## Chapter 12Distributor Pattern

The Fibonacci example below computes the Fibonacci numbers via the collaboration of a set of worker and distributor capsules. The worker capsules, of type FibWorker, iteratively compute the Fibonacci numbers, whereas the distributor capsule, of type Distributor, distributes the computation work among the worker capsules.
Listing 12.1: Fibonacci Example
signature Worker {
Number execute(int num);
}

capsule FibWorker (Worker w) implements Worker {
Number execute(int n) {
if (n < 2) return new Number(n);
if (n < 13) return new Number(helper(n));
return new Sum (w.execute(n-1), w.execute(n-2));
}
private int helper(int n) {
int prev1=0, prev2=1;
for(int i=0; i<n; i++) {
int savePrev1 = prev1;
prev1 = prev2;
prev2 = savePrev1 + prev2;
}
return prev1;
}
}

capsule Distributor (Worker[] workers) implements Worker {
int current = 0;
Number execute(int num) {
Number result = workers[current].execute(num);
current++;
if(current == workers.length) current = 0;
return result;
}
}

capsule Fibonacci (String[] args) {
design {
FibWorker workers;
Distributor d;
d(workers);
wireall(workers, d);
}
void run(){
try {
System.out.println(d.execute(Integer.parseInt(args)).v());
} catch (java.lang.ArrayIndexOutOfBoundsException e) {
System.out.println("Usage:␣panini␣Fibonacci␣<Number>");
}
}
}

class Number {
int number;
Number(int number){ this.number = number; }
int v(){ return number;}
public String toString() { return "" + number; }
}

class Sum extends Number {
Number left; Number right;
Sum(Number left, Number right){ super(0); this.left = left; this.right = right; }
Overrideint v()returnleft.v()+right.v();@@

The system capsule of Fibonacci, lines 3246, declares a system design with 10 Fibworker capsule instances, line 34, and 1 Distributor capsule instance, line 35. Each worker capsule is connected to the distributor capsule, line 37, and vice versa, line 36. The Fibonacci capsule also runs the program to compute the Fibonacci number for the of the input value of args, line 41, by invoking the execute method of the distributor capsule.

The interface for worker and distributor capsules is specified by the signature Worker, lines 13, containing a method execute, implemented by both capsule implementations. The FibWorker capsule implements the signature on lines 13. In the worker capsule, (1) the Fibonacci number for numbers less than 2 is equal to the number itself, line 7; (2) for Fibonacci numbers less than 13, the capsule uses a local helper method, line 8, that iteratively computes the Fibonacci number; and (3) finally for number greater than 13, the computation of the Fibonacci number is sent to worker capsule w, line 9. According the the system design, especially line 37, the worker capsule w is the distributor, which in turn send the computation of the Fibonacci numbers to some other worker capsules. The Distributer capsule, on lines 2230, implements the signature Worker. The distributor invokes execute on the first worker capsules it has available to compute the Fibonacci number num. If the worker number is less than 13, the worker capsule computes the Fibonacci number and returns it. Otherwise, the worker capsule invokes back the execute method of the distributor with messages to compute the Fibonacci for numbers num-1 and num-2. The distributor in turn, invokes other worker capsules to do these computations.

The classes Number, lines 4853 and Sum, lines 5558, are wrapper classes that encapsulate numerical values and their additions.