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21.6. Fibonacci Example: Calculate Rule

rule Calculate
    when
        // Bind f1 and s1
        f1 : Fibonacci( s1 : sequence, value != -1 )
        // Bind f2 and v2; refer to bound variable s1
        f2 : Fibonacci( sequence == (s1 + 1), v2 : value != -1 )
        // Bind f3 and s3; alternative reference of f2.sequence
        f3 : Fibonacci( s3 : sequence == (f2.sequence + 1 ), value == -1 )      
    then
        // Note the various referencing techniques.
        modify ( f3 ) { value = f1.value + v2 };
        System.out.println( s3 + " == " + f3.value );
end
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  • When there are two Fibonacci objects with values not equal to -1, the Calculate rule is able to match them.
  • There are 50 Fibonacci objects in the Working Memory. A suitable triple should be selected to calculate each of value in turn.
  • Using three Fibonacci patterns in a rule without field constraints to confine the possible cross products would result in many incorrect rule firings. The Calculate rule uses field constraints to correctly constraint the Fibonacci patterns in the correct order. This technique is called cross product matching.
  • The first pattern finds any Fibonacci with a value != -1 and binds both the pattern and the field. The second Fibonacci does this too, but it adds an additional field constraint to ensure that its sequence is greater by one than the Fibonacci bound to f1. When this rule fires for the first time, the two constraints ensure that f1 references sequence 1 and f2 references sequence 2. The final pattern finds the Fibonacci with a value equal to -1 and with a sequence one greater than f2.
  • There are three Fibonacci objects correctly selected from the available cross products. You can calculate the value for the third Fibonacci object that's bound to f3.
  • The modify statement updates the value of the Fibonacci object bound to f3. This means there is now another new Fibonacci object with a value not equal to -1, which allows the Calculate rule to rematch and calculate the next Fibonacci number.
  • Switching to the Audit view will show how the firing of the last Bootstrap modifies the Fibonacci object, enabling the "Calculate" rule to match. This then modifies another Fibonacci object allowing the Calculate rule to match again. This continues till the value is set for all Fibonacci objects.
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