Package com.github.tommyettinger.random

Class PcgRXSMXSRandom

java.lang.Object
java.util.Random
com.github.tommyettinger.random.EnhancedRandom
com.github.tommyettinger.random.PcgRXSMXSRandom
All Implemented Interfaces:
Externalizable, Serializable, RandomGenerator
public class PcgRXSMXSRandom extends EnhancedRandom
An exact copy of PCG-Random's RXS-M-XS generator. It has two long states, one of which changes with every generated value and one always-odd state which never changes (the "stream" or "increment"). This uses a linear congruential generator for its changing state (the state changes by multiplying with a large constant and adding the stream), and feeds the resulting value to a specific 64-bit unary hash. The hash is what gives this generator its name; it consists of a random xorshift ("RXS"), a multiplication by a constant ("M"), and another xorshift ("XS"). All shifts are unsigned, and to the right.
This always has a period of 2 to the 64, and there are 2 to the 63 possible sequences that result from changing the stream value. PcgRXSMXSRandom implements all optional methods in EnhancedRandom except EnhancedRandom.skip(long); it does implement previousLong() without using skip().
PcgRXSMXSRandom passes at least 16TB of testing with PractRand, which uses a suite of tests to look for a variety of potential problems. Its original author tested it to 32TB without issues, as well. It has not been tested with hwd or remortality. All the generators here are considered stable. While this is a rather high-quality generator, it is not designed for cryptography, and should not be used for such purposes. Anyone able to see the full output of nextLong() can determine the internal stateA value, and seeing two such full outputs is enough to determine stateB's value as well. There are also some extremely minor and subtle correlations between numerically-similar states that some tests detect, but the ones found so far are essentially insignificant.
See Also:

  • Field Details

    • stateA

      protected long stateA
      The first state, also called the changing state; can be any long.

    • stateB

      protected long stateB
      The second state, also called the stream; can be any odd-number long.

  • Constructor Details

    • PcgRXSMXSRandom

      public PcgRXSMXSRandom()
      Creates a new PcgRXSMXSRandom with a random state.

    • PcgRXSMXSRandom

      public PcgRXSMXSRandom(long seed)
      Creates a new PcgRXSMXSRandom with the given seed; all long values are permitted. The seed will be passed to setSeed(long) to attempt to adequately distribute the seed randomly.
      Parameters:
      seed - any long value

    • PcgRXSMXSRandom

      public PcgRXSMXSRandom(long stateA, long stateB)
      Creates a new PcgRXSMXSRandom with the given two states; all long values are permitted for stateA, and all odd-number long values are permitted for stateB. These states are not changed as long as they are permitted values.
      Parameters:
      stateA - any long value
      stateB - any long value; should be odd, otherwise this will add 1 to make it odd

  • Method Details

    • getTag

      public String getTag()
      Description copied from class: EnhancedRandom
      Gets the tag used to identify this type of EnhancedRandom, as a String. This tag should be unique, and for uniformity purposes, all tags used in this library are 4 characters long. User-defined tags should have a different length.
      Specified by:
      getTag in class EnhancedRandom
      Returns:
      a unique String identifier for this type of EnhancedRandom; usually 4 chars long.

    • getStateCount

      public int getStateCount()
      This generator has 2 long states, so this returns 2.
      Overrides:
      getStateCount in class EnhancedRandom
      Returns:
      2 (two)

    • getSelectedState

      public long getSelectedState(int selection)
      Gets the state determined by selection, as-is. Selections 0 (or any even number) and 1 (or any odd number) refer to states A and B.
      Overrides:
      getSelectedState in class EnhancedRandom
      Parameters:
      selection - used to select which state variable to get; generally 0 or 1
      Returns:
      the value of the selected state

    • setSelectedState

      public void setSelectedState(int selection, long value)
      Sets one of the states, determined by selection, to value, as-is. Selections 0 (or any even number) and 1 (or any odd number) refer to states A and B.
      Overrides:
      setSelectedState in class EnhancedRandom
      Parameters:
      selection - used to select which state variable to set; generally 0 or 1
      value - the exact value to use for the selected state, if valid

    • setSeed

      public void setSeed(long seed)
      This initializes both states of the generator to random values based on the given seed. (2 to the 64) possible initial generator states can be produced here.
      Specified by:
      setSeed in class EnhancedRandom
      Parameters:
      seed - the initial seed; may be any long

    • getStateA

      public long getStateA()
      Gets the first part of the state.
      Returns:
      the first part of the state

    • setStateA

      public void setStateA(long stateA)
      Sets the first part of the state (the changing state).
      Parameters:
      stateA - can be any long

    • getStateB

      public long getStateB()
      Gets the second part of the state (the stream or increment).
      Returns:
      the second part of the state

    • setStateB

      public void setStateB(long stateB)
      Sets the second part of the state (the stream or increment). This must be odd, otherwise this will add 1 to make it odd.
      Parameters:
      stateB - can be any odd-number long; otherwise this adds 1 to make it odd

    • setState

      public void setState(long stateA, long stateB)
      Sets the state completely to the given three state variables. This is the same as calling setStateA(long) and setStateB(long) as a group.
      Overrides:
      setState in class EnhancedRandom
      Parameters:
      stateA - the first state; can be any long
      stateB - the second state; can be any odd-number long

    • nextLong

      public long nextLong()
      Description copied from class: EnhancedRandom
      Returns the next pseudorandom, uniformly distributed long value from this random number generator's sequence. The general contract of nextLong is that one long value is pseudorandomly generated and returned.
      The only methods that need to be implemented by this interface are this and EnhancedRandom.copy(), though other methods can be implemented as appropriate for generators that, for instance, natively produce ints rather than longs.
      Specified by:
      nextLong in interface RandomGenerator
      Specified by:
      nextLong in class EnhancedRandom
      Returns:
      the next pseudorandom, uniformly distributed long value from this random number generator's sequence

    • previousLong

      public long previousLong()
      Description copied from class: EnhancedRandom
      Optional; moves the state to its previous value and returns the previous long that would have been produced by EnhancedRandom.nextLong(). This can be equivalent to calling EnhancedRandom.skip(long) with -1L, but not always; many generators can't efficiently skip long distances, but can step back by one value.
      Generators that natively generate int results typically produce long values by generating an int for the high 32 bits and an int for the low 32 bits. When producing the previous long, the order the high and low bits are generated, such as by EnhancedRandom.previousInt(), should be reversed. Generators that natively produce long values usually don't need to implement EnhancedRandom.previousInt(), but those that produce int usually should implement it, and may optionally call previousInt() twice in this method.
      If you know how to implement the reverse of a particular random number generator, it is recommended you do so here, rather than rely on skip(). This isn't always easy, but should always be possible for any decent PRNG (some historical PRNGs, such as the Middle-Square PRNG, cannot be reversed at all). If a generator cannot be reversed because multiple initial states can transition to the same subsequent state, it is known to have statistical problems that are not necessarily present in a generator that matches one initial state to one subsequent state.
      The public implementation calls EnhancedRandom.skip(long) with -1L, and if skip() has not been implemented differently, then it will throw an UnsupportedOperationException.
      Overrides:
      previousLong in class EnhancedRandom
      Returns:
      the previous number this would have produced with EnhancedRandom.nextLong()

    • next

      public int next(int bits)
      Description copied from class: EnhancedRandom
      Generates the next pseudorandom number with a specific maximum size in bits (not a max number). If you want to get a random number in a range, you should usually use EnhancedRandom.nextInt(int) instead. For some specific cases, this method is more efficient and less biased than EnhancedRandom.nextInt(int). For bits values between 1 and 30, this should be similar in effect to nextInt(1 << bits); though it won't typically produce the same values, they will have the correct range. If bits is 31, this can return any non-negative int; note that nextInt(1 << 31) won't behave this way because 1 << 31 is negative. If bits is 32 (or 0), this can return any int.

      The general contract of next is that it returns an int value and if the argument bits is between 1 and 32 (inclusive), then that many low-order bits of the returned value will be (approximately) independently chosen bit values, each of which is (approximately) equally likely to be 0 or 1.

      Note that you can give this values for bits that are outside its expected range of 1 to 32, but the value used, as long as bits is positive, will effectively be bits % 32. As stated before, a value of 0 for bits is the same as a value of 32.

      Overrides:
      next in class EnhancedRandom
      Parameters:
      bits - the amount of random bits to request, from 1 to 32
      Returns:
      the next pseudorandom value from this random number generator's sequence

    • copy

      public PcgRXSMXSRandom copy()
      Description copied from class: EnhancedRandom
      Creates a new EnhancedRandom with identical states to this one, so if the same EnhancedRandom methods are called on this object and its copy (in the same order), the same outputs will be produced. This is not guaranteed to copy the inherited state of any parent class, so if you call methods that are only implemented by a superclass (like Random) and not this one, the results may differ.
      Specified by:
      copy in class EnhancedRandom
      Returns:
      a deep copy of this EnhancedRandom.

    • equals

      public boolean equals(Object o)
      Overrides:
      equals in class Object

    • toString

      public String toString()
      Overrides:
      toString in class Object