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       1                 :            : // SPDX-License-Identifier: GPL-2.0-or-later
       2                 :            : /* mpihelp-lshift.c  -  MPI helper functions
       3                 :            :  * Copyright (C) 1994, 1996, 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                 :            : 
      18                 :            : /* Shift U (pointed to by UP and USIZE digits long) CNT bits to the left
      19                 :            :  * and store the USIZE least significant digits of the result at WP.
      20                 :            :  * Return the bits shifted out from the most significant digit.
      21                 :            :  *
      22                 :            :  * Argument constraints:
      23                 :            :  * 1. 0 < CNT < BITS_PER_MP_LIMB
      24                 :            :  * 2. If the result is to be written over the input, WP must be >= UP.
      25                 :            :  */
      26                 :            : 
      27                 :            : mpi_limb_t
      28                 :          0 : mpihelp_lshift(mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned int cnt)
      29                 :            : {
      30                 :            :         mpi_limb_t high_limb, low_limb;
      31                 :            :         unsigned sh_1, sh_2;
      32                 :            :         mpi_size_t i;
      33                 :            :         mpi_limb_t retval;
      34                 :            : 
      35                 :            :         sh_1 = cnt;
      36                 :          0 :         wp += 1;
      37                 :          0 :         sh_2 = BITS_PER_MPI_LIMB - sh_1;
      38                 :          0 :         i = usize - 1;
      39                 :          0 :         low_limb = up[i];
      40                 :          0 :         retval = low_limb >> sh_2;
      41                 :            :         high_limb = low_limb;
      42                 :          0 :         while (--i >= 0) {
      43                 :          0 :                 low_limb = up[i];
      44                 :          0 :                 wp[i] = (high_limb << sh_1) | (low_limb >> sh_2);
      45                 :            :                 high_limb = low_limb;
      46                 :            :         }
      47                 :          0 :         wp[i] = high_limb << sh_1;
      48                 :            : 
      49                 :          0 :         return retval;
      50                 :            : }