genesis            83 	}
mpi-genesis            84       else if (c1 < b1) 		/* So 2^31 <= (A/2)/b1 < 2^32 */
mpi-genesis            85 	{
mpi-genesis            86 	  c1 = (b1 - 1) - c1;
mpi-genesis            87 	  c0 = ~c0;			/* logical NOT */
mpi-genesis            88 
mpi-genesis            89 	  sdiv_qrnnd (q, r, c1, c0, b1); /* (A/2) / (d/2) */
mpi-genesis            90 
mpi-genesis            91 	  q = ~q;			/* (A/2)/b1 */
mpi-genesis            92 	  r = (b1 - 1) - r;
mpi-genesis            93 
mpi-genesis            94 	  r = 2*r + (a0 & 1);		/* A/(2*b1) */
mpi-genesis            95 
mpi-genesis            96 	  if ((d & 1) != 0)
mpi-genesis            97 	    {
mpi-genesis            98 	      if (r >= q)
mpi-genesis            99 		r = r - q;
mpi-genesis           100 	      else if (q - r <= d)
mpi-genesis           101 		{
mpi-genesis           102 		  r = r - q + d;
mpi-genesis           103 		  q--;
mpi-genesis           104 		}
mpi-genesis           105 	      else
mpi-genesis           106 		{
mpi-genesis           107 		  r = r - q + 2*d;
mpi-genesis           108 		  q -= 2;
mpi-genesis           109 		}
mpi-genesis           110 	    }
mpi-genesis           111 	}
mpi-genesis           112       else				/* Implies c1 = b1 */
mpi-genesis           113 	{				/* Hence a1 = d - 1 = 2*b1 - 1 */
mpi-genesis           114 	  if (a0 >= -d)
mpi-genesis           115 	    {
mpi-genesis           116 	      q = -1;
mpi-genesis           117 	      r = a0 + d;
mpi-genesis           118 	    }
mpi-genesis           119 	  else
mpi-genesis           120 	    {
mpi-genesis           121 	      q = -2;
mpi-genesis           122 	      r = a0 + 2*d;
mpi-genesis           123 	    }
mpi-genesis           124 	}
mpi-genesis           125     }
mpi-genesis           126 
mpi-genesis           127   *rp = r;
mpi-genesis           128   return q;
mpi-genesis           129 }
mpi-genesis           130 
mpi-genesis           131 #endif
mpi-genesis           132