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<?php
/*

 The Generator class takes one or two closures as contructor parameters
 Each closure passed to it should take a single param
 
 The first closure takes the key and generates the value
 
 The second closure takes the key and decides if the key should exist or not
 
 After it is instantiated, use it like a normal array

*/
class Generator implements Iterator, ArrayAccess
{
    private $position   = 0;
    private $generator  = NULL;
    private $existCheck = NULL;
    
    public function __construct($generator, $existCheck = NULL)
    {
        $this->generator  = $generator;
        $this->existCheck = $existCheck;
    }
    
    //ArrayAccess methods
    
    public function offsetExists($offset = NULL)
    {
        $offset = ($offset === NULL
        ?
            $this->position
        :
            $offset
        );
        
        $exist = $this->existCheck;
        
        return ($exist === NULL
        ?
            true
        :
            $exist($offset)
        );
    }
    
    public function offsetGet($offset)
    {
        $gen = $this->generator;
        
        if($this->offsetExists( $offset ))
        {
            return $gen($offset);
        }
    }
    
    public function offsetSet($offset, $value)
    {
        throw new Exception('Cannot set values on a generator');
    }
    
    public function offsetUnset($offset)
    {
        throw new Exception('Cannot unset values on a generator');
    }
    
    //Ends ArrayAccess methods
    
    //Iterator methods
    
    public function rewind()
    {
        $this->position = 0;
    }
    
    public function current()
    {
        $gen = $this->generator;
        
        return $gen
        ?
            $gen( $this->position )
        :
            NULL
        ;
    }
    
    public function key()
    {
        return $this->position;
    }
    
    function next()
    {
        ++$this->position;
    }
    
    public function valid()
    {
        return $this->offsetExists();
    }
    
    //End Iterator methods
}

/*
 Square Generator
 
 The first closure passed generates the value for a given key
 
*/

$sqr = new Generator(
    function($i)
    {
        return $i*$i;
    }
);

foreach(range(-5, 5) as $it)
{
    //var_dump( $sqr[$it] );
}

/*
 Fibonacci Generator
 
 The first closure passed generates the value for a given key
 
 The second closure passed makes sure the value is nonnegative
 (a negative would cause infinte recursion)
*/

$fib = new Generator(
    $fibGenFunc = function($i) use(&$fibGenFunc)
    {
        if($i == 0)
        {
            return 0;
        }
        elseif($i == 1)
        {
            return $i;
        }
        else
        {
            return $fibGenFunc($i - 1) + $fibGenFunc($i - 2);
        }
    },
    function($i)
    {
        return $i >= 0;
    }
);

foreach(range(-5, 5) as $it)
{
    //var_dump( $fib[$it] );
}