libcruft-util/random.hpp

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/*
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* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/.
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*
* Copyright 2016-2019 Danny Robson <danny@nerdcruft.net>
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*/
#pragma once
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#include "coord/traits.hpp"
#include <algorithm>
#include <array>
#include <random>
#include <limits>
#include <type_traits>
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namespace cruft::random {
///////////////////////////////////////////////////////////////////////////
/// Initialise and return a generator using random_device.
template <typename GeneratorT>
GeneratorT
initialise (void)
{
// Approximate the state size of the generator by its size in bytes.
std::array<unsigned,sizeof (GeneratorT) / sizeof (unsigned) + 1> seed;
std::generate (seed.begin (), seed.end (), std::random_device ());
std::seed_seq seq (seed.begin (), seed.end ());
return GeneratorT (seq);
}
///////////////////////////////////////////////////////////////////////////
/// Returns a correctly pre-initialised reference to a thread-local
/// generator of an unspecified (but not entirely useless) type.
///
/// ie, not LCG.
inline auto&
generator (void)
{
using generator_t = std::mt19937_64;
static thread_local auto gen = initialise<generator_t> ();
return gen;
}
///////////////////////////////////////////////////////////////////////////
/// A convenience typedef that selects between
/// std::uniform_real_distribution and std::uniform_int_distribution
/// depending on the supplied value type.
template <typename ValueT>
using uniform_distribution = std::conditional_t<
std::is_floating_point<ValueT>::value,
std::uniform_real_distribution<ValueT>,
std::uniform_int_distribution<ValueT>
>;
///////////////////////////////////////////////////////////////////////////
/// Returns a value chosen uniformly at random the supplied range.
///
/// This is primarily a convenience helper around the uniform_distribution
/// type.
template <typename ValueT, typename GeneratorT>
decltype(auto)
uniform (ValueT lo, ValueT hi, GeneratorT &&gen)
{
return uniform_distribution<ValueT> { lo, hi } (
std::forward<GeneratorT> (gen)
);
}
///------------------------------------------------------------------------
/// Return a value uniformly random chosen value between lo and hi.
///
/// Interval bounds are treated as per the standard Generator
/// implementations; ie, inclusive for integers, exclusive upper for reals.
template <typename T>
decltype(auto)
uniform (T lo, T hi)
{
return uniform<T> (lo, hi, generator ());
}
///------------------------------------------------------------------------
/// Return a uniformly random value chosen on the interval [0,1)
template <
typename ValueT,
typename GeneratorT,
typename = std::enable_if_t<std::is_floating_point_v<ValueT>>
>
decltype(auto)
uniform (GeneratorT &&gen)
{
return uniform<ValueT> (ValueT{0}, ValueT{1}, std::forward<GeneratorT> (gen));
}
///------------------------------------------------------------------------
/// Return a uniformly random chosen value on the interval [0.f, 1.f)
template <
typename ValueT,
typename = std::enable_if_t<std::is_floating_point_v<ValueT>>
>
decltype(auto)
uniform (void)
{
return uniform<ValueT> (ValueT{0}, ValueT{1}, generator ());
}
///------------------------------------------------------------------------
/// Return a uniformly random chosen value on the interval [0, 1]
template <typename T>
std::enable_if_t<std::is_integral_v<T>,T>
uniform (void)
{
return uniform<T> (
std::numeric_limits<T>::min (),
std::numeric_limits<T>::max ()
);
}
///------------------------------------------------------------------------
/// Returns a uniformly random initialised coordinate type by value.
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template <
typename ValueT,
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typename = std::enable_if_t<
is_coord_v<ValueT> && std::is_floating_point_v<typename ValueT::value_type>
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>
>
ValueT
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uniform (void)
{
ValueT res {};
std::fill (res.begin (), res.end (), uniform<typename ValueT::value_type> ());
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return res;
}
///////////////////////////////////////////////////////////////////////////
/// choose a value at random from an array
template <typename T, size_t N>
T&
choose (T (&t)[N])
{
std::uniform_int_distribution<size_t> dist (0, N - 1);
return t[dist (generator ())];
}
};