Package com.github.tommyettinger.random

Class FourWheelRandom

java.lang.Object
java.util.Random
com.github.tommyettinger.random.EnhancedRandom
com.github.tommyettinger.random.FourWheelRandom
All Implemented Interfaces:
Externalizable, Serializable, RandomGenerator
public class FourWheelRandom extends EnhancedRandom
A random number generator that is extremely fast on Java 16, and has a very large probable period. This generator is measurably faster than TricycleRandom on Java 16 but slightly slower than it on Java 8. It can be considered stable, like the other EnhancedRandom implementations here. Testing performed should be sufficient, but more can always be done; this passes at least 64TB of PractRand and 2PB of hwd without issues. The second test, hwd, only checks for a specific type of quality issue, but also fails if the period is exhausted; going through 2 to the 52 bytes of data (taking over a week to do so) without exhausting the period should be a strong sign that it will have enough period for most tasks. While this is known to fail one test ("remortality," a check for how long it takes for the bitwise AND/OR of sequential results to reach all 0 bits or all 1 bits), it takes 300PB of data processed to reach a failure point, which is astronomically more than most apps will ever produce. StrangerRandom is probably stronger, but not as fast; TrimRandom is much stronger but also not quite as fast as this class (it is close).
The algorithm used here has four states purely to exploit instruction-level parallelism; it isn't trying to extend the period of the generator beyond about 2 to the 64 (the expected bare minimum, though some cycles will likely be much longer). There's a complex tangle of dependencies across the four states, but it is possible to invert the generator given a full 256-bit state; this is vital for its period and quality. State A and state B operate like a staggered LCG that starts with stateD; this part is why 2 to the 64 is expected as the bare minimum period. State C and state D take two of the other states and combine them; C rotates state B and subtracts state D, while D simply XORs states B and C. This returns the state D that the previous step generated. This performs better than TricycleRandom simply because each of the states can be updated in parallel (using ILP) and all the updates depend on either one or two states, instead of one, two, or three with TricycleRandom.
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().
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.

    • stateD

      protected long stateD
      The fourth 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.

  • Constructor Details

    • FourWheelRandom

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

    • FourWheelRandom

      public FourWheelRandom(long seed)
      Creates a new FourWheelRandom 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

    • FourWheelRandom

      public FourWheelRandom(long stateA, long stateB, long stateC, long stateD)
      Creates a new FourWheelRandom 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
      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() (because stateD 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.
      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. Note that if you call nextLong() immediately after this, it will return the given stateD as-is, so you may want to call some random generation methods (such as nextLong()) and discard the results after setting 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. You may want to call nextLong() a few times after setting the states like this, unless the value for stateD (in particular) is already adequately random; the first call to nextLong(), if it is made immediately after calling this, will return stateD as-is.
      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

    • 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 FourWheelRandom 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