Question

George Marsaglia has written an excellent random number generator that is extremely fast, simple, and has a much higher period than the Mersenne Twister. Here is the code with a description:

good C random number generator

I wanted to port the CMWC4096 code to Java, but it uses several unsigned datatypes so I am not sure how to do this properly. Here is the full C code:

/* choose random initial c<809430660 and */ /* 4096 random 32-bit integers for Q[]   */ static unsigned long Q[4096],c=362436;  unsigned long CMWC4096(void) {     unsigned long long t, a=18782LL;     static unsigned long i=4095;     unsigned long x,r=0xfffffffe;     i = (i+1) & 4095;     t = a*Q[i] + c;     c = (t>>32);     x = t + c;     if (x < c) {         x++;         c++;     }     return (Q[i] = r - x); } 

Can anyone port this to Java? How does this work when you only have signed numbers available?

EDIT: Thanks everybody for the quick answers! For the first 100 million numbers this java code seems to produce the same result as the C code. It is 3 times faster than Java’s java.util.Random.

public class ComplimentaryMultiplyWithCarryRandom {      /**      * Choose 4096 random 32-bit integers      */     private long[] Q;      /**      * choose random initial c<809430660      */     private long c = 362436;      private int i;      public ComplimentaryMultiplyWithCarryRandom() {         Random r = new Random(1);         Q = new long[4096];          // TODO initialize with real random 32bit values         for (int i = 0; i < 4096; ++i) {             long v = r.nextInt();             v -= Integer.MIN_VALUE;             Q[i] = v;         }         i = 4095;     }      int next() {         i = (i + 1) & 4095;         long t = 18782 * Q[i] + c;         c = t >>> 32;         long x = (t + c) & 0xffffffffL;         if (x < c) {             ++x;             ++c;         }          long v = 0xfffffffeL - x;         Q[i] = v;         return (int) v;     } } 

Answer 1

Can anyone port this to Java? How does this work when you only have signed numbers available?

No Stress! a=18782 so the largest t could ever be is not large enough to cause signed vs. unsigned problems. You would have to ‘upgrade’ the result of using Q to a value equal to a 32-bit unsigned number before using it anywhere. e.g. if Q is an int (32-bit signed) then you’d have to do this before using it in the t=a*Q[i]+c statement, e.g.

t=a*(((long)Q[i])&0xffffffffL)+c 

where this (((long)Q[i])&0xffffffffL) business promotes Q[i] to a 64-bit # and ensures its high 32 bits are 0’s. (edit: NOTE: you need 0xffffffffL here. Java does the wrong thing if you use 0xffffffff, it seems like it ‘optimizes’ itself to the wrong answer & you get a negative number if Q[i]’s high bit is 1.)

You should be able to verify this by running the algorithms in C++ and Java to compare the outputs.

edit: here’s a shot at it. I tried running it in C++ and Java for N=100000; they both match. Apologies if I used bad Java idioms, I’m still fairly new to Java.

C++:

// marsaglia2003.cpp   #include <stdio.h> #include <stdlib.h> // for atoi  class m2003 {     enum {c0=362436, sz=4096, mask=4095};     unsigned long Q[sz];     unsigned long c;     short i;  public:     m2003()     {         // a real program would seed this with a good random seed         // i'm just putting in something that makes the output interesting         for (int j = 0; j < sz; ++j)             Q[j] = j + (j << 16);         i = 4095;         c = c0;     }      unsigned long next()     {         unsigned long long t, a=18782LL;         unsigned long x;         unsigned long r=0xfffffffe;         i = (i+1)&mask;         t=a*Q[i]+c;         c=(unsigned long)(t>>32);         x=(unsigned long)t + c;         if (x<c)         {             x++;             c++;         }         return (Q[i]=r-x);     } };  int main(int argc, char *argv[]) {     m2003 generator;     int n = 100;     if (argc > 1)         n = atoi(argv[1]);      for (int i = 0; i < n; ++i)     {         printf('%08x\n', generator.next());     }     return 0; } 

java: (slower than compiled C++ but it matches for N=100000)

// Marsaglia2003.java  import java.util.*;  class Marsaglia2003 {     final static private int sz=4096;     final static private int mask=4095;     final private int[] Q = new int[sz];     private int c=362436;     private int i=sz-1;      public Marsaglia2003()     {         // a real program would seed this with a good random seed         // i'm just putting in something that makes the output interesting         for (int j = 0; j < sz; ++j)             Q[j] = j + (j << 16);     }    public int next()      // note: returns a SIGNED 32-bit number.     // if you want to use as unsigned, cast to a (long),      // then AND it with 0xffffffffL     {         long t, a=18782;         int x;         int r=0xfffffffe;         i = (i+1)&mask;         long Qi = ((long)Q[i]) & 0xffffffffL; // treat as unsigned 32-bit         t=a*Qi+c;         c=(int)(t>>32);             // because 'a' is relatively small this result is also small          x=((int)t) + c;         if (x<c && x>=0) // tweak to treat x as unsigned         {             x++;             c++;         }         return (Q[i]=r-x);     }      public static void main(String args[])     {         Marsaglia2003 m2003 = new Marsaglia2003();          int n = 100;         if (args.length > 0)             n = Integer.parseInt(args[0]);         for (int i = 0; i < n; ++i)         {             System.out.printf('%08x\n', m2003.next());         }     } };