File: mersenne.h

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// mersenne.h - written and placed in public domain by Jeffrey Walton.
//              Copyright assigned to Crypto++ project.

//! \file mersenne.h
//! \brief Class file for Mersenne Twister
//! \warning MersenneTwister is suitable for Monte-Carlo simulations, where uniformaly distrubuted
//!   numbers are required quickly. It should not be used for cryptographic purposes.
//! \since Crypto++ 5.6.3
#ifndef CRYPTOPP_MERSENNE_TWISTER_H
#define CRYPTOPP_MERSENNE_TWISTER_H

#include "cryptlib.h"
#include "secblock.h"
#include "misc.h"

NAMESPACE_BEGIN(CryptoPP)

//! \class MersenneTwister
//! \brief Mersenne Twister class for Monte-Carlo simulations
//! \tparam K Magic constant
//! \tparam M Period parameter
//! \tparam N Size of the state vector
//! \tparam F Multiplier constant
//! \tparam S Initial seed
//! \details Provides the MersenneTwister implementation. The class is a header-only implementation.
//! \warning MersenneTwister is suitable for simulations, where uniformaly distrubuted numbers are
//!   required quickly. It should not be used for cryptographic purposes.
//! \sa MT19937, MT19937ar
//! \since Crypto++ 5.6.3
template <unsigned int K, unsigned int M, unsigned int N, unsigned int F, unsigned long S>
class MersenneTwister : public RandomNumberGenerator
{
public:
	//! \brief Construct a Mersenne Twister
	//! \param seed 32-bit seed
	//! \details Defaults to template parameter S due to changing algorithm
	//!   parameters over time
	MersenneTwister(unsigned long seed = S) : m_seed(seed), m_idx(N)
	{
		m_state[0] = seed;
		for (unsigned int i = 1; i < N+1; i++)
			m_state[i] = word32(F * (m_state[i-1] ^ (m_state[i-1] >> 30)) + i);
	}

	//! \brief Generate random array of bytes
	//! \param output byte buffer
	//! \param size length of the buffer, in bytes
	//! \details Bytes are written to output in big endian order. If output length
	//!   is not a multiple of word32, then unused bytes are not accumulated for subsequent
	//!   calls to GenerateBlock. Rather, the unused tail bytes are discarded, and the
	//!   stream is continued at the next word32 boundary from the state array.
	void GenerateBlock(byte *output, size_t size)
	{
		// Handle word32 size blocks
		word32 temp;
		for (size_t i=0; i < size/4; i++, output += 4)
		{
#if defined(CRYPTOPP_ALLOW_UNALIGNED_DATA_ACCESS) && defined(IS_LITTLE_ENDIAN)
			*((word32*)output) = ByteReverse(NextMersenneWord());
#elif defined(CRYPTOPP_ALLOW_UNALIGNED_DATA_ACCESS)
			*((word32*)output) = NextMersenneWord();
#else
			temp = NextMersenneWord();
			output[3] = CRYPTOPP_GET_BYTE_AS_BYTE(temp, 0);
			output[2] = CRYPTOPP_GET_BYTE_AS_BYTE(temp, 1);
			output[1] = CRYPTOPP_GET_BYTE_AS_BYTE(temp, 2);
			output[0] = CRYPTOPP_GET_BYTE_AS_BYTE(temp, 3);
#endif
		}

		// No tail bytes
		if (size%4 == 0)
		{
			// Wipe temp
			*((volatile word32*)&temp) = 0;
			return;
		}

		// Handle tail bytes
		temp = NextMersenneWord();
		switch (size%4)
		{
			case 3: output[2] = CRYPTOPP_GET_BYTE_AS_BYTE(temp, 1); /* fall through */
			case 2: output[1] = CRYPTOPP_GET_BYTE_AS_BYTE(temp, 2); /* fall through */
			case 1: output[0] = CRYPTOPP_GET_BYTE_AS_BYTE(temp, 3); break;

			default: assert(0); ;;
		}

		// Wipe temp
		*((volatile word32*)&temp) = 0;
	}

	//! \brief Generate a random 32-bit word in the range min to max, inclusive
	//! \returns random 32-bit word in the range min to max, inclusive
	//! \details If the 32-bit candidate is not within the range, then it is discarded
	//!   and a new candidate is used.
	word32 GenerateWord32(word32 min=0, word32 max=0xffffffffL)
	{
		const word32 range = max-min;
		if (range == 0xffffffffL)
			return NextMersenneWord();

		const int maxBits = BitPrecision(range);
		word32 value;

		do{
			value = Crop(NextMersenneWord(), maxBits);
		} while (value > range);

		return value+min;
	}

	//! \brief Generate and discard n bytes
	//! \param n the number of bytes to discard, rounded up to a <tt>word32</tt> size
	//! \details If n is not a multiple of <tt>word32</tt>, then unused bytes are
	//!   not accumulated for subsequent calls to GenerateBlock. Rather, the unused
	//!   tail bytes are discarded, and the stream is continued at the next
	//!   <tt>word32</tt> boundary from the state array.
	void DiscardBytes(size_t n)
	{
		for(size_t i=0; i < RoundUpToMultipleOf(n, 4U); i++)
			NextMersenneWord();
	}

protected:

	//! \brief Returns the next 32-bit word from the state array
	//! \returns the next 32-bit word from the state array
	//! \details fetches the next word frm the state array, performs bit operations on
	//!   it, and then returns the value to the caller.
	word32 NextMersenneWord()
	{
		if (m_idx >= N) { Twist(); }

		word32 temp = m_state[m_idx++];

		temp ^= (temp >> 11);
		temp ^= (temp << 7)  & 0x9D2C5680; // 0x9D2C5680 (2636928640)
		temp ^= (temp << 15) & 0xEFC60000; // 0xEFC60000 (4022730752)

		return temp ^ (temp >> 18);
	}

	//! \brief Performs the twist operaton on the state array
	void Twist()
	{
		static const unsigned long magic[2]={0x0UL, K};
		word32 kk, temp;

		assert(N >= M);
		for (kk=0;kk<N-M;kk++)
		{
			temp = (m_state[kk] & 0x80000000)|(m_state[kk+1] & 0x7FFFFFFF);
			m_state[kk] = m_state[kk+M] ^ (temp >> 1) ^ magic[temp & 0x1UL];
		}

		for (;kk<N-1;kk++)
		{
			temp = (m_state[kk] & 0x80000000)|(m_state[kk+1] & 0x7FFFFFFF);
			m_state[kk] = m_state[kk+(M-N)] ^ (temp >> 1) ^ magic[temp & 0x1UL];
		}

		temp = (m_state[N-1] & 0x80000000)|(m_state[0] & 0x7FFFFFFF);
		m_state[N-1] = m_state[M-1] ^ (temp >> 1) ^ magic[temp & 0x1UL];

		// Reset index
		m_idx = 0;

		// Wipe temp
		*((volatile word32*)&temp) = 0;
	}

private:

	//! \brief 32-bit word state array of size N
	FixedSizeSecBlock<word32, N+1> m_state;
	//! \brief the value used to seed the generator
	unsigned int m_seed;
	//! \brief the current index into the state array
	unsigned int m_idx;
};

//! \class MT19937
//! \brief Original MT19937 generator provided in the ACM paper.
//! \details MT19937 uses 4537 as default initial seed.
//! \sa MT19937ar, <A HREF="http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/ARTICLES/mt.pdf">Mersenne twister:
//!   a 623-dimensionally equidistributed uniform pseudo-random number generator</A>
//! \since Crypto++ 5.6.3
#if CRYPTOPP_DOXYGEN_PROCESSING
class MT19937 : public MersenneTwister<0x9908B0DF /*2567483615*/, 397, 624, 0x10DCD /*69069*/, 4537> {};
#else
typedef MersenneTwister<0x9908B0DF /*2567483615*/, 397, 624, 0x10DCD /*69069*/, 4537> MT19937;
#endif

//! \class MT19937ar
//! \brief Updated MT19937 generator adapted to provide an array for initialization.
//! \details MT19937 uses 5489 as default initial seed. Use this generator when interoperating with C++11's
//!   mt19937 class.
//! \sa MT19937, <A HREF="http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/MT2002/emt19937ar.html">Mersenne Twister
//!   with improved initialization</A>
//! \since Crypto++ 5.6.3
#if CRYPTOPP_DOXYGEN_PROCESSING
class MT19937ar : public MersenneTwister<0x9908B0DF /*2567483615*/, 397, 624, 0x6C078965 /*1812433253*/, 5489> {};
#else
typedef MersenneTwister<0x9908B0DF /*2567483615*/, 397, 624, 0x6C078965 /*1812433253*/, 5489> MT19937ar;
#endif

NAMESPACE_END

#endif // CRYPTOPP_MERSENNE_TWISTER_H