CC-GNU LGPL.
*
* @version 1.0
*/
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;
}
}