include/xrpl/basics/random.h

#pragma once
 
#include <xrpl/beast/utility/instrumentation.h>
#include <xrpl/beast/xor_shift_engine.h>
 
#include <cstddef>
#include <cstdint>
#include <limits>
#include <mutex>
#include <random>
#include <type_traits>
 
namespace xrpl {
 
#ifndef __INTELLISENSE__
static_assert(
    // NOLINTNEXTLINE(misc-redundant-expression)
    std::is_integral_v<beast::xor_shift_engine::result_type> &&
        std::is_unsigned_v<beast::xor_shift_engine::result_type>,
    "The XRPL default PRNG engine must return an unsigned integral type.");
 
static_assert(
    // NOLINTNEXTLINE(misc-redundant-expression)
    std::numeric_limits<beast::xor_shift_engine::result_type>::max() >=
        std::numeric_limits<std::uint64_t>::max(),
    "The XRPL default PRNG engine return must be at least 64 bits wide.");
#endif
 
namespace detail {
 
// Determines if a type can be called like an Engine
// NOLINTNEXTLINE(readability-redundant-typename): typename required by MSVC
template <class Engine, class Result = typename Engine::result_type>
using is_engine = std::is_invocable_r<Result, Engine>;
}  // namespace detail

inline beast::xor_shift_engine&
defaultPrng()
{
    // This is used to seed the thread-specific PRNGs on demand
    static beast::xor_shift_engine kSeeder = [] {
        std::random_device rng;
        std::uniform_int_distribution<std::uint64_t> distribution{1};
        return beast::xor_shift_engine(distribution(rng));
    }();
 
    // This protects the seeder
    static std::mutex kM;
 
    // The thread-specific PRNGs:
    thread_local beast::xor_shift_engine kEngine = [] {
        std::uint64_t seed = 0;
        {
            std::scoped_lock const lk(kM);
            std::uniform_int_distribution<std::uint64_t> distribution{1};
            seed = distribution(kSeeder);
        }
        return beast::xor_shift_engine{seed};
    }();
 
    return kEngine;
}

template <class Engine, class Integral>
std::enable_if_t<std::is_integral_v<Integral> && detail::is_engine<Engine>::value, Integral>
randInt(Engine& engine, Integral min, Integral max)
{
    XRPL_ASSERT(max > min, "xrpl::randInt : max over min inputs");
 
    // This should have no state and constructing it should
    // be very cheap. If that turns out not to be the case
    // it could be hand-optimized.
    return std::uniform_int_distribution<Integral>(min, max)(engine);
}
 
template <class Integral>
std::enable_if_t<std::is_integral_v<Integral>, Integral>
randInt(Integral min, Integral max)
{
    return randInt(defaultPrng(), min, max);
}
 
template <class Engine, class Integral>
std::enable_if_t<std::is_integral_v<Integral> && detail::is_engine<Engine>::value, Integral>
randInt(Engine& engine, Integral max)
{
    return randInt(engine, Integral(0), max);
}
 
template <class Integral>
std::enable_if_t<std::is_integral_v<Integral>, Integral>
randInt(Integral max)
{
    return randInt(defaultPrng(), max);
}
 
template <class Integral, class Engine>
std::enable_if_t<std::is_integral_v<Integral> && detail::is_engine<Engine>::value, Integral>
randInt(Engine& engine)
{
    return randInt(engine, std::numeric_limits<Integral>::max());
}
 
template <class Integral = int>
std::enable_if_t<std::is_integral_v<Integral>, Integral>
randInt()
{
    return randInt(defaultPrng(), std::numeric_limits<Integral>::max());
}


template <class Byte, class Engine>
std::enable_if_t<
    (std::is_same_v<Byte, unsigned char> || std::is_same_v<Byte, std::uint8_t>) &&
        detail::is_engine<Engine>::value,
    Byte>
randByte(Engine& engine)
{
    return static_cast<Byte>(randInt<Engine, std::uint32_t>(
        engine, std::numeric_limits<Byte>::min(), std::numeric_limits<Byte>::max()));
}
 
template <class Byte = std::uint8_t>
std::enable_if_t<(std::is_same_v<Byte, unsigned char> || std::is_same_v<Byte, std::uint8_t>), Byte>
randByte()
{
    return randByte<Byte>(defaultPrng());
}


template <class Engine>
inline bool
randBool(Engine& engine)
{
    return randInt(engine, 1) == 1;
}
 
inline bool
randBool()
{
    return randBool(defaultPrng());
}

 
}  // namespace xrpl