Question

I have a function that gives me a number as result:

Now to make it simple I’ll use an invented example:

   function generate($a) {return $a*2;}
   //this is just an example, the real generate function is really expensive in terms of speed and resources

I have also an array with the values to pass to that funcion:

$array = array(1,3,4,6,8,9,11);

I’d like to find the value of $array, that, passed to generate(), gives as output the number nearest to 5 and minor of it.

With a progressive search I would get this:

$array[0] => 2;
$array[1] => 6;
$array[2] => 8;
etc.

In this case I’d expect my search function to give as output the number 1 as this is value that, passed into generate(), gives 2 as output, the number nearest and minor to 5.

Since the generate() function is really slow ( 1,5 sec on average ) I want to do a binary search with the hope of reducing the use of my function.

So basically what I want to do is: slice $array into 2 pieces, use generate(), then slice againg etc.

I’m not an expert both in recursive function and binary search (it’s my first script trying to do that).

However I tried to wrote some code I’m pasting below, however it doesn’t work and honestrly I don’t have a clear idea about it.

function generate($a) {return $a*2;}

$array = array(1,2,3,4,5,6,7,8,9);

function find($array) {
    $first_half = array_slice($array,0,round(count($array)/2,0));
    $second_half = array_slice($array,round(count($array)/2,0));
    echo "<pre>";
    print_r($first_half);
    print_r($second_half);
    echo "</pre>";
    $last = end($first_half);
    $last = generate($last);

    if($last > 4) {find($first_half);}
    else {}
}

find($array);

Can you help me?

Best regards,
Giorgio

Answer 1

While this may be a good try for some academic research, in practice I think it’s slower than a plain progressive approach.

What you would want is to optimize generate() (or the way it is called*) rather than the traversal algorithm.

---

*One such improvement is to replace this:

foreach($array as $k=>$v) $array[$k] = generate($v);

With this:

$array = array_map('generate', $array);