package Fibonacci2;

/**
 * Fibonacci Number Calculator.
 *
 * @author Wayne Eberly
 */

public class Fibonacci {

 public static long fib(int i) throws NumericalOverflowException {
   if (i<0) {
     throw new IllegalArgumentException("Negative Input: " + i);
   } else {                          // i >= 0
     if (i==0) {
				return 0;
     } else {                        // i >= 1
       if (i==1) {
         return 1;
       } else {                      // i >= 2
         if (i >= 92) {              // Too large for input to
                                     // represent the output
           throw new NumericalOverflowException("Overflow when computing the " + i + "th Fibonacci number");
         } else {                    // Output can be represented
           long previous = 0;
           long current = 1;
           long next;
           for (int j=1; j < i; j++) {
           /*
           * Loop Invariant:
           *  a) j is an integer such that 1 <= j <= i-1
           *  b) previous is equal to the j-1'st Fibonacci
           *     number
           *  c) current is equal to the j'th Fibonacci number
           * Loop Variant: i-j
           */
             next = previous + current;
             previous = current;
             current = next;
             assert (previous == SFibonacci.fib(j))
                        && (current == SFibonacci.fib(j+1));
           };
           return current;
         }
       }
     }
   }
 } 

}