# JRuby: Stochastic Simulation with SSJ

I needed to work on a simulation project that involved working with the SSJ java library.
If I haven’t mentioned before I love Ruby (the programming language) and Java, though a tool in my toolkit, does not evoke the same feeling.

So my first reaction was I can use JRuby to benefit from the existing java library and beauty of Ruby.
The SSJ library comes with a comprehensive sets of examples and documentation. I started exploring that.

Fortunately at RubyConf 2008 with some help from Charles Nutter I was able to install jruby on my laptop, getting jruby installed and running was not trivial. I wish somebody can do a one click installer for jruby same as they did for Ruby and Rails.

I tried the following example from examples.pdf

import umontreal.iro.lecuyer.rng.*;
import umontreal.iro.lecuyer.stat.*;
public class Collision {
int k; // Number of locations.
int m; // Number of items.
double lambda; // Theoretical expectation of C (asymptotic).
boolean[] used; // Locations already used.
public Collision (int k, int m) {
this.k = k;
this.m = m;
lambda = (double) m * m / (2.0 * k);
used = new boolean[k];
}
// Generates and returns the number of collisions.
public int generateC (RandomStream stream) {
int C = 0;
for (int i = 0; i < k; i++) used&#91;i&#93; = false;
for (int j = 0; j < m; j++) {
int loc = stream.nextInt (0, k-1);
if (used&#91;loc&#93;) C++;
else used&#91;loc&#93; = true;
}
return C;
}
// Performs n indep. runs using stream and collects statistics in statC.
public void simulateRuns (int n, RandomStream stream, Tally statC) {
statC.init();
for (int i=0; i<n; i++) statC.add (generateC (stream));
statC.setConfidenceIntervalStudent();
System.out.println (statC.report (0.95, 3));
System.out.println (" Theoretical mean: " + lambda);
}
public static void main (String&#91;&#93; args) {
Tally statC = new Tally ("Statistics on collisions");
Collision col = new Collision (10000, 500);
col.simulateRuns (100000, new MRG32k3a(), statC);
}
}
&#91;/sourcecode&#93;

I compiled and ran the Java code above to ensure I was getting the same results using the following steps:
<pre>
javac -cp "C:\Program Files\Java\jdk1.6.0_02\lib\ext\ssj.jar" Collision.java
java -cp "C:\Program Files\Java\jdk1.6.0_02\lib\ext\ssj.jar;C:\code\ruby\jruby" Collision
</pre>

As you can notice my code was in the C:\code\ruby\jruby directory and I was in this directory when I ran the above program.

Here is the same code example in jruby code:

require 'java'
require 'ssj.jar'

import 'umontreal.iro.lecuyer.rng.RandomStream'
import 'umontreal.iro.lecuyer.stat.Tally'
import 'umontreal.iro.lecuyer.rng.MRG32k3a'

class Collision
def initialize(k, m)
@k = k
@m = m
@lambda = m * m / (2.0 * k)
@used = Array.new(k, false)
end

def generate_c(stream)
c = 0
@k.times { |i| @used[i] = false }
@m.times do |j|
loc = stream.nextInt(0, @k-1)
if @used[loc]
c += 1
else
@used[loc] = true
end
end
return c
end

def simulate_runs(n, stream, stat_c)
stat_c.init
stat_c.setConfidenceIntervalStudent()
puts stat_c.report(0.95, 3)
puts " Theoretical mean: #{@lambda} "
end

def self.run
stat_c = Tally.new("Statistics on collision")
col = Collision.new(10000,500)
col.simulate_runs(100000, MRG32k3a.new, stat_c)
end

end

Collision.run

The jruby code runs comparatively slower (my feeling is 5 times slower but I will measure and add it later). I am using the latest available jruby 1.1.5 at the time of this post.

Update: The JRuby version takes around 13 minutes while the equivalent Java code above takes 17 seconds. This seems to be unacceptable performance, I am not sure if this is specific to SSJ or other libraries too. If I understand correctly, JRuby should be running inside the JVM and when I call SSJ library that is the same Java code that runs so I don’t understand why it is so slow unless I have used some technique that slows down my JRuby version.

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