Loretta Macklem wrote:

> I am working with ECJ and I have made a shell script to do 30 runs in a 

> row. I am running into a seeding issue. It is always using the same 

> seed. Is there a way to set the seed to use a random number?



The ec.util.MersenneTwister class has some deficiencies that have caused

us to develop an improved version for our project. One of these is the

default seed method, which is just;



public MersenneTwister() {

   this(System.currentTimeMillis());

}



If you create several instances in quick succession you get identical

RNGs, and System.currentTimeMillis() doesn't have much entropy anyway.

Here's the default seed method in our version;



   private final static long ISEED_MULT = 0x5DEECE66DL;

   private final static long ISEED_INC  = 0xBL;

   private final static long ISEED_MASK = (1L<<48)-1L;



   private static volatile long iseed = 0xE5524F178035A3C6L;



   private long defaultSeed() {



     final long xseed = ((iseed*ISEED_MULT)+ISEED_INC)&ISEED_MASK;

     final long nanos = System.nanoTime(); iseed = xseed;

     return (nanos>>>32)^(nanos<<32)^System.currentTimeMillis()^

            xseed^(((long)super.hashCode())<<32)^

            Runtime.getRuntime().freeMemory();



   }



It combines entropy from the current real time, the number of clock

ticks since last startup, the new object's memory address (via 

super.hashCode()), the amount of free memory in the JVM and the

output of an embedded static RNG which uses the same algorithm as

java.util.Random. This produces a reasonable amount of entropy

for experimental purposes (we have a couple of 'high quality' seeding

methods too but they take longer to run). Feel free to replace the

default seeding algorithm in MersenneTwister and/or MersenneTwisterFast

with this one. Unfortunately the MersenneTwister setSeed(long seed)

method only uses the low 32 bits of the seed supplied, so for use in

the current ECJ class this version will give slightly better results;



   private final static long ISEED_MULT = 0x5DEECE66DL;

   private final static long ISEED_INC  = 0xBL;

   private final static long ISEED_MASK = (1L<<48)-1L;



   private static volatile long iseed = 0xE5524F178035A3C6L;



   /**

    * Constructor using an improved default seed.

    */

   public MersenneTwister() {



     final long xseed = ((iseed*ISEED_MULT)+ISEED_INC)&ISEED_MASK;

     iseed = xseed;

     this(System.nanoTime()^System.currentTimeMillis()^xseed^

          super.hashCode()^Runtime.getRuntime().freeMemory());



   }



Access to iseed isn't atomic or synchronized, but that's ok because

super.hashCode() and System.nanoTime() still pretty much guarantee

uniqueness even in the unlikely event of overlapping updates.



We've made various other improvements in performance and statistical

quality over the current ECJ RNG classes (plus we've added lots of

extra utility methods). If anyone is interested I'll try to find the

time to do a proper performance comparison and an open-source release

of the code.



Michael Wilson

Systems Engineer

Purple Frog Text Ltd

+44 (0)114 2015707