libcruft-util/random.hpp

/*
 * 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/.
 *
 * Copyright 2016-2019 Danny Robson <danny@nerdcruft.net>
 */

#pragma once

#include "coord/traits.hpp"

#include <algorithm>
#include <array>
#include <random>
#include <limits>
#include <type_traits>

#include <chrono>


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 + 1> seed;
        std::generate_n (
            seed.begin (),
            seed.size () - 1,
            std::random_device ()
        );

        // Append the current time with the highest resolution we have
        // available. This defends against instances where the implementation
        // of std::random_device is actually deterministic.
        seed[seed.size () - 1] = static_cast<unsigned> (
            std::chrono::high_resolution_clock::now ().time_since_epoch ().count ()
        );

        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. As such, the interval is the same as the std library; ie, closed
    /// for integers, half-open for reals.
    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 value uniformly chosen between 0 and the given value.
    ///
    /// 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 hi)
    {
        return uniform<T> (T{0}, hi);
    }


    ///------------------------------------------------------------------------
    /// 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.
    template <
        typename ValueT,
        typename = std::enable_if_t<
            is_coord_v<ValueT> && std::is_floating_point_v<typename ValueT::value_type>
        >
    >
    ValueT
    uniform (void)
    {
        ValueT res;
        for (auto &v: res)
            v = uniform<typename ValueT::value_type> ();
        return res;
    }


    ///////////////////////////////////////////////////////////////////////////
    /// Choose a value at random from an array
    ///
    /// We return pointers (rather than values) so that we can return values
    /// for empty containers without invalid dereferences.
    ///
    /// \return A pointer to the chosen value.
    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 ())];
    }


    ///------------------------------------------------------------------------
    /// Choose a value at random from a container.
    ///
    /// We return iterators (rather than values) so that we can return values
    /// for empty containers without invalid dereferences.
    //
    /// \return An iterator to the chosen value.
    template <typename ContainerT, typename GeneratorT>
    typename ContainerT::iterator
    choose (ContainerT &data, GeneratorT &&gen)
    {
        if (data.empty ())
            return data.end ();

        auto const offset = uniform (
            typename ContainerT::size_type {0},
            data.size () - 1,
            gen
        );

        return std::next (data.begin (), offset);
    }


    ///------------------------------------------------------------------------
    /// Choose a value at random from a container.
    ///
    /// We return iterators (rather than values) so that we can return values
    /// for empty containers without invalid dereferences.
    ///
    /// \return An iterator to the chosen value.
    template <typename ContainerT>
    typename ContainerT::iterator
    choose (ContainerT &data)
    {
        return choose (data, generator ());
    }
}