gnupg/mpi/mpi-scan.c

/* mpi-scan.c  -  MPI functions
 *	Copyright (C) 1998 Free Software Foundation, Inc.
 *
 * This file is part of GnuPG.
 *
 * GnuPG 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 of the License, or
 * (at your option) any later version.
 *
 * GnuPG 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 this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
 */

#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include "mpi-internal.h"
#include "longlong.h"

/****************
 * Scan through an mpi and return byte for byte. a -1 is returned to indicate
 * the end of the mpi. Scanning is done from the lsb to the msb, returned
 * values are in the range of 0 .. 255.
 *
 * FIXME: This code is VERY ugly!
 */
int
mpi_getbyte( MPI a, unsigned idx )
{
    int i, j;
    unsigned n;
    mpi_ptr_t ap;
    mpi_limb_t limb;

    ap = a->d;
    for(n=0,i=0; i < a->nlimbs; i++ ) {
	limb = ap[i];
	for( j=0; j < BYTES_PER_MPI_LIMB; j++, n++ )
	    if( n == idx )
		return (limb >> j*8) & 0xff;
    }
    return -1;
}


/****************
 * Put a value at position IDX into A. idx counts from lsb to msb
 */
void
mpi_putbyte( MPI a, unsigned idx, int xc )
{
    int i, j;
    unsigned n;
    mpi_ptr_t ap;
    mpi_limb_t limb, c;

    c = xc & 0xff;
    ap = a->d;
    for(n=0,i=0; i < a->alloced; i++ ) {
	limb = ap[i];
	for( j=0; j < BYTES_PER_MPI_LIMB; j++, n++ )
	    if( n == idx ) {
	      #if BYTES_PER_MPI_LIMB == 4
		if( j == 0 )
		    limb = (limb & 0xffffff00) | c;
		else if( j == 1 )
		    limb = (limb & 0xffff00ff) | (c<<8);
		else if( j == 2 )
		    limb = (limb & 0xff00ffff) | (c<<16);
		else
		    limb = (limb & 0x00ffffff) | (c<<24);
	      #elif BYTES_PER_MPI_LIMB == 8
		if( j == 0 )
		    limb = (limb & 0xffffffffffffff00) | c;
		else if( j == 1 )
		    limb = (limb & 0xffffffffffff00ff) | (c<<8);
		else if( j == 2 )
		    limb = (limb & 0xffffffffff00ffff) | (c<<16);
		else if( j == 3 )
		    limb = (limb & 0xffffffff00ffffff) | (c<<24);
		else if( j == 4 )
		    limb = (limb & 0xffffff00ffffffff) | (c<<32);
		else if( j == 5 )
		    limb = (limb & 0xffff00ffffffffff) | (c<<40);
		else if( j == 6 )
		    limb = (limb & 0xff00ffffffffffff) | (c<<48);
		else
		    limb = (limb & 0x00ffffffffffffff) | (c<<56);
	      #else
		 #error please enhance this function, its ugly - i know.
	      #endif
		if( a->nlimbs <= i )
		    a->nlimbs = i+1;
		ap[i] = limb;
		return;
	    }
    }
    abort(); /* index out of range */
}


/****************
 * Count the number of zerobits at the low end of A
 */
unsigned
mpi_trailing_zeros( MPI a )
{
    unsigned n, count = 0;

    for(n=0; n < a->nlimbs; n++ ) {
	if( a->d[n] ) {
	    unsigned nn;
	    mpi_limb_t alimb = a->d[n];

	    count_trailing_zeros( nn, alimb );
	    count += nn;
	    break;
	}
	count += BITS_PER_MPI_LIMB;
    }
    return count;

}