Class FastRandom

A fast random number generator for .NET Colin Green, January 2005

September 4th 2005 Added NextBytesUnsafe() - commented out by default. Fixed bug in Reinitialise() - y,z and w variables were not being reset.

Key points:

  1. Based on a simple and fast xor-shift pseudo random number generator (RNG) specified in: Marsaglia, George. (2003). Xorshift RNGs. http://www.jstatsoft.org/v08/i14/xorshift.pdf

This particular implementation of xorshift has a period of 2^128-1. See the above paper to see how this can be easily extened if you need a longer period. At the time of writing I could find no information on the period of System.Random for comparison.

  1. Faster than System.Random. Up to 8x faster, depending on which methods are called.

  2. Direct replacement for System.Random. This class implements all of the methods that System.Random does plus some additional methods. The like named methods are functionally equivalent.

  3. Allows fast re-initialisation with a seed, unlike System.Random which accepts a seed at construction time which then executes a relatively expensive initialisation routine. This provides a vast speed improvement if you need to reset the pseudo-random number sequence many times, e.g. if you want to re-generate the same sequence many times. An alternative might be to cache random numbers in an array, but that approach is limited by memory capacity and the fact that you may also want a large number of different sequences cached. Each sequence can each be represented by a single seed value (int) when using FastRandom.

Notes. A further performance improvement can be obtained by declaring local variables as static, thus avoiding re-allocation of variables on each call. However care should be taken if multiple instances of FastRandom are in use or if being used in a multi-threaded environment.

Inheritance
System.Object
FastRandom
Namespace: OpenAPI.Utils
Assembly: OpenAPI.dll
Syntax
public class FastRandom : object

Constructors

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FastRandom()

Initialises a new instance using time dependent seed.

Declaration
public FastRandom()
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FastRandom(Int32)

Initialises a new instance using an int value as seed. This constructor signature is provided to maintain compatibility with System.Random

Declaration
public FastRandom(int seed)
Parameters
Type Name Description
System.Int32 seed

Methods

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Next()

Generates a random int over the range 0 to int.MaxValue-1. MaxValue is not generated in order to remain functionally equivalent to System.Random.Next(). This does slightly eat into some of the performance gain over System.Random, but not much. For better performance see:

Call NextInt() for an int over the range 0 to int.MaxValue.

Call NextUInt() and cast the result to an int to generate an int over the full Int32 value range including negative values.

Declaration
public int Next()
Returns
Type Description
System.Int32
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Next(Int32)

Generates a random int over the range 0 to upperBound-1, and not including upperBound.

Declaration
public int Next(int upperBound)
Parameters
Type Name Description
System.Int32 upperBound
Returns
Type Description
System.Int32
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Next(Int32, Int32)

Generates a random int over the range lowerBound to upperBound-1, and not including upperBound. upperBound must be >= lowerBound. lowerBound may be negative.

Declaration
public int Next(int lowerBound, int upperBound)
Parameters
Type Name Description
System.Int32 lowerBound
System.Int32 upperBound
Returns
Type Description
System.Int32
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NextBool()

Generates a single random bit. This method's performance is improved by generating 32 bits in one operation and storing them ready for future calls.

Declaration
public bool NextBool()
Returns
Type Description
System.Boolean
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NextBytes(Byte[])

Fills the provided byte array with random bytes. This method is functionally equivalent to System.Random.NextBytes().

Declaration
public void NextBytes(byte[] buffer)
Parameters
Type Name Description
System.Byte[] buffer
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NextDouble()

Generates a random double. Values returned are from 0.0 up to but not including 1.0.

Declaration
public double NextDouble()
Returns
Type Description
System.Double
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NextInt()

Generates a random int over the range 0 to int.MaxValue, inclusive. This method differs from Next() only in that the range is 0 to int.MaxValue and not 0 to int.MaxValue-1.

The slight difference in range means this method is slightly faster than Next() but is not functionally equivalent to System.Random.Next().

Declaration
public int NextInt()
Returns
Type Description
System.Int32
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NextUInt()

Generates a uint. Values returned are over the full range of a uint, uint.MinValue to uint.MaxValue, inclusive.

This is the fastest method for generating a single random number because the underlying random number generator algorithm generates 32 random bits that can be cast directly to a uint.

Declaration
public uint NextUInt()
Returns
Type Description
System.UInt32
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Reinitialise(Int32)

Reinitialises using an int value as a seed.

Declaration
public void Reinitialise(int seed)
Parameters
Type Name Description
System.Int32 seed