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package at.chrl.nutils;
import java.util.Random;
/**
* @author David Beaumont, Copyright 2005
*
* A Java implementation of the MT19937 (Mersenne Twister) pseudo random
* number generator algorithm based upon the original C code by Makoto
* Matsumoto and Takuji Nishimura (see
* http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/emt.html for more
* information.
*
* As a subclass of java.util.Random this class provides a single
* canonical method next() for generating bits in the pseudo random
* number sequence. Anyone using this class should invoke the public
* inherited methods (nextInt(), nextFloat etc.) to obtain values as
* normal. This class should provide a drop-in replacement for the
* standard implementation of java.util.Random with the additional
* advantage of having a far longer period and the ability to use a far
* larger seed value.
*
* This is not a cryptographically strong source of randomness
* and should not be used for cryptographic systems or in any
* other situation where true random numbers are required.
*
* 
* This software is licensed under the CC-GNU LGPL.
*
* @version 1.0
*/
public class MTRandom extends Random {
/**
* Auto-generated serial version UID. Note that MTRandom does NOT support
* serialisation of its internal state and it may even be necessary to
* implement read/write methods to re-seed it properly. This is only here to
* make Eclipse shut up about it being missing.
*/
private static final long serialVersionUID = -515082678588212038L;
// Constants used in the original C implementation
private final static int UPPER_MASK = 0x80000000;
private final static int LOWER_MASK = 0x7fffffff;
private final static int N = 624;
private final static int M = 397;
private final static int[] MAGIC = { 0x0, 0x9908b0df };
private final static int MAGIC_FACTOR1 = 1812433253;
private final static int MAGIC_FACTOR2 = 1664525;
private final static int MAGIC_FACTOR3 = 1566083941;
private final static int MAGIC_MASK1 = 0x9d2c5680;
private final static int MAGIC_MASK2 = 0xefc60000;
private final static int MAGIC_SEED = 19650218;
private final static long DEFAULT_SEED = 5489L;
// Internal state
private transient int[] mt;
private transient int mti;
private transient boolean compat = false;
// Temporary buffer used during setSeed(long)
private transient int[] ibuf;
/**
* The default constructor for an instance of MTRandom. This invokes the
* no-argument constructor for java.util.Random which will result in the
* class being initialised with a seed value obtained by calling
* System.currentTimeMillis().
*/
public MTRandom() {
}
/**
* This version of the constructor can be used to implement identical
* behaviour to the original C code version of this algorithm including
* exactly replicating the case where the seed value had not been set prior
* to calling genrand_int32.
*
* If the compatibility flag is set to true, then the algorithm will be
* seeded with the same default value as was used in the original C code.
* Furthermore the setSeed() method, which must take a 64 bit long value,
* will be limited to using only the lower 32 bits of the seed to facilitate
* seamless migration of existing C code into Java where identical behaviour
* is required.
*
* Whilst useful for ensuring backwards compatibility, it is advised that
* this feature not be used unless specifically required, due to the
* reduction in strength of the seed value.
*
* @param compatible
* Compatibility flag for replicating original behaviour.
*/
public MTRandom(boolean compatible) {
super(0L);
compat = compatible;
setSeed(compat ? DEFAULT_SEED : System.currentTimeMillis());
}
/**
* This version of the constructor simply initialises the class with the
* given 64 bit seed value. For a better random number sequence this seed
* value should contain as much entropy as possible.
*
* @param seed
* The seed value with which to initialise this class.
*/
public MTRandom(long seed) {
super(seed);
}
/**
* This version of the constructor initialises the class with the given byte
* array. All the data will be used to initialise this instance.
*
* @param buf
* The non-empty byte array of seed information.
* @throws NullPointerException
* if the buffer is null.
* @throws IllegalArgumentException
* if the buffer has zero length.
*/
public MTRandom(byte[] buf) {
super(0L);
setSeed(buf);
}
/**
* This version of the constructor initialises the class with the given
* integer array. All the data will be used to initialise this instance.
*
* @param buf
* The non-empty integer array of seed information.
* @throws NullPointerException
* if the buffer is null.
* @throws IllegalArgumentException
* if the buffer has zero length.
*/
public MTRandom(int[] buf) {
super(0L);
setSeed(buf);
}
// Initializes mt[N] with a simple integer seed. This method is
// required as part of the Mersenne Twister algorithm but need
// not be made public.
/**
* @param seed
*
*/
private void setSeed(int seed) {
// Annoying runtime check for initialisation of internal data
// caused by java.util.Random invoking setSeed() during init.
// This is unavoidable because no fields in our instance will
// have been initialised at this point, not even if the code
// were placed at the declaration of the member variable.
if (mt == null)
mt = new int[N];
// ---- Begin Mersenne Twister Algorithm ----
mt[0] = seed;
for (mti = 1; mti < N; mti++) {
mt[mti] = (MAGIC_FACTOR1 * (mt[mti - 1] ^ (mt[mti - 1] >>> 30)) + mti);
}
// ---- End Mersenne Twister Algorithm ----
}
/**
* This method resets the state of this instance using the 64 bits of seed
* data provided. Note that if the same seed data is passed to two different
* instances of MTRandom (both of which share the same compatibility state)
* then the sequence of numbers generated by both instances will be
* identical.
*
* If this instance was initialised in 'compatibility' mode then this method
* will only use the lower 32 bits of any seed value passed in and will
* match the behaviour of the original C code exactly with respect to state
* initialisation.
*
* @param seed
* The 64 bit value used to initialise the random number
* generator state.
*/
@Override
public final synchronized void setSeed(long seed) {
if (compat) {
setSeed((int) seed);
} else {
// Annoying runtime check for initialisation of internal data
// caused by java.util.Random invoking setSeed() during init.
// This is unavoidable because no fields in our instance will
// have been initialised at this point, not even if the code
// were placed at the declaration of the member variable.
if (ibuf == null)
ibuf = new int[2];
ibuf[0] = (int) seed;
ibuf[1] = (int) (seed >>> 32);
setSeed(ibuf);
}
}
/**
* This method resets the state of this instance using the byte array of
* seed data provided. Note that calling this method is equivalent to
* calling "setSeed(pack(buf))" and in particular will result in a new
* integer array being generated during the call. If you wish to retain this
* seed data to allow the pseudo random sequence to be restarted then it
* would be more efficient to use the "pack()" method to convert it into an
* integer array first and then use that to re-seed the instance. The
* behaviour of the class will be the same in both cases but it will be more
* efficient.
*
* @param buf
* The non-empty byte array of seed information.
* @throws NullPointerException
* if the buffer is null.
* @throws IllegalArgumentException
* if the buffer has zero length.
*/
public final void setSeed(byte[] buf) {
setSeed(pack(buf));
}
/**
* This method resets the state of this instance using the integer array of
* seed data provided. This is the canonical way of resetting the pseudo
* random number sequence.
*
* @param buf
* The non-empty integer array of seed information.
* @throws NullPointerException
* if the buffer is null.
* @throws IllegalArgumentException
* if the buffer has zero length.
*/
public final synchronized void setSeed(int[] buf) {
int length = buf.length;
if (length == 0)
throw new IllegalArgumentException("Seed buffer may not be empty");
// ---- Begin Mersenne Twister Algorithm ----
int i = 1, j = 0, k = (N > length ? N : length);
setSeed(MAGIC_SEED);
for (; k > 0; k--) {
mt[i] = (mt[i] ^ ((mt[i - 1] ^ (mt[i - 1] >>> 30)) * MAGIC_FACTOR2)) + buf[j] + j;
i++;
j++;
if (i >= N) {
mt[0] = mt[N - 1];
i = 1;
}
if (j >= length)
j = 0;
}
for (k = N - 1; k > 0; k--) {
mt[i] = (mt[i] ^ ((mt[i - 1] ^ (mt[i - 1] >>> 30)) * MAGIC_FACTOR3)) - i;
i++;
if (i >= N) {
mt[0] = mt[N - 1];
i = 1;
}
}
mt[0] = UPPER_MASK; // MSB is 1; assuring non-zero initial array
// ---- End Mersenne Twister Algorithm ----
}
/**
* This method forms the basis for generating a pseudo random number
* sequence from this class. If given a value of 32, this method behaves
* identically to the genrand_int32 function in the original C code and
* ensures that using the standard nextInt() function (inherited from
* Random) we are able to replicate behaviour exactly.
*
* Note that where the number of bits requested is not equal to 32 then bits
* will simply be masked out from the top of the returned integer value.
* That is to say that:
*
*
*
* mt.setSeed(12345);
* int foo = mt.nextInt(16) + (mt.nextInt(16) << 16);
*
*
*
* will not give the same result as
*
*
*
* mt.setSeed(12345);
* int foo = mt.nextInt(32);
*
*
* @param bits
* The number of significant bits desired in the output.
* @return The next value in the pseudo random sequence with the specified
* number of bits in the lower part of the integer.
*/
@Override
protected final synchronized int next(int bits) {
// ---- Begin Mersenne Twister Algorithm ----
int y, kk;
if (mti >= N) { // generate N words at one time
// In the original C implementation, mti is checked here
// to determine if initialisation has occurred; if not
// it initialises this instance with DEFAULT_SEED (5489).
// This is no longer necessary as initialisation of the
// Java instance must result in initialisation occurring
// Use the constructor MTRandom(true) to enable backwards
// compatible behaviour.
for (kk = 0; kk < N - M; kk++) {
y = (mt[kk] & UPPER_MASK) | (mt[kk + 1] & LOWER_MASK);
mt[kk] = mt[kk + M] ^ (y >>> 1) ^ MAGIC[y & 0x1];
}
for (; kk < N - 1; kk++) {
y = (mt[kk] & UPPER_MASK) | (mt[kk + 1] & LOWER_MASK);
mt[kk] = mt[kk + (M - N)] ^ (y >>> 1) ^ MAGIC[y & 0x1];
}
y = (mt[N - 1] & UPPER_MASK) | (mt[0] & LOWER_MASK);
mt[N - 1] = mt[M - 1] ^ (y >>> 1) ^ MAGIC[y & 0x1];
mti = 0;
}
y = mt[mti++];
// Tempering
y ^= (y >>> 11);
y ^= (y << 7) & MAGIC_MASK1;
y ^= (y << 15) & MAGIC_MASK2;
y ^= (y >>> 18);
// ---- End Mersenne Twister Algorithm ----
return (y >>> (32 - bits));
}
// This is a fairly obscure little code section to pack a
// byte[] into an int[] in little endian ordering.
/**
* This simply utility method can be used in cases where a byte array of
* seed data is to be used to repeatedly re-seed the random number sequence.
* By packing the byte array into an integer array first, using this method,
* and then invoking setSeed() with that; it removes the need to re-pack the
* byte array each time setSeed() is called.
*
* If the length of the byte array is not a multiple of 4 then it is
* implicitly padded with zeros as necessary. For example:
*
*
*
* byte[] { 0x01, 0x02, 0x03, 0x04, 0x05, 0x06 }
*
*
*
* becomes
*
*
*
* int[] { 0x04030201, 0x00000605 }
*
*
*
*
* Note that this method will not complain if the given byte array is empty
* and will produce an empty integer array, but the setSeed() method will
* throw an exception if the empty integer array is passed to it.
*
* @param buf
* The non-null byte array to be packed.
* @return A non-null integer array of the packed bytes.
* @throws NullPointerException
* if the given byte array is null.
*/
public static int[] pack(byte[] buf) {
int k, blen = buf.length, ilen = ((buf.length + 3) >>> 2);
int[] ibuf = new int[ilen];
for (int n = 0; n < ilen; n++) {
int m = (n + 1) << 2;
if (m > blen)
m = blen;
for (k = buf[--m] & 0xff; (m & 0x3) != 0; k = (k << 8) | buf[--m] & 0xff)
;
ibuf[n] = k;
}
return ibuf;
}
}