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https://github.com/cryb-to/cryb-to.git
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115 lines
3.3 KiB
C
115 lines
3.3 KiB
C
/*
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* Copyright (c) 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 <errno.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 quotient and remainder of A divided by B in Q and R.
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*
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* Assumes Q != R, but any combination of {Q,R} == {A,B} is fine. Either
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* Q or R or both may be NULL, althought the latter is pointless.
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*/
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int
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mpi_div_abs(cryb_mpi *Q, cryb_mpi *R, const cryb_mpi *A, const cryb_mpi *B)
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{
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cryb_mpi AA = CRYB_MPI_ZERO, QQ = CRYB_MPI_ZERO;
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int cmp;
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/* trivial cases */
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if (B->msb == 0) {
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/* divide something by zero */
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errno = EINVAL;
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return (-1);
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} else if (A->msb == 0) {
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/* divide zero by something */
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if (Q != NULL)
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mpi_zero(Q);
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if (R != NULL)
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mpi_zero(R);
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return (0);
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} else if (B->msb == 1) {
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/* divide something by one */
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if (Q != A && Q != NULL && mpi_copy(Q, A) != 0)
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return (-1);
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if (R != NULL)
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mpi_zero(R);
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return (0);
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} else if ((cmp = mpi_cmp_abs(A, B)) == 0) {
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/* divide something by itself */
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if (Q != NULL)
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mpi_set(Q, 1);
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if (R != NULL)
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mpi_zero(R);
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return (0);
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} else if (cmp < 0) {
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/* divide something by something larger */
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if (Q != NULL)
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mpi_zero(Q);
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if (R != A && R != NULL && mpi_copy(R, A) != 0)
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return (-1);
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return (0);
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}
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/* division is destructive, so we work on copies */
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if (mpi_copy(&AA, A) != 0)
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return (-1);
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mpi_zero(&QQ);
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/*
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* Repeatedly subtract B from A until they are the same length,
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* then one last time if A is still greater than B.
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*/
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while (AA.msb > B->msb) {
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mpi_sub_abs(&AA, &AA, B);
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mpi_inc_abs(&QQ);
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}
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if (mpi_cmp_abs(&AA, B) >= 0) {
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mpi_sub_abs(&AA, &AA, B);
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mpi_inc_abs(&QQ);
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}
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/* store result where requested and clean up */
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if (Q != NULL)
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mpi_swap(Q, &QQ);
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if (R != NULL)
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mpi_swap(R, &AA);
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mpi_destroy(&AA);
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mpi_destroy(&QQ);
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return (0);
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}
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