This story is about a problem we found in JAS. It was really subtle, so I may need a brief introduction to the project I worked on: LocusEqu. That project retrieves the equation of “any” geometrical locus. If you don’t know what a locus is, Wolfram Math World defines it as “the set of all points (usually forming a curve or surface) satisfying some condition” . LocusEqu generates a few EquationExpression, and then, by using a EquationTranslator, it generates a GenPolynomial<BigRational> object.

For translating doubles, it calls the BigRational constructor whose only argument is a String. This is the code:

Can you spot the bug? Look at the 21st line. If s is “-0.5″, then n is new BigInteger(“-0″), and that’s what makes “-0.5″ be parsed as 1/2.

 

General improvements

I made a few general improvements on GeoGebra:

1. Refactored build.dir in ant build file. Previously, build.dir wasn’t in the root directory.
2. A few ant tasks were added, such compile-grammar, compile-oe (outside Eclipse), run-easyb and run-easyb-outside-eclipse.
3. SVN properties were set in order to work outside Eclipse. This way, .class files will be kept out of the repo without the intervention of any Eclipse plugin.
4. Easyb, a BDD groovy-based framework, has been included in order to test GeoGebra. It is not RSpec, but I guess it’ll do.

Coding

First, I started creating a few EquationPoint classes,  currently there are six EquationPoint children classes:

• EquationFreePoint represents an independent point.
• EquationSymbolicPoint represents a dependent point,  EquationSpecialSymbolicPoint standing only for the locus point.
• EquationNormalPoint and EquationPointVectorPoint are only auxiliar elements.

Then, a few EquationElement classes were added, these stand for the different constructions:

EquationElement is an abstract class containing a few basic methods:

• forPoint: Given an EquationPoint, returns a String with the equation that means that the point is in the construction.
• isAlgebraic: returns true if the construction is algebraic, and false otherwise.

Both EquationGenericCircle and EquationGenericLine are abstractions of specific line and circle contructions, all of them algebraic. EquationGenericSegment is to segment what EquationGenericLine is to lines. Obviously, EquationGenericSegment is not algebraic.

All of these classes are used together by EquationScope.

A pause for a screenshot.

Click for enlarge.

A glimpse into the future.

What to do next?:

• Maybe Equation should be a proper class, not just a String.
• More equations.
• Working out the locus equation.
• Not using an algorithm twice.

 

Do you remember Java’s classes hierarchy? If so, you’ll realize  that Java objects inherit from java.lang.Object by default but that is not actually what we need. But how can we tell JRuby that our object inherits from Object, the king of Ruby world? The answer is so simple: just extend RubyObject! (There is a RubyBasicObject too) Taken from Nokogiri::XML::Node:

Of course, you can extend any other class, as long as it is a “Ruby object”. For example, Nokogiri::XML::Document extends from Nokogiri::XML::Node, and we do not need to do anything special to reflect it, just extend XmlNode like XmlDocument does:

After talking a bit about hierarchy, let’s talk about Java constructors. At least two parameters are needed: a org.jruby.Ruby object and a org.jruby.RubyClass object. Being the importance of the former quite obvious, the reason for the latter may not be so clear. Let me show you some real world code: Nokogiri::XML::Node’s dup method. Take a look at the following test:

Both new and dup methods in subclass rely on Nokogiri::XML::Node’s. In here, you can see the this snippet of code:

The rb_obj_class method returns the class of an object, in this case, self. This way, the new node will be an instance of the same class as the original node. That’s the reason a RubyClass is needed in the constructor, in order to know which is the actual class being instantiated.

By the way, do not forget to call super with the Ruby and RubyClass objects.

Next step: creating methods.

photo credit: quinn.anya

 

photo credit: cesarastudillo