random

/*
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_VISION_CONSENSUS_SAMPLE_RANDOM_HPP
#define GTL_VISION_CONSENSUS_SAMPLE_RANDOM_HPP

// Summary: Implementation of a random dataset sampler. [wip]

#ifndef NDEBUG
#   if defined(_MSC_VER)
#       define __builtin_trap() __debugbreak()
#   endif
/// @brief A simple assert macro to break the program if the random is misused.
#   define GTL_RANDOM_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_ASSERT(ASSERTION, MESSAGE) static_cast<void>(0)
#endif

#include <vision/consensus/sampler>

namespace {
    using size_t = decltype(sizeof(0));
}

namespace gtl {
    template <size_t sample_size>
    class random final
        : public sampler<sample_size> {
    private:
        class random_pcg final {
        private:
            unsigned long long int state = 0x853C49E6748FEA9Bull;
            unsigned long long int increment = 0xDA3E39CB94B95BDBull;
        public:
            unsigned int get_random() {
                unsigned long long int state_previous = this->state;
                this->state = state_previous * 0x5851F42D4C957F2Dull + this->increment;
                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));
            }
        };

    private:
        random_pcg rng;
        size_t size = 0;

    public:
        virtual void prepare(
            const size_t data_size
        ) override final {
            GTL_RANDOM_ASSERT(data_size > 0, "Data size must be greater than zero.");
            GTL_RANDOM_ASSERT(data_size >= sample_size, "Data size must be greater or equal to the sample size.");
            this->size = data_size;
        }

    public:
        virtual void sample(
            size_t* const __restrict indices
        ) override final {
            GTL_RANDOM_ASSERT(this->size > 0, "Data size must be greater than zero.");
            GTL_RANDOM_ASSERT(this->size >= sample_size, "Data size must be greater or equal to the sample size.");
            for (size_t i = 0; i < sample_size; ++i) {
                // Get a random index.
                indices[i] = static_cast<size_t>(this->rng.get_random()) % this->size;
                // Ensure it is unique.
                for (size_t j = 0; j < i; ++j) {
                    if (indices[j] == indices[i]) {
                        indices[i] = (indices[i] + 1) % this->size;
                        j = static_cast<size_t>(-1);
                    }
                }
            }
        }
    };
}

#undef GTL_RANDOM_ASSERT

#endif // GTL_VISION_CONSENSUS_SAMPLE_RANDOM_HPP