Timothy Boronczyk/zaemis

<?php
class Fibonacci implements Iterator, ArrayAccess
{
    const PHI = 1.618033989;

    private $i;

    private $a;
    private $b;

    public function __construct() {
    }

    public function rewind() {
        $this->i = 0;
        $this->a = 1;
        $this->b = 0;
    }

    public function current() {
        return $this->b;
    }

    public function key() {
       return $this->i;
    }

    public function next() {
        $this->i++;
        $tmp = $this->a + $this->b;
        $this->a = $this->b;
        $this->b = $tmp;
    }

    public function valid() {
        return $this->offsetExists($this->i);
    }

    public function offsetExists($i) {
        // Fib(1476) = float(1.30698922376E+308)
        // Fib(1477) = float(INF)
        return $i > -1 && $i < 1478;
    }

    public function offsetGet($i) {
        // http://en.wikipedia.org/wiki/Fibonacci_number#Computation_by_rounding
        return floor((pow(self::PHI, $i) / sqrt(5)) + 0.5);
    }

    public function offsetSet($i, $val) {
        throw new Exception("sequence is read-only");
    }

    public function offsetUnset($i) {
        throw new Exception("sequence is read-only");
    }
}