Question

I have an array $used_logins with set of logins (can be large), and I need to generate an array of three unique logins like $login+rand(1, 1000); which wouldn’t be in $used_logins array.

How can I do this fastly?

Answer 1

If you can’t do it in a database or such: Use the keys of $used_logins to store the data. And then check whether an element with the key exists.

$k_used_logins = array_flip($used_logins);  // Complexity is O(n)

$logins = array();
do {
    $l = $login+rand(1, 1000);
    if (!isset($k_used_logins[$l])) { // Complexity O(1)
        $logins[] = $l;
    }
} while (sizeof($logins) != 3);

Depending on the sie of the array this can be faster thant the naive way using array search each time. (One creates a copy of the array, but array_search slower than a key access)