uniform_real_distribution.hpp

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// SPDX-License-Identifier: BSL-1.0
 
#ifndef TETL_RANDOM_UNIFORM_REAL_DISTRIBUTION_HPP
#define TETL_RANDOM_UNIFORM_REAL_DISTRIBUTION_HPP
 
#include <etl/_limits/numeric_limits.hpp>
#include <etl/_random/generate_canonical.hpp>
 
namespace etl {

template <typename RealType = double>
struct uniform_real_distribution {
    using result_type = RealType;
 
    struct param_type {
        using distribution_type = uniform_real_distribution;
 
        constexpr param_type() noexcept = default;
 
        explicit constexpr param_type(result_type min, result_type max = result_type(1)) noexcept
            : _min{min}
            , _max{max}
        {
        }
 
        [[nodiscard]] constexpr auto a() const noexcept -> result_type { return _min; }
 
        [[nodiscard]] constexpr auto b() const noexcept -> result_type { return _max; }
 
        [[nodiscard]] friend constexpr auto operator==(param_type const& lhs, param_type const& rhs) noexcept -> bool
        {
            return (lhs._min == rhs._min) and (lhs._max == rhs._max);
        }
 
    private:
        result_type _min{0};
        result_type _max{1};
    };
 
    constexpr uniform_real_distribution()
        : uniform_real_distribution{static_cast<RealType>(0)}
    {
    }
 
    explicit constexpr uniform_real_distribution(param_type const& parm)
        : _param{parm}
    {
    }
 
    explicit constexpr uniform_real_distribution(RealType a, RealType b = RealType(1))
        : uniform_real_distribution(param_type{a, b})
    {
    }
 
    constexpr auto param(param_type const& parm) -> void { _param = parm; }
 
    [[nodiscard]] constexpr auto param() const -> param_type { return _param; }
 
    [[nodiscard]] constexpr auto a() const -> result_type { return _param.a(); }
 
    [[nodiscard]] constexpr auto b() const -> result_type { return _param.b(); }
 
    [[nodiscard]] constexpr auto min() const -> result_type { return a(); }
 
    [[nodiscard]] constexpr auto max() const -> result_type { return b(); }
 
    constexpr auto reset() -> void { (void)this; }
 
    template <typename URBG>
    [[nodiscard]] constexpr auto operator()(URBG& g) noexcept(noexcept(g())) -> result_type
    {
        return (*this)(g, _param);
    }
 
    template <typename URBG>
    [[nodiscard]] constexpr auto operator()(URBG& g, param_type const& parm) noexcept(noexcept(g())) -> result_type
    {
        constexpr auto digits  = static_cast<size_t>(numeric_limits<RealType>::digits);
        constexpr auto bits    = ~size_t{0};
        constexpr auto minBits = digits < bits ? digits : bits;
        static_assert(minBits <= 64);
 
        // x = a + u * (b - a)
        auto const a = parm.a();
        auto const b = parm.b();
        auto const u = etl::generate_canonical<RealType, minBits>(g);
        return a + u * (b - a);
    }
 
    friend constexpr auto operator==(uniform_real_distribution const& x, uniform_real_distribution const& y) -> bool
    {
        return x.param() == y.param();
    }
 
private:
    param_type _param;
};
 
} // namespace etl
 
#endif // TETL_RANDOM_UNIFORM_REAL_DISTRIBUTION_HPP