# FastRandom

A fast random number generator for .NET written by Colin Green, January 2005. I'm putting it here just to make it easier to link to it from other articles.

using System; namespace Shimirel.Utilities.Random { /// /// 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. /// /// 2) Faster than System.Random. Up to 8x faster, depending on which methods are called. /// /// 3) 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. /// /// 4) 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. /// /// public class FastRandom : IFastRandom { // The +1 ensures NextDouble doesn't generate 1.0 const double REAL_UNIT_INT = 1.0 / ((double)int.MaxValue + 1.0); const double REAL_UNIT_UINT = 1.0 / ((double)uint.MaxValue + 1.0); const uint Y = 842502087, Z = 3579807591, W = 273326509; uint x, y, z, w; #region Constructors /// /// Initialises a new instance using time dependent seed. /// public FastRandom() { // Initialise using the system tick count. Reinitialise((int)Environment.TickCount); } /// /// Initialises a new instance using an int value as seed. /// This constructor signature is provided to maintain compatibility with /// System.Random /// public FastRandom(int seed) { Reinitialise(seed); } #endregion #region Public Methods [Reinitialisation] /// /// Reinitialises using an int value as a seed. /// /// public void Reinitialise(int seed) { // The only stipulation stated for the xorshift RNG is that at least one of // the seeds x,y,z,w is non-zero. We fulfill that requirement by only allowing // resetting of the x seed x = (uint)seed; y = Y; z = Z; w = W; } #endregion #region Public Methods [System.Random functionally equivalent methods] /// /// 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. /// /// public int Next() { uint t = (x ^ (x << 11)); x = y; y = z; z = w; w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)); // Handle the special case where the value int.MaxValue is generated. This is outside of // the range of permitted values, so we therefore call Next() to try again. uint rtn = w & 0x7FFFFFFF; if (rtn == 0x7FFFFFFF) return Next(); return (int)rtn; } /// /// Generates a random int over the range 0 to upperBound-1, and not including upperBound. /// /// /// public int Next(int upperBound) { if (upperBound < 0) throw new ArgumentOutOfRangeException("upperBound", upperBound, "upperBound must be >=0"); uint t = (x ^ (x << 11)); x = y; y = z; z = w; // The explicit int cast before the first multiplication gives better performance. // See comments in NextDouble. return (int)((REAL_UNIT_INT * (int)(0x7FFFFFFF & (w = (w ^ (w >> 19)) ^ (t ^ (t >> 8))))) * upperBound); } /// /// Generates a random int over the range lowerBound to upperBound-1, and not including upperBound. /// upperBound must be >= lowerBound. lowerBound may be negative. /// /// /// /// public int Next(int lowerBound, int upperBound) { if (lowerBound > upperBound) throw new ArgumentOutOfRangeException("upperBound", upperBound, "upperBound must be >=lowerBound"); uint t = (x ^ (x << 11)); x = y; y = z; z = w; // The explicit int cast before the first multiplication gives better performance. // See comments in NextDouble. int range = upperBound - lowerBound; if (range < 0) { // If range is <0 then an overflow has occured and must resort to using long integer arithmetic instead (slower). // We also must use all 32 bits of precision, instead of the normal 31, which again is slower. return lowerBound + (int)((REAL_UNIT_UINT * (double)(w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)))) * (double)((long)upperBound - (long)lowerBound)); } // 31 bits of precision will suffice if range<=int.MaxValue. This allows us to cast to an int and gain // a little more performance. return lowerBound + (int)((REAL_UNIT_INT * (double)(int)(0x7FFFFFFF & (w = (w ^ (w >> 19)) ^ (t ^ (t >> 8))))) * (double)range); } /// /// Generates a random double. Values returned are from 0.0 up to but not including 1.0. /// /// public double NextDouble() { uint t = (x ^ (x << 11)); x = y; y = z; z = w; // Here we can gain a 2x speed improvement by generating a value that can be cast to // an int instead of the more easily available uint. If we then explicitly cast to an // int the compiler will then cast the int to a double to perform the multiplication, // this final cast is a lot faster than casting from a uint to a double. The extra cast // to an int is very fast (the allocated bits remain the same) and so the overall effect // of the extra cast is a significant performance improvement. // // Also note that the loss of one bit of precision is equivalent to what occurs within // System.Random. return (REAL_UNIT_INT * (int)(0x7FFFFFFF & (w = (w ^ (w >> 19)) ^ (t ^ (t >> 8))))); } /// /// Fills the provided byte array with random bytes. /// This method is functionally equivalent to System.Random.NextBytes(). /// /// public void NextBytes(byte[] buffer) { // Fill up the bulk of the buffer in chunks of 4 bytes at a time. uint x = this.x, y = this.y, z = this.z, w = this.w; int i = 0; uint t; for (int bound = buffer.Length - 3; i < bound;) { // Generate 4 bytes. // Increased performance is achieved by generating 4 random bytes per loop. // Also note that no mask needs to be applied to zero out the higher order bytes before // casting because the cast ignores thos bytes. Thanks to Stefan Troschütz for pointing this out. t = (x ^ (x << 11)); x = y; y = z; z = w; w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)); buffer[i++] = (byte)w; buffer[i++] = (byte)(w >> 8); buffer[i++] = (byte)(w >> 16); buffer[i++] = (byte)(w >> 24); } // Fill up any remaining bytes in the buffer. if (i < buffer.Length) { // Generate 4 bytes. t = (x ^ (x << 11)); x = y; y = z; z = w; w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)); buffer[i++] = (byte)w; if (i < buffer.Length) { buffer[i++] = (byte)(w >> 8); if (i < buffer.Length) { buffer[i++] = (byte)(w >> 16); if (i < buffer.Length) { buffer[i] = (byte)(w >> 24); } } } } this.x = x; this.y = y; this.z = z; this.w = w; } // /// // /// A version of NextBytes that uses a pointer to set 4 bytes of the byte buffer in one operation // /// thus providing a nice speedup. The loop is also partially unrolled to allow out-of-order-execution, // /// this results in about a x2 speedup on an AMD Athlon. Thus performance may vary wildly on different CPUs // /// depending on the number of execution units available. // /// // /// Another significant speedup is obtained by setting the 4 bytes by indexing pDWord (e.g. pDWord[i++]=w) // /// instead of adjusting it dereferencing it (e.g. *pDWord++=w). // /// // /// Note that this routine requires the unsafe compilation flag to be specified and so is commented out by default. // /// // /// // public unsafe void NextBytesUnsafe(byte[] buffer) // { // if(buffer.Length % 8 != 0) // throw new ArgumentException("Buffer length must be divisible by 8", "buffer"); // // uint x=this.x, y=this.y, z=this.z, w=this.w; // // fixed(byte* pByte0 = buffer) // { // uint* pDWord = (uint*)pByte0; // for(int i=0, len=buffer.Length>>2; i < len; i+=2) // { // uint t=(x^(x<<11)); // x=y; y=z; z=w; // pDWord[i] = w = (w^(w>>19))^(t^(t>>8)); // // t=(x^(x<<11)); // x=y; y=z; z=w; // pDWord[i+1] = w = (w^(w>>19))^(t^(t>>8)); // } // } // // this.x=x; this.y=y; this.z=z; this.w=w; // } #endregion #region Public Methods [Methods not present on System.Random] /// /// 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. /// /// public uint NextUInt() { uint t = (x ^ (x << 11)); x = y; y = z; z = w; return (w = (w ^ (w >> 19)) ^ (t ^ (t >> 8))); } /// /// 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(). /// /// public int NextInt() { uint t = (x ^ (x << 11)); x = y; y = z; z = w; return (int)(0x7FFFFFFF & (w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)))); } // Buffer 32 bits in bitBuffer, return 1 at a time, keep track of how many have been returned // with bitBufferIdx. uint bitBuffer; uint bitMask = 1; /// /// 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. /// /// public bool NextBool() { if (bitMask == 1) { // Generate 32 more bits. uint t = (x ^ (x << 11)); x = y; y = z; z = w; bitBuffer = w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)); // Reset the bitMask that tells us which bit to read next. bitMask = 0x80000000; return (bitBuffer & bitMask) == 0; } return (bitBuffer & (bitMask >>= 1)) == 0; } #endregion } }

namespace Shimirel.Utilities.Random { public interface IFastRandom { /// /// Reinitialises using an int value as a seed. /// /// void Reinitialise(int seed); /// /// 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. /// /// int Next(); /// /// Generates a random int over the range 0 to upperBound-1, and not including upperBound. /// /// /// int Next(int upperBound); /// /// Generates a random int over the range lowerBound to upperBound-1, and not including upperBound. /// upperBound must be >= lowerBound. lowerBound may be negative. /// /// /// /// int Next(int lowerBound, int upperBound); /// /// Generates a random double. Values returned are from 0.0 up to but not including 1.0. /// /// double NextDouble(); /// /// Fills the provided byte array with random bytes. /// This method is functionally equivalent to System.Random.NextBytes(). /// /// void NextBytes(byte[] buffer); /// /// 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. /// /// uint NextUInt(); /// /// 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(). /// /// int NextInt(); /// /// 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. /// /// bool NextBool(); } }

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As a Tech Lead for Sagittarius marketing who I have been with for the last twelve years. I oversee a team of seven working pods, including numerous developers and contractors in multiple global locations. This involves supporting the developers with coding issues, meetings and phone calls with their clients and going out of pitches with potential new clients.

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About the author

Richard Brisley

I'm a multi-award winning Sitecore developer. Currently working for Sagittarius Marketing as a solutions architect to understand customer needs and produce multi-national high-performance websites.