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division_scs.c

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00001 /** Functions for SCS inverse and division
00002 
00003 @file division_scs.c
00004 
00005 @author Defour David David.Defour@ens-lyon.fr
00006 @author Florent de Dinechin Florent.de.Dinechin@ens-lyon.fr 
00007  
00008 This file is part of the SCS library.
00009 
00010 
00011 */
00012 
00013 /*
00014 Copyright (C) 2002  David Defour and Florent de Dinechin
00015 
00016     This library is free software; you can redistribute it and/or
00017     modify it under the terms of the GNU Lesser General Public
00018     License as published by the Free Software Foundation; either
00019     version 2.1 of the License, or (at your option) any later version.
00020 
00021     This library is distributed in the hope that it will be useful,
00022     but WITHOUT ANY WARRANTY; without even the implied warranty of
00023     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
00024     Lesser General Public License for more details.
00025 
00026     You should have received a copy of the GNU Lesser General Public
00027     License along with this library; if not, write to the Free Software
00028     Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
00029 
00030  */
00031 #include "scs.h"
00032 #include "scs_private.h"
00033 
00034 
00035 /*
00036  * Compute 1/x with a Newton scheme
00037  */
00038 void scs_inv(scs_ptr result, scs_ptr x){
00039   scs_t tmp, res, res1, scstwo;
00040   double app_x, inv;
00041   
00042   scs_set(tmp, x);  tmp->index = 0;  scs_get_d(&app_x, tmp);
00043 
00044   scs_set_si(scstwo, 2);
00045   /* inv is a 53-bit approximation of 1/x */
00046   inv = 1/app_x;
00047   
00048   scs_set_d(res, inv);
00049   res->index -= x->index;
00050 
00051   /* First Newton Iteration */
00052   scs_mul(res1, x, res);
00053   scs_sub(res1, scstwo, res1);
00054   scs_mul(res, res, res1);
00055 
00056   /* Second Newton Iteration */
00057   scs_mul(res1, x, res);
00058   scs_sub(res1, scstwo, res1);
00059   scs_mul(result, res, res1); 
00060 
00061   return;
00062 }
00063 
00064 /*
00065  * Compute result = x/y; 
00066  */
00067 void scs_div(scs_ptr result, scs_ptr x, scs_ptr y){ 
00068   scs_t res;
00069  
00070   if (X_EXP != 1){
00071     R_EXP = X_EXP / Y_EXP;
00072     return;
00073   }
00074 
00075   scs_inv(res, y);
00076   scs_mul(result, res, x);
00077   return;
00078 }

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