Alepha/fastRandom.cc

static_assert( __cplusplus > 2023'00 );

#include "fastRandom.h"

#include <random>

#include <boost/random/taus88.hpp>

namespace Alepha::Hydrogen  ::detail::  fastRandom_m
{
	namespace
	{
		struct FastRandomState
		{
			boost::random::taus88 fastRandomState{ std::random_device{}() };
			/* The choice of Taus88 is deliberate.  The more familiar Mt19937
			   is only slightly slower and its cousin Mt11213b is effectively
			   no slower, but they require 100x to 200x as many integers to
			   maintain state.  Thus with even only one random variable, the
			   latter two will exceed cache line size and create cache load,
			   but with several/many the cache pressure will be significant and
			   affect performance not just for this code but for the system.
			*/
			std::uint32_t pool;
			int remainingBits= 0;

			void
			next()
			{
				pool= fastRandomState();
				remainingBits= 32;
			}

			void
			refresh()
			{
				if( remainingBits == 0 ) next();
			}

			std::uint32_t
			getBit()
			{
				refresh();
				const std::uint32_t rv= pool & 1;
				--remainingBits;
				pool>>= 1;
				return rv;
			}

			std::uint32_t
			get( int count )
			{
				std::uint32_t rv= 0;
				while( count-- )
				{
					rv<<= 1;
					rv|= getBit();
				}
				return rv;
			}
		};

		thread_local FastRandomState fastRandom;
	}

	std::uint32_t
	exports::fastRandomBits( const int numBits )
	{
		return fastRandom.get( numBits );
	}
}