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       1                 :            : // SPDX-License-Identifier: GPL-2.0-or-later
       2                 :            : /* mpihelp-add_2.c  -  MPI helper functions
       3                 :            :  * Copyright (C) 1994, 1996, 1997, 1998, 2001 Free Software Foundation, Inc.
       4                 :            :  *
       5                 :            :  * This file is part of GnuPG.
       6                 :            :  *
       7                 :            :  * Note: This code is heavily based on the GNU MP Library.
       8                 :            :  *       Actually it's the same code with only minor changes in the
       9                 :            :  *       way the data is stored; this is to support the abstraction
      10                 :            :  *       of an optional secure memory allocation which may be used
      11                 :            :  *       to avoid revealing of sensitive data due to paging etc.
      12                 :            :  *       The GNU MP Library itself is published under the LGPL;
      13                 :            :  *       however I decided to publish this code under the plain GPL.
      14                 :            :  */
      15                 :            : 
      16                 :            : #include "mpi-internal.h"
      17                 :            : #include "longlong.h"
      18                 :            : 
      19                 :            : mpi_limb_t
      20                 :        414 : mpihelp_sub_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
      21                 :            :               mpi_ptr_t s2_ptr, mpi_size_t size)
      22                 :            : {
      23                 :        414 :         mpi_limb_t x, y, cy;
      24                 :        414 :         mpi_size_t j;
      25                 :            : 
      26                 :            :         /* The loop counter and index J goes from -SIZE to -1.  This way
      27                 :            :            the loop becomes faster.  */
      28                 :        414 :         j = -size;
      29                 :            : 
      30                 :            :         /* Offset the base pointers to compensate for the negative indices.  */
      31                 :        414 :         s1_ptr -= j;
      32                 :        414 :         s2_ptr -= j;
      33                 :        414 :         res_ptr -= j;
      34                 :            : 
      35                 :        414 :         cy = 0;
      36                 :       6144 :         do {
      37                 :       6144 :                 y = s2_ptr[j];
      38                 :       6144 :                 x = s1_ptr[j];
      39                 :       6144 :                 y += cy;        /* add previous carry to subtrahend */
      40                 :       6144 :                 cy = y < cy; /* get out carry from that addition */
      41                 :       6144 :                 y = x - y;      /* main subtract */
      42                 :       6144 :                 cy += y > x; /* get out carry from the subtract, combine */
      43                 :       6144 :                 res_ptr[j] = y;
      44         [ +  + ]:       6144 :         } while (++j);
      45                 :            : 
      46                 :        414 :         return cy;
      47                 :            : }