PHP in_array（）表现糟糕。 搜索数组的最快方法

I have the following simple code to test against collision on a primary key I am creating:

$machine_ids = array();

for($i = 0; $i < 100000; $i++) {
    //Generate machine id returns a 15 character alphanumeric string
    $mid = Functions::generate_machine_id();

    if(in_array($mid, $machine_ids)) {
        die("Collision!");
    } else {
        $machine_ids[] = $mid;  
    }
}

die("Success!");

Any idea why this is taking many minutes to run? Anyway to speed it up?

for($i = 0; $i < 100000; $i++) 
{
  //Generate machine id returns a 15 character alphanumeric string
  $mid = Functions::generate_machine_id();
  if (isset($machine_ids[$mid]))
  {
    die("Collision!");
  }
  $machine_ids[$mid] = true;
}

For this, use $mid as keys, and dummy value as value. Specifically, instead of

if(in_array($mid, $machine_ids)) {
    die("Collision!");
} else {
    $machine_ids[] = $mid;  
}

use

if(isset($machine_ids[$mid])) {
    die("Collision!");
} else {
    $machine_ids[$mid] = 1;  
}

At the end you can extract the array you originally wanted with array_keys($machine_ids).

This should be much faster. If it is still slow, then your Functions::generate_machine_id() is slow.

EDITED to add isset as per comments.

Checking for array membership is a O(n) operation, since you have to compare the value to every element in the array. After you add a whole bunch of stuff to the array, naturally it gets slower.

If you need to do a whole bunch of membership tests, as is the case here, you should use a different data structure that supports O(1) membership tests, such as a hash.

Refactor your code so that it uses a associated array to hold the machine IDs and use isset to check

if( isset($machine_id[$mid]) ) die("Collision");

$machine_ids[$mid] = $mid;

Using isset should be faster