/// \file
// Range v3 library
//
//  Copyright Casey Carter 2016
//
//  Use, modification and distribution is subject to the
//  Boost Software License, Version 1.0. (See accompanying
//  file LICENSE_1_0.txt or copy at
//  http://www.boost.org/LICENSE_1_0.txt)
//
// Project home: https://github.com/ericniebler/range-v3
//

/*
 * Random-Number Utilities (randutil)
 *     Addresses common issues with C++11 random number generation.
 *     Makes good seeding easier, and makes using RNGs easy while retaining
 *     all the power.
 *
 * The MIT License (MIT)
 *
 * Copyright (c) 2015 Melissa E. O'Neill
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#ifndef RANGES_V3_UTILITY_RANDOM_HPP
#define RANGES_V3_UTILITY_RANDOM_HPP

#include <array>
#include <cstddef>
#include <cstdint>
#include <initializer_list>
#include <new>
#include <random>
#include <meta/meta.hpp>
#include <range/v3/range_fwd.hpp>
#include <range/v3/algorithm/copy.hpp>
#include <range/v3/algorithm/generate.hpp>
#include <concepts/concepts.hpp>
#include <range/v3/functional/invoke.hpp>
#include <range/v3/functional/reference_wrapper.hpp>
#include <range/v3/iterator/concepts.hpp>

#if !RANGES_CXX_THREAD_LOCAL
#include <mutex>
#endif

RANGES_DIAGNOSTIC_PUSH
RANGES_DIAGNOSTIC_IGNORE_CXX17_COMPAT

namespace ranges
{
    /// \addtogroup group-numerics
    /// @{
    CPP_def
    (
        template(typename Gen)
        concept UniformRandomNumberGenerator,
            requires(int)
            (
                uncvref_t<Gen>::min(),
                uncvref_t<Gen>::max(),
                concepts::requires_<Same<invoke_result_t<Gen&>, decltype(uncvref_t<Gen>::min())>>,
                concepts::requires_<Same<invoke_result_t<Gen&>, decltype(uncvref_t<Gen>::max())>>
            ) &&
            Invocable<Gen &> &&
            UnsignedIntegral<invoke_result_t<Gen&>> &&
            uncvref_t<Gen>::min() < uncvref_t<Gen>::max()
    );
    /// @}

    /// \cond
    namespace detail
    {
        namespace randutils
        {
            inline std::array<std::uint32_t, 8> get_entropy()
            {
                std::array<std::uint32_t, 8> seeds;

                // Hopefully high-quality entropy from random_device.
#if defined(__GLIBCXX__) && defined(RANGES_WORKAROUND_VALGRIND_RDRAND)
                std::random_device rd{"/dev/urandom"};
#else
                std::random_device rd;
#endif
                std::uniform_int_distribution<std::uint32_t> dist{};
                ranges::generate(seeds, [&] { return dist(rd); });

                return seeds;
            }

            template<typename I>
            constexpr auto fast_exp(I x, I power, I result = I{1}) ->
                CPP_ret(I)(
                    requires UnsignedIntegral<I>)
            {
                return power == I{0} ? result
                    : randutils::fast_exp(x * x, power >> 1, result * (power & I{1} ? x : 1));
            }

            //////////////////////////////////////////////////////////////////////////////
            //
            // seed_seq_fe
            //
            //////////////////////////////////////////////////////////////////////////////

            /*
            * seed_seq_fe implements a fixed-entropy seed sequence; it conforms to all
            * the requirements of a Seed Sequence concept.
            *
            * seed_seq_fe<N> implements a seed sequence which seeds based on a store of
            * N * 32 bits of entropy.  Typically, it would be initialized with N or more
            * integers.
            *
            * seed_seq_fe128 and seed_seq_fe256 are provided as convenience typedefs for
            * 128- and 256-bit entropy stores respectively.  These variants outperform
            * std::seed_seq, while being better mixing the bits it is provided as entropy.
            * In almost all common use cases, they serve as better drop-in replacements
            * for seed_seq.
            *
            * Technical details
            *
            * Assuming it constructed with M seed integers as input, it exhibits the
            * following properties
            *
            * * Diffusion/Avalanche:  A single-bit change in any of the M inputs has a
            *   50% chance of flipping every bit in the bitstream produced by generate.
            *   Initializing the N-word entropy store with M words requires O(N * M)
            *   time precisely because of the avalanche requirements.  Once constructed,
            *   calls to generate are linear in the number of words generated.
            *
            * * Bias freedom/Bijection: If M == N, the state of the entropy store is a
            *   bijection from the M inputs (i.e., no states occur twice, none are
            *   omitted). If M > N the number of times each state can occur is the same
            *   (each state occurs 2**(32*(M-N)) times, where ** is the power function).
            *   If M < N, some states cannot occur (bias) but no state occurs more
            *   than once (it's impossible to avoid bias if M < N; ideally N should not
            *   be chosen so that it is more than M).
            *
            *   Likewise, the generate function has similar properties (with the entropy
            *   store as the input data).  If more outputs are requested than there is
            *   entropy, some outputs cannot occur.  For example, the Mersenne Twister
            *   will request 624 outputs, to initialize its 19937-bit state, which is
            *   much larger than a 128-bit or 256-bit entropy pool.  But in practice,
            *   limiting the Mersenne Twister to 2**128 possible initializations gives
            *   us enough initializations to give a unique initialization to trillions
            *   of computers for billions of years.  If you really have 624 words of
            *   *real* high-quality entropy you want to use, you probably don't need
            *   an entropy mixer like this class at all.  But if you *really* want to,
            *   nothing is stopping you from creating a randutils::seed_seq_fe<624>.
            *
            * * As a consequence of the above properties, if all parts of the provided
            *   seed data are kept constant except one, and the remaining part is varied
            *   through K different states, K different output sequences will be produced.
            *
            * * Also, because the amount of entropy stored is fixed, this class never
            *   performs dynamic allocation and is free of the possibility of generating
            *   an exception.
            *
            * Ideas used to implement this code include hashing, a simple PCG generator
            * based on an MCG base with an XorShift output function and permutation
            * functions on tuples.
            *
            * More detail at
            *     http://www.pcg-random.org/posts/developing-a-seed_seq-alternative.html
            */

            template<std::size_t count, typename IntRep = std::uint32_t>
            struct seed_seq_fe {
            public:
                CPP_assert(UnsignedIntegral<IntRep>);
                typedef IntRep result_type;

            private:
                static constexpr std::size_t mix_rounds = 1 + (count <= 2);

                static constexpr std::uint32_t INIT_A = 0x43b0d7e5;
                static constexpr std::uint32_t MULT_A = 0x931e8875;

                static constexpr std::uint32_t INIT_B = 0x8b51f9dd;
                static constexpr std::uint32_t MULT_B = 0x58f38ded;

                static constexpr std::uint32_t MIX_MULT_L = 0xca01f9dd;
                static constexpr std::uint32_t MIX_MULT_R = 0x4973f715;
                static constexpr std::uint32_t XSHIFT = sizeof(IntRep)*8/2;

                std::array<IntRep, count> mixer_;

                template<typename I, typename S>
                auto mix_entropy(I begin, S end) ->
                    CPP_ret(void)(
                        requires InputIterator<I> && Sentinel<S, I> &&
                            ConvertibleTo<iter_reference_t<I>, IntRep>)
                {
                    auto hash_const = INIT_A;
                    auto hash = [&](IntRep value) RANGES_INTENDED_MODULAR_ARITHMETIC
                    {
                        value ^= hash_const;
                        hash_const *= MULT_A;
                        value *= hash_const;
                        value ^= value >> XSHIFT;
                        return value;
                    };
                    auto mix = [](IntRep x, IntRep y) RANGES_INTENDED_MODULAR_ARITHMETIC
                    {
                        IntRep result = MIX_MULT_L*x - MIX_MULT_R*y;
                        result ^= result >> XSHIFT;
                        return result;
                    };

                    for(auto& elem : mixer_)
                    {
                        if(begin != end) {
                            elem = hash(static_cast<IntRep>(*begin));
                            ++begin;
                        } else
                            elem = hash(IntRep{0});
                    }
                    for(auto& src : mixer_)
                        for(auto& dest : mixer_)
                            if(&src != &dest)
                                dest = mix(dest, hash(src));
                    for(; begin != end; ++begin)
                        for(auto& dest : mixer_)
                            dest = mix(dest, hash(static_cast<IntRep>(*begin)));
                }

            public:
                seed_seq_fe(const seed_seq_fe&)    = delete;
                void operator=(const seed_seq_fe&) = delete;

                template<typename T>
                CPP_ctor(seed_seq_fe)(std::initializer_list<T> init)(
                    requires ConvertibleTo<T const&, IntRep>)
                {
                    seed(init.begin(), init.end());
                }

                template<typename I, typename S>
                CPP_ctor(seed_seq_fe)(I begin, S end)(
                    requires InputIterator<I> && Sentinel<S, I> &&
                        ConvertibleTo<iter_reference_t<I>, IntRep>)
                {
                    seed(begin, end);
                }

                // generating functions
                template<typename I, typename S>
                RANGES_INTENDED_MODULAR_ARITHMETIC
                auto generate(I first, S const last) const ->
                    CPP_ret(void)(
                        requires RandomAccessIterator<I> && Sentinel<S, I>)
                {
                    auto src_begin = mixer_.begin();
                    auto src_end   = mixer_.end();
                    auto src       = src_begin;
                    auto hash_const = INIT_B;
                    for(; first != last; ++first)
                    {
                        auto dataval = *src;
                        if(++src == src_end)
                            src = src_begin;
                        dataval ^= hash_const;
                        hash_const *= MULT_B;
                        dataval *= hash_const;
                        dataval ^= dataval >> XSHIFT;
                        *first = dataval;
                    }
                }

                constexpr std::size_t size() const
                {
                    return count;
                }

                template<typename O>
                RANGES_INTENDED_MODULAR_ARITHMETIC
                auto param(O dest) const ->
                    CPP_ret(void)(
                        requires WeaklyIncrementable<O> &&
                            IndirectlyCopyable<decltype(mixer_.begin()), O>)
                {
                    constexpr IntRep INV_A = randutils::fast_exp(MULT_A, IntRep(-1));
                    constexpr IntRep MIX_INV_L = randutils::fast_exp(MIX_MULT_L, IntRep(-1));

                    auto mixer_copy = mixer_;
                    for(std::size_t round = 0; round < mix_rounds; ++round)
                    {
                        // Advance to the final value.  We'll backtrack from that.
                        auto hash_const = INIT_A*randutils::fast_exp(MULT_A, IntRep(count * count));

                        for(auto src = mixer_copy.rbegin(); src != mixer_copy.rend(); ++src)
                            for(auto dest = mixer_copy.rbegin(); dest != mixer_copy.rend(); ++dest)
                                if(src != dest)
                                {
                                    IntRep revhashed = *src;
                                    auto mult_const = hash_const;
                                    hash_const *= INV_A;
                                    revhashed ^= hash_const;
                                    revhashed *= mult_const;
                                    revhashed ^= revhashed >> XSHIFT;
                                    IntRep unmixed = *dest;
                                    unmixed ^= unmixed >> XSHIFT;
                                    unmixed += MIX_MULT_R*revhashed;
                                    unmixed *= MIX_INV_L;
                                    *dest = unmixed;
                                }

                        for(auto i = mixer_copy.rbegin(); i != mixer_copy.rend(); ++i)
                        {
                            IntRep unhashed = *i;
                            unhashed ^= unhashed >> XSHIFT;
                            unhashed *= randutils::fast_exp(hash_const, IntRep(-1));
                            hash_const *= INV_A;
                            unhashed ^= hash_const;
                            *i = unhashed;
                        }
                    }
                    ranges::copy(mixer_copy, dest);
                }

                template<typename I, typename S>
                auto seed(I begin, S end) ->
                    CPP_ret(void)(
                    requires InputIterator<I> && Sentinel<S, I> &&
                        ConvertibleTo<iter_reference_t<I>, IntRep>)
                {
                    mix_entropy(begin, end);
                    // For very small sizes, we do some additional mixing.  For normal
                    // sizes, this loop never performs any iterations.
                    for(std::size_t i = 1; i < mix_rounds; ++i)
                        stir();
                }

                seed_seq_fe& stir()
                {
                    mix_entropy(mixer_.begin(), mixer_.end());
                    return *this;
                }
            };

            using seed_seq_fe128 = seed_seq_fe<4, std::uint32_t>;
            using seed_seq_fe256 = seed_seq_fe<8, std::uint32_t>;

            //////////////////////////////////////////////////////////////////////////////
            //
            // auto_seeded
            //
            //////////////////////////////////////////////////////////////////////////////

            /*
            * randutils::auto_seeded
            *
            *   Extends a seed sequence class with a nondeterministic default constructor.
            *   Uses a variety of local sources of entropy to portably initialize any
            *   seed sequence to a good default state.
            *
            *   In normal use, it's accessed via one of the following type aliases, which
            *   use seed_seq_fe128 and seed_seq_fe256 above.
            *
            *       randutils::auto_seed_128
            *       randutils::auto_seed_256
            *
            *   It's discussed in detail at
            *       http://www.pcg-random.org/posts/simple-portable-cpp-seed-entropy.html
            *   and its motivation (why you can't just use std::random_device) here
            *       http://www.pcg-random.org/posts/cpps-random_device.html
            */

            template<typename SeedSeq>
            struct auto_seeded : public SeedSeq {
                auto_seeded()
                    : auto_seeded(randutils::get_entropy())
                {}
                template<std::size_t N>
                auto_seeded(std::array<std::uint32_t, N> const& seeds)
                    : SeedSeq(seeds.begin(), seeds.end())
                {}
                using SeedSeq::SeedSeq;

                const SeedSeq& base() const
                {
                    return *this;
                }
                SeedSeq& base()
                {
                    return *this;
                }
            };

            using auto_seed_128 = auto_seeded<seed_seq_fe128>;
            using auto_seed_256 = auto_seeded<seed_seq_fe256>;
        }

        using default_URNG = meta::if_c<(sizeof(void*) >= sizeof(long long)),
            std::mt19937_64, std::mt19937>;

#if !RANGES_CXX_THREAD_LOCAL
        template<typename URNG>
        class sync_URNG
          : private URNG
        {
            mutable std::mutex mtx_;
        public:
            using URNG::URNG;
            sync_URNG() = default;
            using typename URNG::result_type;
            result_type operator()()
            {
                std::lock_guard<std::mutex> guard{mtx_};
                return static_cast<URNG &>(*this)();
            }
            using URNG::min;
            using URNG::max;
        };
        using default_random_engine = sync_URNG<default_URNG>;
#else
        using default_random_engine = default_URNG;
#endif

        template<typename T = void>
        default_random_engine& get_random_engine()
        {
            using Seeder = meta::if_c<
                (sizeof(default_URNG) > 16),
                randutils::auto_seed_256,
                randutils::auto_seed_128>;

#if RANGES_CXX_THREAD_LOCAL >= RANGES_CXX_THREAD_LOCAL_11
            static thread_local default_random_engine engine{Seeder{}.base()};

#elif RANGES_CXX_THREAD_LOCAL
            static __thread bool initialized = false;
            static __thread meta::_t<std::aligned_storage<
                sizeof(default_random_engine),
                alignof(default_random_engine)>> storage;

            if(!initialized)
            {
                ::new(static_cast<void*>(&storage)) default_random_engine{Seeder{}.base()};
                initialized = true;
            }
            auto& engine = reinterpret_cast<default_random_engine&>(storage);
#else
            static default_random_engine engine{Seeder{}.base()};
#endif // RANGES_CXX_THREAD_LOCAL

            return engine;
        }
    }
    /// \endcond
}

RANGES_DIAGNOSTIC_POP

#endif
