last digit of sum of first N Fibonacci number

FibonacciSumLastDigit.java

   /**
    * Get the last digit of the nth fibonocci number.
    */
    private static long getFibonacciSumNaive(long n) {
        if (n <= 1)
            return n;
        
        // we are using array length = 60  because 
        // fib[i] % 10 has a pissano period of 10 
        // i.e fib[i]%10 repeats itself after every 60 times.
        int [] array = new int[60];
        array[0]=0;
        array[1]=1;
         
        //fib[n-1] = fib[n+1] - fib[n]
        //fib[n-2] = fib[n] - fib[n-3]
        //fib[n-3] = fib[n-3] - fib[n-4]
        // ..      = ..       - ..
        // ..      = ..       - ..
        //fib[0]   = fib[2]   - fib[1]
        //-------------------------------

        // LHS = fib[0] + fib[1] + .... fib[n-2] + fib[n-1] = Sn[n-1]
        // RHS = fib[n+1] - fib[1] = fib[n+1] - 1

        // Sn[n-1] = fib[n+1]-1 
        // Therefore Sn[n] = fib[n+2] - 1

        // first populate all the modulo remaininder up to pisano number 60
        for(int i=2;i<60;i++){
            array[i] = (array[i-1]+array[i-2])%60;
        }
        // get index of fib[n+2]%60
        int index= (int)((n+2)%60);


        // array[index] = fib[n+2] 
        // Sn[n] = array[index]-1 = fib[n+2]-1
        int val = array[index]-1;
        
        // finally get the last digit of Sn[n]
        return val%10;
 
    }