random_pcg

/*
Copyright (C) 2018-2024 Geoffrey Daniels. https://gpdaniels.com/

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, version 3 of the License only.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/

#pragma once
#ifndef GTL_RANDOM_RANDOM_PCG_HPP
#define GTL_RANDOM_RANDOM_PCG_HPP

// Summary: PCG pseudo-random number generator.

#ifndef NDEBUG
#   if defined(_MSC_VER)
#       define __builtin_trap() __debugbreak()
#   endif
/// @brief A simple assert macro to break the program if the random_pcg is misused.
#   define GTL_RANDOM_PCG_ASSERT(ASSERTION, MESSAGE) static_cast<void>((ASSERTION) || (__builtin_trap(), 0))
#else
/// @brief At release time the assert macro is implemented as a nop.
#   define GTL_RANDOM_PCG_ASSERT(ASSERTION, MESSAGE) static_cast<void>(0)
#endif

namespace gtl {
    /// @brief  The random_pcg class implements the pcg pseudo-random-number generator.
    class random_pcg final {
    private:
        /// @brief  Current state of the random number generator.
        unsigned long long int state;

        /// @brief  Increment added to the state during pseudo-random-number generation.
        unsigned long long int increment;

    public:
        /// @brief  Defaulted destructor.
        ~random_pcg() = default;

        /// @brief  Empty constructor.
        random_pcg()
            : state(0x853C49E6748FEA9Bull)
            , increment(0xDA3E39CB94B95BDBull) {
        }

        /// @brief  Defaulted copy constructor.
        random_pcg(const random_pcg&) = default;

        /// @brief  Defaulted move constructor.
        random_pcg(random_pcg&&) = default;

        /// @brief  Defaulted copy assignment.
        random_pcg& operator=(const random_pcg&) = default;

        /// @brief  Defaulted move assignment.
        random_pcg& operator=(random_pcg&&) = default;

        /// @brief  Constructor with 64 bit seed.
        /// @param  seed_value Seed value used to initialise the state.
        random_pcg(unsigned long long int seed_value) {
            this->seed(seed_value);
        }

        /// @brief  Initialise the state from a 64 bit seed.
        /// @param  seed_value Seed value used to initialise the state.
        void seed(unsigned long long int seed_value) {
            this->state = 0;
            this->increment = (seed_value << 1) | 1;
            this->get_random_raw();
            this->state += seed_value;
            this->get_random_raw();
        }

        /// @brief  Get the next random number from the generator.
        /// @return A pseudo-random number.
        unsigned int get_random_raw() {
            // Save current state for output calculation.
            unsigned long long int state_previous = this->state;
            // Advance internal state.
            this->state = state_previous * 0x5851F42D4C957F2Dull + this->increment;
            // Calculate output function.
            unsigned int state_shift_xor_shift = static_cast<unsigned int>(((state_previous >> 18u) ^ state_previous) >> 27u);
            int rotation = state_previous >> 59u;
            return (state_shift_xor_shift >> rotation) | (state_shift_xor_shift << ((-rotation) & 31));
        }

    public:
        /// @brief  Get a random number in the open interval 0 < value < 1.
        /// @return A pseudo-random number.
        double get_random_exclusive() {
            return (static_cast<double>(this->get_random_raw()) + 0.5) * (1.0 / static_cast<double>(1ull << 32));
        }

        /// @brief  Get a random number in the half-open interval 0 <= value < 1.
        /// @return A pseudo-random number.
        double get_random_exclusive_top() {
            return static_cast<double>(this->get_random_raw()) * (1.0 / static_cast<double>(1ull << 32));
        }

        /// @brief  Get a random number in the closed interval 0 <= value <= 1.
        /// @return A pseudo-random number.
        double get_random_inclusive() {
            return static_cast<double>(this->get_random_raw()) * (1.0 / static_cast<double>((1ull << 32) - 1));
        }

        /// @brief  Get a random number between two bounds.
        /// @param  inclusive_min Minimum number that can be returned.
        /// @param  inclusive_max Max number than can be returned.
        /// @return A pseudo-random number.
        unsigned int get_random(unsigned int inclusive_min, unsigned int inclusive_max)  {
            GTL_RANDOM_PCG_ASSERT(inclusive_min < inclusive_max, "Minimum bound must be lower than maximum bound.");
            return (this->get_random_raw() % (1 + inclusive_max - inclusive_min)) + inclusive_min;
        }

        /// @brief  Get a random number between two bounds.
        /// @param  inclusive_min Minimum number that can be returned.
        /// @param  inclusive_max Max number than can be returned.
        /// @return A pseudo-random number.
        double get_random(double inclusive_min, double inclusive_max) {
            GTL_RANDOM_PCG_ASSERT(inclusive_min < inclusive_max, "Minimum bound must be lower than maximum bound.");
            return (this->get_random_inclusive() * (inclusive_max - inclusive_min)) + inclusive_min;
        }
    };
}

#undef GTL_RANDOM_PCG_ASSERT

#endif // GTL_RANDOM_RANDOM_PCG_HPP