mirror of
https://github.com/cryb-to/cryb-to.git
synced 2024-11-30 09:35:43 +00:00
a09cdc6318
The count we passed to memcmp() in mpi_eq() and mpi_eq_abs() was actually the number of significant words in the MPI, rather than the number of bytes we wanted to compare. Multiply by 4 to get the correct value.
To make the intent of the code more apparent, introduce a private MPI_MSW() macro which evaluates to the number of significant words (or 1-based index of the most significant word). This also comes in handy in mpi_{add,sub,mul}_abs().
Add a couple of test cases which not only demonstrate the bug we fixed here but also demonstrate why we must compare whole words: on a big-endian machine, we would be comparing the unused upper bytes of the first and only word instead of the lower bytes which actually hold a value...
102 lines
2.9 KiB
C
102 lines
2.9 KiB
C
/*
|
|
* Copyright (c) 2014-2017 Dag-Erling Smørgrav
|
|
* All rights reserved.
|
|
*
|
|
* Redistribution and use in source and binary forms, with or without
|
|
* modification, are permitted provided that the following conditions
|
|
* are met:
|
|
* 1. Redistributions of source code must retain the above copyright
|
|
* notice, this list of conditions and the following disclaimer.
|
|
* 2. Redistributions in binary form must reproduce the above copyright
|
|
* notice, this list of conditions and the following disclaimer in the
|
|
* documentation and/or other materials provided with the distribution.
|
|
* 3. The name of the author may not be used to endorse or promote
|
|
* products derived from this software without specific prior written
|
|
* permission.
|
|
*
|
|
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
|
|
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
|
|
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
* SUCH DAMAGE.
|
|
*/
|
|
|
|
#include "cryb/impl.h"
|
|
|
|
#include <stddef.h>
|
|
#include <stdint.h>
|
|
#include <strings.h>
|
|
|
|
#include <cryb/bitwise.h>
|
|
#include <cryb/mpi.h>
|
|
|
|
#include "cryb_mpi_impl.h"
|
|
|
|
/*
|
|
* Store the absolute value of the difference of the absolute values of A
|
|
* and B in X.
|
|
*/
|
|
int
|
|
mpi_sub_abs(cryb_mpi *X, const cryb_mpi *A, const cryb_mpi *B)
|
|
{
|
|
const cryb_mpi *L, *G;
|
|
unsigned int i;
|
|
uint32_t c, cn;
|
|
|
|
/*
|
|
* Trivial cases: A and B are the same or equal or at least one of
|
|
* them is zero.
|
|
*/
|
|
if (A == B || mpi_eq_abs(A, B)) {
|
|
mpi_zero(X);
|
|
return (0);
|
|
}
|
|
if (A->msb == 0) {
|
|
if (X != B && mpi_copy(X, B) != 0)
|
|
return (-1);
|
|
X->neg = 0;
|
|
return (0);
|
|
}
|
|
if (B->msb == 0) {
|
|
if (X != A && mpi_copy(X, A) != 0)
|
|
return (-1);
|
|
X->neg = 0;
|
|
return (0);
|
|
}
|
|
|
|
/* we want to subtract the smaller number from the larger */
|
|
if (mpi_cmp_abs(A, B) < 0)
|
|
L = A, G = B;
|
|
else
|
|
L = B, G = A;
|
|
|
|
/* make sure X is large enough for the largest possible result */
|
|
if (mpi_grow(X, G->msb) != 0)
|
|
return (-1);
|
|
|
|
/* subtract L from G word by word until we run out of L */
|
|
for (c = i = 0; i < MPI_MSW(L); ++i) {
|
|
cn = G->words[i] < c ||
|
|
G->words[i] - c < L->words[i];
|
|
X->words[i] = G->words[i] - L->words[i] - c;
|
|
c = cn;
|
|
}
|
|
/* keep propagating carry */
|
|
while (c) {
|
|
cn = (G->words[i] < c);
|
|
X->words[i] = G->words[i] - c;
|
|
c = cn;
|
|
++i;
|
|
}
|
|
while (X->words[i] == 0)
|
|
--i;
|
|
X->msb = i * 32 + flsl(X->words[i]);
|
|
X->neg = 0;
|
|
return (0);
|
|
}
|