Question

In a browser game we have items that occur based on their probabilities.

P(i1) = 0.8

P(i2) = 0.45

P(i3) = 0.33

P(i4) = 0.01

How do we implement a function in php that returns a random item based on its probability chance?

edit

The items have a property called rarity which varies from 1 to 100 and represents the probability to occcur. The item that occurs is chosen from a set of all items of a certain type. (e.x the given example above represents all artifacts tier 1)

Answer 1

I don’t know if its the best solution but when I had to solve this a while back this is what I found:

Function taken from this blog post:

// Given an array of values, and weights for those values (any positive int) 
// it will select a value randomly as often as the given weight allows. 
// for example:
// values(A, B, C, D)
// weights(30, 50, 100, 25)
// Given these values C should come out twice as often as B, and 4 times as often as D. 
function weighted_random($values, $weights){ 
    $count = count($values); 
    $i = 0; 
    $n = 0; 
    $num = mt_rand(0, array_sum($weights)); 
    while($i < $count){
        $n += $weights[$i]; 
        if($n >= $num){
            break; 
        }
        $i++; 
    } 
    return $values[$i]; 
}

Example call:

$values = array('A','B','C');
$weights = array(1,50,100);

$weighted_value = weighted_random($values, $weights);

It’s somewhat unwieldy as obviously the values and weights need to be supplied separately but this could probably be refactored to suit your needs.