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ircd::math: Add modular inversion utility.

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This commit is contained in:
Jason Volk 2021-02-17 10:15:21 -08:00
parent 014d0281d7
commit 015ef698a3
2 changed files with 39 additions and 0 deletions
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38
include/ircd/math/inv.h Normal file
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@ -0,0 +1,38 @@
// Matrix Construct
//
// Copyright (C) Matrix Construct Developers, Authors & Contributors
// Copyright (C) 2016-2021 Jason Volk <jason@zemos.net>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice is present in all copies. The
// full license for this software is available in the LICENSE file.
#pragma once
#define HAVE_IRCD_MATH_INV_H
namespace ircd::math
{
template<class Z> constexpr Z inv(Z a, const Z m);
}
/// Modular Inverse
template<class Z>
inline constexpr Z
ircd::math::inv(Z a,
const Z m)
{
auto b{m}, x0{0}, x1{1};
if(likely(m != 1)) while(a > 1)
{
auto t{b}, q{a / b};
b = a % b;
a = t;
t = x0;
x0 = x1 - q * x0;
x1 = t;
}
x1 += boolmask<Z>(x1 < 0) & m;
return x1;
}

1
include/ircd/math/math.h
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@ -12,3 +12,4 @@
#define HAVE_IRCD_MATH_MATH_H
#include "log2.h"
#include "inv.h"