163 lines
3.9 KiB
C
163 lines
3.9 KiB
C
/* mpn_sub  Subtract two lowlevel naturalnumber integers.




Copyright (C) 1991 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 the GNU General Public License as published by


the Free Software Foundation; either version 2, or (at your option)


any later version.




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 a copy of the GNU General Public License


along with the GNU MP Library; see the file COPYING. If not, write to


the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. */




#include "gmp.h"


#include "gmpimpl.h"




/* Subtract SUB_PTR/SUB_SIZE from MIN_PTR/MIN_SIZE and store the


result (MIN_SIZE words) at DIF_PTR.




Return 1 if min < sub (result is negative). Otherwise, return the


negative difference between the number of words in dif and min.


(I.e. return 0 if the result has MIN_SIZE words, 1 if it has


MIN_SIZE  1 words, etc.)




Argument constraint: MIN_SIZE >= SUB_SIZE.




The size of DIF can be calculated as MIN_SIZE + the return value. */




mp_size


#ifdef __STDC__


mpn_sub (mp_ptr dif_ptr,


mp_srcptr min_ptr, mp_size min_size,


mp_srcptr sub_ptr, mp_size sub_size)


#else


mpn_sub (dif_ptr, min_ptr, min_size, sub_ptr, sub_size)


mp_ptr dif_ptr;


mp_srcptr min_ptr;


mp_size min_size;


mp_srcptr sub_ptr;


mp_size sub_size;


#endif


{


mp_limb m, s, dif;


mp_size j;




/* The loop counter and index J goes from some negative value to zero.


This way the loops are faster. Need to offset the base pointers


to take care of the negative indices. */




j = sub_size;


if (j == 0)


goto sub_finished;




min_ptr = j;


sub_ptr = j;


dif_ptr = j;




/* There are two doloops, marked NONCARRY LOOP and CARRY LOOP that


jump between each other. The first loop is for when the previous


subtraction didn't produce a carryout; the second is for the


complementary case. */




/* NONCARRY LOOP */


do


{


m = min_ptr[j];


s = sub_ptr[j];


dif = m  s;


dif_ptr[j] = dif;


if (dif > m)


goto cy_loop;


ncy_loop:


j++;


}


while (j < 0);




/* We have exhausted SUB, with no carry out. Copy remaining part of


MIN to DIF. */




sub_finished:


j = sub_size  min_size;




/* If there's no difference between the length of the operands, the


last words might have become zero, and renormalization is needed. */


if (j == 0)


goto normalize;




min_ptr = j;


dif_ptr = j;




goto copy;




/* CARRY LOOP */


do


{


m = min_ptr[j];


s = sub_ptr[j];


dif = m  s  1;


dif_ptr[j] = dif;


if (dif < m)


goto ncy_loop;


cy_loop:


j++;


}


while (j < 0);




/* We have exhausted SUB, but need to propagate carry. */




j = sub_size  min_size;


if (j == 0)


return 1; /* min < sub. Flag it to the caller */




min_ptr = j;


dif_ptr = j;




/* Propagate carry. Sooner or later the carry will cancel with a


nonzero word, because the minuend is normalized. Considering this,


there's no need to test the index J. */


for (;;)


{


m = min_ptr[j];


dif = m  1;


dif_ptr[j] = dif;


j++;


if (dif < m)


break;


}




if (j == 0)


goto normalize;




copy:


/* Don't copy the remaining words of MIN to DIF if MIN_PTR and DIF_PTR


are equal. It would just be a noop copying. Return 0, as the length


of the result equals that of the minuend. */


if (dif_ptr == min_ptr)


return 0;




do


{


dif_ptr[j] = min_ptr[j];


j++;


}


while (j < 0);


return 0;




normalize:


for (j = 1; j >= min_size; j)


{


if (dif_ptr[j] != 0)


return j + 1;


}




return min_size;


}
