lib/mpi/mpi-scan.c

   1 /* mpi-scan.c  -  MPI functions
   2  * Copyright (C) 1998, 1999, 2000, 2001 Free Software Foundation, Inc.
   3  *
   4  * This file is part of GnuPG.
   5  *
   6  * GnuPG is free software; you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License as published by
   8  * the Free Software Foundation; either version 2 of the License, or
   9  * (at your option) any later version.
  10  *
  11  * GnuPG is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14  * GNU General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU General Public License
  17  * along with this program; if not, write to the Free Software
  18  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  19  */
  20
  21 #include "mpi-internal.h"
  22 #include "longlong.h"
  23
  24 /****************
  25  * Scan through an mpi and return byte for byte. a -1 is returned to indicate
  26  * the end of the mpi. Scanning is done from the lsb to the msb, returned
  27  * values are in the range of 0 .. 255.
  28  *
  29  * FIXME: This code is VERY ugly!
  30  */
  31 int mpi_getbyte(const MPI a, unsigned idx)
  32 {
  33         int i, j;
  34         unsigned n;
  35         mpi_ptr_t ap;
  36         mpi_limb_t limb;
  37
  38         ap = a->d;
  39         for (n = 0, i = 0; i < a->nlimbs; i++) {
  40                 limb = ap[i];
  41                 for (j = 0; j < BYTES_PER_MPI_LIMB; j++, n++)
  42                         if (n == idx)
  43                                 return (limb >> j * 8) & 0xff;
  44         }
  45         return -1;
  46 }
  47
  48 /****************
  49  * Put a value at position IDX into A. idx counts from lsb to msb
  50  */
  51 void mpi_putbyte(MPI a, unsigned idx, int xc)
  52 {
  53         int i, j;
  54         unsigned n;
  55         mpi_ptr_t ap;
  56         mpi_limb_t limb, c;
  57
  58         c = xc & 0xff;
  59         ap = a->d;
  60         for (n = 0, i = 0; i < a->alloced; i++) {
  61                 limb = ap[i];
  62                 for (j = 0; j < BYTES_PER_MPI_LIMB; j++, n++)
  63                         if (n == idx) {
  64 #if BYTES_PER_MPI_LIMB == 4
  65                                 if (j == 0)
  66                                         limb = (limb & 0xffffff00) | c;
  67                                 else if (j == 1)
  68                                         limb = (limb & 0xffff00ff) | (c << 8);
  69                                 else if (j == 2)
  70                                         limb = (limb & 0xff00ffff) | (c << 16);
  71                                 else
  72                                         limb = (limb & 0x00ffffff) | (c << 24);
  73 #elif BYTES_PER_MPI_LIMB == 8
  74                                 if (j == 0)
  75                                         limb = (limb & 0xffffffffffffff00) | c;
  76                                 else if (j == 1)
  77                                         limb =
  78                                             (limb & 0xffffffffffff00ff) | (c <<
  79                                                                            8);
  80                                 else if (j == 2)
  81                                         limb =
  82                                             (limb & 0xffffffffff00ffff) | (c <<
  83                                                                            16);
  84                                 else if (j == 3)
  85                                         limb =
  86                                             (limb & 0xffffffff00ffffff) | (c <<
  87                                                                            24);
  88                                 else if (j == 4)
  89                                         limb =
  90                                             (limb & 0xffffff00ffffffff) | (c <<
  91                                                                            32);
  92                                 else if (j == 5)
  93                                         limb =
  94                                             (limb & 0xffff00ffffffffff) | (c <<
  95                                                                            40);
  96                                 else if (j == 6)
  97                                         limb =
  98                                             (limb & 0xff00ffffffffffff) | (c <<
  99                                                                            48);
 100                                 else
 101                                         limb =
 102                                             (limb & 0x00ffffffffffffff) | (c <<
 103                                                                            56);
 104 #else
 105 #error please enhance this function, its ugly - i know.
 106 #endif
 107                                 if (a->nlimbs <= i)
 108                                         a->nlimbs = i + 1;
 109                                 ap[i] = limb;
 110                                 return;
 111                         }
 112         }
 113         log_bug("index out of range\n");
 114 }
 115
 116 /****************
 117  * Count the number of zerobits at the low end of A
 118  */
 119 unsigned mpi_trailing_zeros(const MPI a)
 120 {
 121         unsigned n, count = 0;
 122
 123         for (n = 0; n < a->nlimbs; n++) {
 124                 if (a->d[n]) {
 125                         unsigned nn;
 126                         mpi_limb_t alimb = a->d[n];
 127
 128                         count_trailing_zeros(nn, alimb);
 129                         count += nn;
 130                         break;
 131                 }
 132                 count += BITS_PER_MPI_LIMB;
 133         }
 134         return count;
 135
 136 }