Viel neues
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<?php
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/**
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* BCMath Barrett Modular Exponentiation Engine
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*
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* PHP version 5 and 7
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*
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* @author Jim Wigginton <terrafrost@php.net>
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* @copyright 2017 Jim Wigginton
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* @license http://www.opensource.org/licenses/mit-license.html MIT License
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* @link http://pear.php.net/package/Math_BigInteger
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*/
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declare(strict_types=1);
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namespace phpseclib3\Math\BigInteger\Engines\BCMath\Reductions;
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use phpseclib3\Math\BigInteger\Engines\BCMath\Base;
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/**
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* PHP Barrett Modular Exponentiation Engine
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*
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* @author Jim Wigginton <terrafrost@php.net>
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*/
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abstract class Barrett extends Base
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{
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/**
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* Cache constants
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*
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* $cache[self::VARIABLE] tells us whether or not the cached data is still valid.
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*/
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public const VARIABLE = 0;
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/**
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* $cache[self::DATA] contains the cached data.
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*/
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public const DATA = 1;
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/**
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* Barrett Modular Reduction
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*
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* See {@link http://www.cacr.math.uwaterloo.ca/hac/about/chap14.pdf#page=14 HAC 14.3.3} /
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* {@link http://math.libtomcrypt.com/files/tommath.pdf#page=165 MPM 6.2.5} for more information. Modified slightly,
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* so as not to require negative numbers (initially, this script didn't support negative numbers).
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*
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* Employs "folding", as described at
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* {@link http://www.cosic.esat.kuleuven.be/publications/thesis-149.pdf#page=66 thesis-149.pdf#page=66}. To quote from
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* it, "the idea [behind folding] is to find a value x' such that x (mod m) = x' (mod m), with x' being smaller than x."
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*
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* Unfortunately, the "Barrett Reduction with Folding" algorithm described in thesis-149.pdf is not, as written, all that
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* usable on account of (1) its not using reasonable radix points as discussed in
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* {@link http://math.libtomcrypt.com/files/tommath.pdf#page=162 MPM 6.2.2} and (2) the fact that, even with reasonable
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* radix points, it only works when there are an even number of digits in the denominator. The reason for (2) is that
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* (x >> 1) + (x >> 1) != x / 2 + x / 2. If x is even, they're the same, but if x is odd, they're not. See the in-line
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* comments for details.
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*/
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protected static function reduce(string $n, string $m): string
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{
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static $cache = [
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self::VARIABLE => [],
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self::DATA => [],
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];
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$m_length = strlen($m);
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if (strlen($n) >= 2 * $m_length) {
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return bcmod($n, $m);
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}
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// if (m.length >> 1) + 2 <= m.length then m is too small and n can't be reduced
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if ($m_length < 5) {
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return self::regularBarrett($n, $m);
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}
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// n = 2 * m.length
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if (($key = array_search($m, $cache[self::VARIABLE])) === false) {
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$key = count($cache[self::VARIABLE]);
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$cache[self::VARIABLE][] = $m;
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$lhs = '1' . str_repeat('0', $m_length + ($m_length >> 1));
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$u = bcdiv($lhs, $m, 0);
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$m1 = bcsub($lhs, bcmul($u, $m));
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$cache[self::DATA][] = [
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'u' => $u, // m.length >> 1 (technically (m.length >> 1) + 1)
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'm1' => $m1, // m.length
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];
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} else {
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extract($cache[self::DATA][$key]);
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}
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$cutoff = $m_length + ($m_length >> 1);
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$lsd = substr($n, -$cutoff);
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$msd = substr($n, 0, -$cutoff);
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$temp = bcmul($msd, $m1); // m.length + (m.length >> 1)
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$n = bcadd($lsd, $temp); // m.length + (m.length >> 1) + 1 (so basically we're adding two same length numbers)
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//if ($m_length & 1) {
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// return self::regularBarrett($n, $m);
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//}
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// (m.length + (m.length >> 1) + 1) - (m.length - 1) == (m.length >> 1) + 2
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$temp = substr($n, 0, -$m_length + 1);
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// if even: ((m.length >> 1) + 2) + (m.length >> 1) == m.length + 2
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// if odd: ((m.length >> 1) + 2) + (m.length >> 1) == (m.length - 1) + 2 == m.length + 1
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$temp = bcmul($temp, $u);
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// if even: (m.length + 2) - ((m.length >> 1) + 1) = m.length - (m.length >> 1) + 1
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// if odd: (m.length + 1) - ((m.length >> 1) + 1) = m.length - (m.length >> 1)
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$temp = substr($temp, 0, -($m_length >> 1) - 1);
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// if even: (m.length - (m.length >> 1) + 1) + m.length = 2 * m.length - (m.length >> 1) + 1
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// if odd: (m.length - (m.length >> 1)) + m.length = 2 * m.length - (m.length >> 1)
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$temp = bcmul($temp, $m);
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// at this point, if m had an odd number of digits, we'd be subtracting a 2 * m.length - (m.length >> 1) digit
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// number from a m.length + (m.length >> 1) + 1 digit number. ie. there'd be an extra digit and the while loop
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// following this comment would loop a lot (hence our calling _regularBarrett() in that situation).
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$result = bcsub($n, $temp);
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//if (bccomp($result, '0') < 0) {
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if ($result[0] == '-') {
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$temp = '1' . str_repeat('0', $m_length + 1);
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$result = bcadd($result, $temp);
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}
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while (bccomp($result, $m) >= 0) {
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$result = bcsub($result, $m);
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}
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return $result;
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}
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/**
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* (Regular) Barrett Modular Reduction
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*
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* For numbers with more than four digits BigInteger::_barrett() is faster. The difference between that and this
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* is that this function does not fold the denominator into a smaller form.
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*/
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private static function regularBarrett(string $x, string $n): string
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{
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static $cache = [
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self::VARIABLE => [],
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self::DATA => [],
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];
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$n_length = strlen($n);
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if (strlen($x) > 2 * $n_length) {
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return bcmod($x, $n);
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}
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if (($key = array_search($n, $cache[self::VARIABLE])) === false) {
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$key = count($cache[self::VARIABLE]);
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$cache[self::VARIABLE][] = $n;
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$lhs = '1' . str_repeat('0', 2 * $n_length);
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$cache[self::DATA][] = bcdiv($lhs, $n, 0);
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}
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$temp = substr($x, 0, -$n_length + 1);
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$temp = bcmul($temp, $cache[self::DATA][$key]);
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$temp = substr($temp, 0, -$n_length - 1);
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$r1 = substr($x, -$n_length - 1);
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$r2 = substr(bcmul($temp, $n), -$n_length - 1);
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$result = bcsub($r1, $r2);
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//if (bccomp($result, '0') < 0) {
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if ($result[0] == '-') {
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$q = '1' . str_repeat('0', $n_length + 1);
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$result = bcadd($result, $q);
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}
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while (bccomp($result, $n) >= 0) {
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$result = bcsub($result, $n);
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}
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return $result;
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}
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}
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@@ -0,0 +1,104 @@
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<?php
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/**
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* BCMath Dynamic Barrett Modular Exponentiation Engine
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*
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* PHP version 5 and 7
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*
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* @author Jim Wigginton <terrafrost@php.net>
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* @copyright 2017 Jim Wigginton
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* @license http://www.opensource.org/licenses/mit-license.html MIT License
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* @link http://pear.php.net/package/Math_BigInteger
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*/
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declare(strict_types=1);
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namespace phpseclib3\Math\BigInteger\Engines\BCMath\Reductions;
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use phpseclib3\Math\BigInteger\Engines\BCMath;
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use phpseclib3\Math\BigInteger\Engines\BCMath\Base;
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/**
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* PHP Barrett Modular Exponentiation Engine
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*
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* @author Jim Wigginton <terrafrost@php.net>
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*/
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abstract class EvalBarrett extends Base
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{
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/**
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* Custom Reduction Function
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*
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* @see self::generateCustomReduction
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*/
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private static $custom_reduction;
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/**
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* Barrett Modular Reduction
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*
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* This calls a dynamically generated loop unrolled function that's specific to a given modulo.
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* Array lookups are avoided as are if statements testing for how many bits the host OS supports, etc.
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*/
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protected static function reduce(string $n, string $m): string
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{
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$inline = self::$custom_reduction;
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return $inline($n);
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}
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/**
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* Generate Custom Reduction
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*
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* @return callable|void
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*/
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protected static function generateCustomReduction(BCMath $m, string $class)
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{
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$m_length = strlen($m);
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if ($m_length < 5) {
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$code = 'return bcmod($x, $n);';
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eval('$func = function ($n) { ' . $code . '};');
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self::$custom_reduction = $func;
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return;
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}
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$lhs = '1' . str_repeat('0', $m_length + ($m_length >> 1));
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$u = bcdiv($lhs, $m, 0);
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$m1 = bcsub($lhs, bcmul($u, $m));
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$cutoff = $m_length + ($m_length >> 1);
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$m = "'$m'";
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$u = "'$u'";
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$m1 = "'$m1'";
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$code = '
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$lsd = substr($n, -' . $cutoff . ');
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$msd = substr($n, 0, -' . $cutoff . ');
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$temp = bcmul($msd, ' . $m1 . ');
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$n = bcadd($lsd, $temp);
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$temp = substr($n, 0, ' . (-$m_length + 1) . ');
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$temp = bcmul($temp, ' . $u . ');
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$temp = substr($temp, 0, ' . (-($m_length >> 1) - 1) . ');
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$temp = bcmul($temp, ' . $m . ');
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$result = bcsub($n, $temp);
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if ($result[0] == \'-\') {
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$temp = \'1' . str_repeat('0', $m_length + 1) . '\';
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$result = bcadd($result, $temp);
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}
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while (bccomp($result, ' . $m . ') >= 0) {
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$result = bcsub($result, ' . $m . ');
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}
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return $result;';
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eval('$func = function ($n) { ' . $code . '};');
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self::$custom_reduction = $func;
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return $func;
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}
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}
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