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https://github.com/cryb-to/cryb-to.git
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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...
112 lines
3.1 KiB
C
112 lines
3.1 KiB
C
/*
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* Copyright (c) 2014-2017 Dag-Erling Smørgrav
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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* 3. The name of the author may not be used to endorse or promote
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* products derived from this software without specific prior written
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* permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*/
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#include "cryb/impl.h"
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#include <stddef.h>
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#include <stdint.h>
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#include <strings.h>
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#include <cryb/bitwise.h>
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#include <cryb/mpi.h>
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#include "cryb_mpi_impl.h"
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/*
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* Store the sum of the absolutes values of A and B in X.
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*/
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int
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mpi_add_abs(cryb_mpi *X, const cryb_mpi *A, const cryb_mpi *B)
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{
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unsigned int i;
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uint32_t c;
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/*
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* Trivial cases: A and B are the same or equal or at least one of
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* them is zero.
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*/
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if (A == B || mpi_eq_abs(A, B)) {
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if (X != A && X != B && mpi_copy(X, A) != 0)
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return (-1);
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if (mpi_lshift(X, 1) != 0)
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return (-1);
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X->neg = 0;
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return (0);
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}
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if (A->msb == 0) {
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if (X != B && mpi_copy(X, B) != 0)
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return (-1);
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X->neg = 0;
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return (0);
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}
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if (B->msb == 0) {
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if (X != A && mpi_copy(X, A) != 0)
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return (-1);
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X->neg = 0;
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return (0);
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}
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/*
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* Normalize our operands: if X is identical to either A or B, we
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* want it to be A. Otherwise, copy A into X. In either case,
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* make sure X is large enough for the largest possible result.
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*/
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if (X == B) {
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B = A;
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A = X;
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}
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if (mpi_grow(X, A->msb + 1) != 0 || mpi_grow(X, B->msb + 1) != 0)
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return (-1);
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if (X != A)
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/* this cannot fail */
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mpi_copy(X, A);
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/*
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* From now on, we are adding B to X and A is irrelevant.
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*/
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/* add B into X word by word until we run out of B */
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for (c = i = 0; i < MPI_MSW(B); ++i) {
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X->words[i] += c;
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c = (X->words[i] < c);
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X->words[i] += B->words[i];
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c += (X->words[i] < B->words[i]);
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}
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/* keep propagating carry */
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while (c) {
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X->words[i] += c;
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c = (X->words[i] < c);
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++i;
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}
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if (X->words[i] == 0)
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--i;
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X->msb = i * 32 + flsl(X->words[i]);
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X->neg = 0;
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return (0);
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}
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