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 */
 31int 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 */
 51void 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 */
119unsigned 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}