Package com.github.tommyettinger.random

Class Xoshiro256StarStarRandom

java.lang.Object
java.util.Random
com.github.tommyettinger.random.EnhancedRandom
com.github.tommyettinger.random.Xoshiro256StarStarRandom
All Implemented Interfaces:
Externalizable, Serializable, RandomGenerator
public class Xoshiro256StarStarRandom extends EnhancedRandom
A random number generator that is fairly fast and guarantees 4-dimensional equidistribution (except for the quartet with four zeroes in a row, every quartet of long results is produced exactly once over the period). It has a period of (2 to the 256) - 1, which would take millennia to exhaust on current-generation hardware (at least). It can be considered stable, like the other EnhancedRandom implementations here. This passes heavy testing, but isn't a cryptographic generator, and it does have known issues when its output is multiplied by certain specific constants (any of a lot) and tests are then run. The only invalid state is the one with 0 in each state variable, and this won't ever occur in the normal period of that contains all other states. You should generally seed this with setSeed(long), rather than setState(long, long, long, long), because if you give similar states to the latter, it tends to produce severely flawed output on at least the low-order bits. This can't happen with setSeed().
The main reasons you could prefer this generator to the typically-faster WhiskerRandom are:
  • This generator is 4D equidistributed, so groups of four coordinates will always be unique.
  • This generator is well-studied and appeared in a peer-reviewed paper.
  • You will never use Java 16, and if you use Java 17, you would rather use the implementation in the JDK there.
  • You need a regular structure to the generated numbers, with guarantees about that structure.

This implements all optional methods in EnhancedRandom except EnhancedRandom.skip(long); it does implement previousLong() without using skip().
Xoshiro256** was written in 2018 by David Blackman and Sebastiano Vigna. You can consult their paper for technical details: PDF link here.
See Also:

  • Field Details

    • stateA

      protected long stateA
      The first state; can be any long, as long as all states are not 0.

    • stateB

      protected long stateB
      The second state; can be any long, as long as all states are not 0. This is the state that is scrambled and returned; if it is 0 before a number is generated, then the next number will be 0.

    • stateC

      protected long stateC
      The third state; can be any long, as long as all states are not 0.

    • stateD

      protected long stateD
      The fourth state; can be any long, as long as all states are not 0.

  • Constructor Details

    • Xoshiro256StarStarRandom

      public Xoshiro256StarStarRandom()
      Creates a new Xoshiro256StarStarRandom with a random state.

    • Xoshiro256StarStarRandom

      public Xoshiro256StarStarRandom(long seed)
      Creates a new Xoshiro256StarStarRandom 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

    • Xoshiro256StarStarRandom

      public Xoshiro256StarStarRandom(long stateA, long stateB, long stateC, long stateD)
      Creates a new Xoshiro256StarStarRandom with the given four states; all long values are permitted. These states will be used verbatim, as long as they are not all 0. In that case, stateD is changed.
      Parameters:
      stateA - any long value
      stateB - any long value
      stateC - any long value
      stateD - any long value

  • 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 4 long states, so this returns 4.
      Overrides:
      getStateCount in class EnhancedRandom
      Returns:
      4 (four)

    • getSelectedState

      public long getSelectedState(int selection)
      Gets the state determined by selection, as-is. The value for selection should be between 0 and 3, inclusive; if it is any other value this gets state D as if 3 was given.
      Overrides:
      getSelectedState in class EnhancedRandom
      Parameters:
      selection - used to select which state variable to get; generally 0, 1, 2, or 3
      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, 1, 2, and 3 refer to states A, B, C, and D, and if the selection is anything else, this treats it as 3 and sets stateD. If this would cause all states to be 0, it instead sets the selected state to 0x9E3779B97F4A7C15L.
      Overrides:
      setSelectedState in class EnhancedRandom
      Parameters:
      selection - used to select which state variable to set; generally 0, 1, 2, or 3
      value - the exact value to use for the selected state, if valid

    • setSeed

      public void setSeed(long seed)
      This initializes all 4 states of the generator to random values based on the given seed. (2 to the 64) possible initial generator states can be produced here, all with a different first value returned by nextLong() (because stateB is guaranteed to be different for every different seed).
      Specified by:
      setSeed in class EnhancedRandom
      Parameters:
      seed - the initial seed; may be any long

    • getStateA

      public long getStateA()

    • setStateA

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

    • getStateB

      public long getStateB()

    • setStateB

      public void setStateB(long stateB)
      Sets the second part of the state. Note that if you set this state to 0, the next random long (or most other types) will be 0, regardless of the other states.
      Parameters:
      stateB - can be any long

    • getStateC

      public long getStateC()

    • setStateC

      public void setStateC(long stateC)
      Sets the third part of the state.
      Parameters:
      stateC - can be any long

    • getStateD

      public long getStateD()

    • setStateD

      public void setStateD(long stateD)
      Sets the fourth part of the state.
      Parameters:
      stateD - can be any long

    • setState

      public void setState(long stateA, long stateB, long stateC, long stateD)
      Sets the state completely to the given four state variables. This is the same as calling setStateA(long), setStateB(long), setStateC(long), and setStateD(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 long
      stateC - the third state; can be any long
      stateD - the fourth state; this will be returned as-is if the next call is to nextLong()

    • 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

    • 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

    • 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()

    • leap

      public long leap()
      Jumps extremely far in the generator's sequence, such that it requires Math.pow(2, 64) calls to leap() to complete a cycle through the generator's entire sequence. This can be used to create over 18 quintillion substreams of this generator's sequence, each with a period of Math.pow(2, 192).
      Returns:
      the result of what nextLong() would return if it was called at the state this jumped to

    • copy

      public Xoshiro256StarStarRandom 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