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I’m just updating the Fibonacci example as we overhaul and start to document drools-examples. For a bit of nostolgia I thought I’d show how Fibonacci looked 2 years ago with Drools 2.0 and how it looks now with Drools 4.0 and the new MVEL dialect.

Drools 4.0

package org.drools.examples

import org.drools.examples.FibonacciExample.Fibonacci;

dialect "mvel"

rule Recurse
   salience 10
   when
       f : Fibonacci ( value == -1 )
       not ( Fibonacci ( sequence == 1 ) )
   then
       insert( new Fibonacci( f.sequence - 1 ) );
       System.out.println( "recurse for " + f.sequence );
end

rule Bootstrap
   when
       f : Fibonacci( sequence == 1 || == 2, value == -1 ) // this is a multi-restriction || on a single field
   then
       modify ( f ){ value = 1 };
       System.out.println( f.sequence + " == " + f.value );
end

rule Calculate
   when
       f1 : Fibonacci( s1 : sequence, value != -1 ) // here we bind sequence
       f2 : Fibonacci( sequence == (s1 + 1 ), value != -1 ) // here we don't, just to demonstrate the different way bindings can be used
       f3 : Fibonacci( s3 : sequence == (f2.sequence + 1 ), value == -1 )            
   then  
       modify ( f3 ) { value = f1.value + f2.value };
       System.out.println( s3 + " == " + f3.value ); // see how you can access pattern and field  bindings
end

Drools 2.0

<rule-set name="fibonacci"
          xmlns="http://drools.org/rules"
          xmlns:java="http://drools.org/semantics/java"
          xmlns:xs="http://www.w3.org/2001/XMLSchema-instance"
          xs:schemaLocation="http://drools.org/rules rules.xsd
                             http://drools.org/semantics/java java.xsd">

  <import>org.drools.examples.fibonacci.Fibonacci</import>

  <rule name="Bootstrap 1" salience="20">
    <parameter identifier="f">
      <class>Fibonacci</class>
    </parameter>

    <java:condition>f.getSequence() == 1</java:condition>
    <java:condition>f.getValue() == -1</java:condition>
    <java:consequence>
      f.setValue( 1 );
      System.err.println( f.getSequence() + " == " + f.getValue() );
      drools.modifyObject( f );
    </java:consequence>
  </rule>

  <rule name="Bootstrap 2">
    <parameter identifier="f">
      <class>Fibonacci</class>
    </parameter>
    <java:condition>f.getSequence() == 2</java:condition>
    <java:condition>f.getValue() == -1</java:condition>
    <java:consequence>
      f.setValue( 1 );
      System.err.println( f.getSequence() + " == " + f.getValue() );
      drools.modifyObject( f );
    </java:consequence>
  </rule>

  <rule name="Recurse" salience="10">
    <parameter identifier="f">
      <class>Fibonacci</class>
    </parameter>
    <java:condition>f.getValue() == -1</java:condition>
    <java:consequence>
      System.err.println( "recurse for " + f.getSequence() );
      drools.assertObject( new Fibonacci( f.getSequence() - 1 ) );
    </java:consequence>
  </rule>

  <rule name="Calculate">
    <parameter identifier="f1">
      <class>Fibonacci</class>
    </parameter>
    <parameter identifier="f2">
      <class>Fibonacci</class>
    </parameter>
    <parameter identifier="f3">
      <class>Fibonacci</class>
    </parameter>
    <java:condition>f2.getSequence() == (f1.getSequence() + 1)</java:condition>
    <java:condition>f3.getSequence() == (f2.getSequence() + 1)</java:condition>
    <java:condition>f1.getValue() != -1</java:condition>
    <java:condition>f2.getValue() != -1</java:condition>
    <java:condition>f3.getValue() == -1</java:condition>
    <java:consequence>
      f3.setValue( f1.getValue() + f2.getValue() );
      System.err.println( f3.getSequence() + " == " + f3.getValue() );
      drools.modifyObject( f3 );
      drools.retractObject( f1 );
    </java:consequence>
  </rule>

</rule-set>