Random.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

/*============================================================
**
**
**
** Purpose: A random number generator.
**
** 
===========================================================*/
namespace System {
    
    using System;
    using System.Runtime;
    using System.Runtime.CompilerServices;
    using System.Globalization;
    using System.Diagnostics.Contracts;
[System.Runtime.InteropServices.ComVisible(true)]
    [Serializable]
    public class Random {
      //
      // Private Constants 
      //
      private const int MBIG =  Int32.MaxValue;
      private const int MSEED = 161803398;
      private const int MZ = 0;
    
      
      //
      // Member Variables
      //
      private int inext;
      private int inextp;
      private int[] SeedArray = new int[56];
    
      //
      // Public Constants
      //
    
      //
      // Native Declarations
      //
    
      //
      // Constructors
      //
    
      public Random() 
        : this(Environment.TickCount) {
      }
    
      public Random(int Seed) {
        int ii;
        int mj, mk;
    
        //Initialize our Seed array.
        int subtraction = (Seed == Int32.MinValue) ? Int32.MaxValue : Math.Abs(Seed);
        mj = MSEED - subtraction;
        SeedArray[55]=mj;
        mk=1;
        for (int i=1; i<55; i++) {  //Apparently the range [1..55] is special (Knuth) and so we're wasting the 0'th position.
          ii = (21*i)%55;
          SeedArray[ii]=mk;
          mk = mj - mk;
          if (mk<0) mk+=MBIG;
          mj=SeedArray[ii];
        }
        for (int k=1; k<5; k++) {
          for (int i=1; i<56; i++) {
        SeedArray[i] -= SeedArray[1+(i+30)%55];
        if (SeedArray[i]<0) SeedArray[i]+=MBIG;
          }
        }
        inext=0;
        inextp = 21;
        Seed = 1;
      }
    
      //
      // Package Private Methods
      //
    
      /*====================================Sample====================================
      **Action: Return a new random number [0..1) and reSeed the Seed array.
      **Returns: A double [0..1)
      **Arguments: None
      **Exceptions: None
      ==============================================================================*/
      protected virtual double Sample() {
          //Including this division at the end gives us significantly improved
          //random number distribution.
          return (InternalSample()*(1.0/MBIG));
      }
    
      private int InternalSample() {
          int retVal;
          int locINext = inext;
          int locINextp = inextp;

          if (++locINext >=56) locINext=1;
          if (++locINextp>= 56) locINextp = 1;
          
          retVal = SeedArray[locINext]-SeedArray[locINextp];

          if (retVal == MBIG) retVal--;          
          if (retVal<0) retVal+=MBIG;
          
          SeedArray[locINext]=retVal;

          inext = locINext;
          inextp = locINextp;
                    
          return retVal;
      }

      //
      // Public Instance Methods
      // 
    
    
      /*=====================================Next=====================================
      **Returns: An int [0..Int32.MaxValue)
      **Arguments: None
      **Exceptions: None.
      ==============================================================================*/
      public virtual int Next() {
        return InternalSample();
      }
    
      private double GetSampleForLargeRange() {
          // The distribution of double value returned by Sample 
          // is not distributed well enough for a large range.
          // If we use Sample for a range [Int32.MinValue..Int32.MaxValue)
          // We will end up getting even numbers only.

          int result = InternalSample();
          // Note we can't use addition here. The distribution will be bad if we do that.
          bool negative = (InternalSample()%2 == 0) ? true : false;  // decide the sign based on second sample
          if( negative) {
              result = -result;
          }
          double d = result;
          d += (Int32.MaxValue - 1); // get a number in range [0 .. 2 * Int32MaxValue - 1)
          d /= 2*(uint)Int32.MaxValue - 1  ;              
          return d;
      }


      /*=====================================Next=====================================
      **Returns: An int [minvalue..maxvalue)
      **Arguments: minValue -- the least legal value for the Random number.
      **           maxValue -- One greater than the greatest legal return value.
      **Exceptions: None.
      ==============================================================================*/
      public virtual int Next(int minValue, int maxValue) {
          if (minValue>maxValue) {
              throw new ArgumentOutOfRangeException("minValue",Environment.GetResourceString("Argument_MinMaxValue", "minValue", "maxValue"));
          }
          Contract.EndContractBlock();
          
          long range = (long)maxValue-minValue;
          if( range <= (long)Int32.MaxValue) {  
              return ((int)(Sample() * range) + minValue);
          }          
          else { 
              return (int)((long)(GetSampleForLargeRange() * range) + minValue);
          }
      }
    
    
      /*=====================================Next=====================================
      **Returns: An int [0..maxValue)
      **Arguments: maxValue -- One more than the greatest legal return value.
      **Exceptions: None.
      ==============================================================================*/
      public virtual int Next(int maxValue) {
          if (maxValue<0) {
              throw new ArgumentOutOfRangeException("maxValue", Environment.GetResourceString("ArgumentOutOfRange_MustBePositive", "maxValue"));
          }
          Contract.EndContractBlock();
          return (int)(Sample()*maxValue);
      }
    
    
      /*=====================================Next=====================================
      **Returns: A double [0..1)
      **Arguments: None
      **Exceptions: None
      ==============================================================================*/
      public virtual double NextDouble() {
        return Sample();
      }
    
    
      /*==================================NextBytes===================================
      **Action:  Fills the byte array with random bytes [0..0x7f].  The entire array is filled.
      **Returns:Void
      **Arugments:  buffer -- the array to be filled.
      **Exceptions: None
      ==============================================================================*/
      public virtual void NextBytes(byte [] buffer){
        if (buffer==null) throw new ArgumentNullException("buffer");
        Contract.EndContractBlock();
        for (int i=0; i<buffer.Length; i++) {
          buffer[i]=(byte)(InternalSample()%(Byte.MaxValue+1)); 
        }
      }
    }



}