mirror of
https://github.com/cryb-to/cryb-to.git
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96 lines
2.9 KiB
C
96 lines
2.9 KiB
C
/*
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* Copyright (c) 2014 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 <stdint.h>
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#include <stdlib.h>
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#include <cryb/mpi.h>
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/*
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* Store the difference between the absolute values of A and B in X.
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*/
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int
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mpi_sub_abs(cryb_mpi *X, cryb_mpi *A, 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 identical and / or both zero.
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*/
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if (A == B || (A->msb == 0 && B->msb == 0)) {
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mpi_zero(X);
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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 sum.
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*/
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if (X == B && A != 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, X->msb) != 0 || mpi_grow(X, B->msb) != 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 subtracting B from X and A is irrelevant.
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*/
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/* subtract B from X word by word until we run out of B */
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for (c = i = 0; i < (B->msb + 31) / 32; ++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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/* compute msb of msw */
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/* XXX use flsl() */
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for (X->msb = 31; X->msb > 0; --X->msb)
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if (X->words[i] & (1 << X->msb))
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break;
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/* add msw offset */
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X->msb += i * 32 + 1;
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return (0);
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}
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