Package com.github.tommyettinger.random

Class TrimRandom

java.lang.Object
java.util.Random
com.github.tommyettinger.random.EnhancedRandom
com.github.tommyettinger.random.TrimRandom
All Implemented Interfaces:
Externalizable, Serializable, RandomGenerator
public class TrimRandom extends EnhancedRandom
A random number generator that is very fast on Java 16+, has both a very large probable period and a large guaranteed minimum period, and uses only add, bitwise-rotate, and XOR operations (no multiplication). This generator is not as fast as FourWheelRandom or WhiskerRandom on machines that can multiply long values quickly, but is faster than just about everything else (except TricycleRandom and DistinctRandom on Java 8 with HotSpot, or DistinctRandom on most OpenJ9 versions). If this algorithm is run on a GPU, on most hardware it will be significantly faster than FourWheelRandom (indeed, it was faster than any other algorithm I tested on a low-end GPU, though it's been reported that it performs the same as FourWheelRandom on a much better GPU, hitting some bottleneck other than calculation speed).
This can now be considered stable, like the other EnhancedRandom subclasses here. Testing performed should be sufficient, but more can always be done; this passes at least 64TB of PractRand without issues, and passes a much more rigorous single test ("Remortality," which measures how often the bitwise AND/bitwise OR of sequential numbers become all 0 bits or all 1 bits) through over 150 PB. The test in question runs on the GPU using CUDA, so was able to generate far more numbers in a timeframe of days than most CPU approaches could. Earlier versions of Remortality incorrectly measured byte length and reported a higher size, so reports of 1 exabyte by earlier versions are roughly equivalent to 150 petabytes now. This is still a tremendous amount of data, but the space of possible states for a 256-bit generator is even more sizeable. Unfortunately, some initial states appear to be statistically weaker than others, and some may have suspect results on or fail a Remortality test after a few PB. WhiskerRandom seems stronger and faster on desktop CPUs, but could be slower instead if you use its algorithm on a GPU.
This was changed a few times; when the algorithm could be strengthened, I took the chance to do so. The most recent change made the first number returned a little more robust; where before it was always the incoming value of stateC (which would change for the next returned number, but not the current one), now it is the outgoing value of stateC, which is slightly-less obviously-related to one state only. The first result of nextLong() incorporates states A, B, and C, but not D; the second and later results will incorporate stateD. This doesn't seem to have any performance penalty, and may actually improve performance in some cases.
The algorithm used here has four states purely to exploit instruction-level parallelism; one state is a counter (this gives the guaranteed minimum period of 2 to the 64), and the others combine the values of the four states across three variables. There's a complex tangle of dependencies across the states, but it is possible to invert the generator given a full 256-bit state; this is vital for its period and quality.
It is strongly recommended that you seed this with setSeed(long) instead of setState(long, long, long, long), because if you give sequential seeds to both setSeed() and setState(), the former will start off random, while the latter will start off repeating the seed sequence. After about 20-40 random numbers generated, any correlation between similarly seeded generators will probably be completely gone, though.
This implements all optional methods in EnhancedRandom except EnhancedRandom.skip(long); it does implement previousLong() without using skip().
This is called TrimRandom because it uses a trimmed-down set of operations, purely "ARX" -- add, rotate, XOR.
See Also:

  • Field Details

    • stateA

      protected long stateA
      The first state; can be any long.

    • stateB

      protected long stateB
      The second state; can be any long.

    • stateC

      protected long stateC
      The third state; can be any long. If this has just been set to some value, then the next call to nextLong() will return that value as-is. Later calls will be more random.

    • stateD

      protected long stateD
      The fourth state; can be any long.

  • Constructor Details

    • TrimRandom

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

    • TrimRandom

      public TrimRandom(long seed)
      Creates a new TrimRandom 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

    • TrimRandom

      public TrimRandom(long stateA, long stateB, long stateC, long stateD)
      Creates a new TrimRandom with the given four states; all long values are permitted. These states will be used verbatim.
      Parameters:
      stateA - any long value
      stateB - any long value
      stateC - any long value; will be returned exactly on the first call to nextLong()
      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.
      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().
      This uses MX3 by Jon Maiga to mix seed, then only does a little distribution of the mixed long so that 128 of 256 bits are always set across the four states. Because this uses MX3, it uses long multiplication; this is the only part of TrimRandom that does so.
      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.
      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; can be any 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 TrimRandom 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.

    • leap

      public long leap()
      Jumps extremely far in the generator's sequence, such that one call to leap() advances the state as many as Math.pow(2, 48) calls to nextLong(). This can be used to create 65536 substreams of this generator's sequence, each with a period of at least Math.pow(2, 48) but likely much more.
      Returns:
      the result of what nextLong() would return if it was called at the state this jumped to

    • equals

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

    • toString

      public String toString()
      Overrides:
      toString in class Object