Coverage Report

Created: 2024-11-21 07:03

/src/mpdecimal-4.0.0/libmpdec/crt.c
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/*
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 * Copyright (c) 2008-2024 Stefan Krah. 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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 *
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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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 *
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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 <assert.h>
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#include "constants.h"
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#include "crt.h"
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#include "numbertheory.h"
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#include "mpdecimal.h"
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#include "typearith.h"
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#include "umodarith.h"
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/* Bignum: Chinese Remainder Theorem, extends the maximum transform length. */
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/* Multiply P1P2 by v, store result in w. */
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static inline void
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_crt_mulP1P2_3(mpd_uint_t w[3], mpd_uint_t v)
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0
{
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    mpd_uint_t hi1, hi2, lo;
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    _mpd_mul_words(&hi1, &lo, LH_P1P2, v);
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    w[0] = lo;
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    _mpd_mul_words(&hi2, &lo, UH_P1P2, v);
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    lo = hi1 + lo;
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    if (lo < hi1) hi2++;
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    w[1] = lo;
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    w[2] = hi2;
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}
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/* Add 3 words from v to w. The result is known to fit in w. */
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static inline void
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_crt_add3(mpd_uint_t w[3], mpd_uint_t v[3])
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{
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    mpd_uint_t carry;
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    w[0] = w[0] + v[0];
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    carry = (w[0] < v[0]);
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    w[1] = w[1] + v[1];
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    if (w[1] < v[1]) w[2]++;
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    w[1] = w[1] + carry;
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    if (w[1] < carry) w[2]++;
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    w[2] += v[2];
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}
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/* Divide 3 words in u by v, store result in w, return remainder. */
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static inline mpd_uint_t
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_crt_div3(mpd_uint_t *w, const mpd_uint_t *u, mpd_uint_t v)
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{
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    mpd_uint_t r1 = u[2];
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    mpd_uint_t r2;
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    if (r1 < v) {
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        w[2] = 0;
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    }
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    else {
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        _mpd_div_word(&w[2], &r1, u[2], v); /* GCOV_NOT_REACHED */
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    }
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    _mpd_div_words(&w[1], &r2, r1, u[1], v);
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    _mpd_div_words(&w[0], &r1, r2, u[0], v);
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    return r1;
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}
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/*
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 * Chinese Remainder Theorem:
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 * Algorithm from Joerg Arndt, "Matters Computational",
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 * Chapter 37.4.1 [http://www.jjj.de/fxt/]
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 *
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 * See also Knuth, TAOCP, Volume 2, 4.3.2, exercise 7.
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 */
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/*
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 * CRT with carry: x1, x2, x3 contain numbers modulo p1, p2, p3. For each
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 * triple of members of the arrays, find the unique z modulo p1*p2*p3, with
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 * zmax = p1*p2*p3 - 1.
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 *
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 * In each iteration of the loop, split z into result[i] = z % MPD_RADIX
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 * and carry = z / MPD_RADIX. Let N be the size of carry[] and cmax the
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 * maximum carry.
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 *
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 * Limits for the 32-bit build:
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 *
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 *   N    = 2**96
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 *   cmax = 7711435591312380274
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 *
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 * Limits for the 64 bit build:
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 *
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 *   N    = 2**192
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 *   cmax = 627710135393475385904124401220046371710
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 *
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 * The following statements hold for both versions:
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 *
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 *   1) cmax + zmax < N, so the addition does not overflow.
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 *
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 *   2) (cmax + zmax) / MPD_RADIX == cmax.
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 *
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 *   3) If c <= cmax, then c_next = (c + zmax) / MPD_RADIX <= cmax.
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 */
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void
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crt3(mpd_uint_t *x1, mpd_uint_t *x2, mpd_uint_t *x3, mpd_size_t rsize)
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{
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    mpd_uint_t p1 = mpd_moduli[P1];
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    mpd_uint_t umod;
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#ifdef PPRO
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    double dmod;
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    uint32_t dinvmod[3];
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#endif
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    mpd_uint_t a1, a2, a3;
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    mpd_uint_t s;
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    mpd_uint_t z[3], t[3];
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    mpd_uint_t carry[3] = {0,0,0};
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    mpd_uint_t hi, lo;
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    mpd_size_t i;
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    for (i = 0; i < rsize; i++) {
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        a1 = x1[i];
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        a2 = x2[i];
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        a3 = x3[i];
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        SETMODULUS(P2);
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        s = ext_submod(a2, a1, umod);
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        s = MULMOD(s, INV_P1_MOD_P2);
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        _mpd_mul_words(&hi, &lo, s, p1);
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        lo = lo + a1;
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        if (lo < a1) hi++;
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        SETMODULUS(P3);
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        s = dw_submod(a3, hi, lo, umod);
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        s = MULMOD(s, INV_P1P2_MOD_P3);
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        z[0] = lo;
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        z[1] = hi;
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        z[2] = 0;
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        _crt_mulP1P2_3(t, s);
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        _crt_add3(z, t);
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        _crt_add3(carry, z);
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        x1[i] = _crt_div3(carry, carry, MPD_RADIX);
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    }
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    assert(carry[0] == 0 && carry[1] == 0 && carry[2] == 0);
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}