Object comparison and array sorting in PHP

Question

I have a problem with object comparison in PHP. What seems like a straightforward code actually runs way too slow for my liking and as I am not that advanced in the language I would like some feedback and suggestions regarding the following code:

class TestTokenGroup {
    private $tokens;
    ...

    public static function create($tokens) {
        $instance = new static();
        $instance->tokens = $tokens;
        ...
        return $instance;
    }

    public function getTokens() {
        return $this->tokens;
    }

    public static function compare($tokenGroup1, $tokenGroup2) {
        $i = 0;
        $minLength = min(array(count($tokenGroup1->getTokens()), count($tokenGroup2->getTokens())));
        $equalLengths = (count($tokenGroup1->getTokens()) == count($tokenGroup2->getTokens()));
        $comparison = strcmp($tokenGroup1->getTokens()[$i], $tokenGroup2->getTokens()[$i]);
        while ($comparison == 0) {
            $i++;
            if (($i == $minLength) && ($equalLengths == true)) {
                return 0;
            }
            $comparison = strcmp($tokenGroup1->getTokens()[$i], $tokenGroup2->getTokens()[$i]);
        }
        $result = $comparison;
        if ($result < 0)
            return -1;
        elseif ($result > 0)
            return 1;
        else
            return 0;
    }
    ...

}

In the code above $tokens is just a simple array of strings.

Using the method above through usort() for an array of TestTokenGroup consisting of around 40k objects takes ~2secs.

Is there a sensible way to speed that up? Where is the bottleneck here?

EDIT: Added the getTokens() method I initially forgot to include.

Answer 1

You know that objects are "pass by reference", and arrays are "pass by value"?

If getTokens() returns $this->tokens , the array is copied every time you invoke that method.

Try accessing $tokens directly via $tokenGroup1->tokens . You could also use references ( & ) although returning a reference doesn't work in all PHP versions.

Alternatively, make one copy only:

$tokens1 = $tokenGroup1->getTokens();
$tokens2 = $tokenGroup2->getTokens();

Even if each token group is relatively small, it will save at least 40000 * ( 6 + $average_token_group_length * 2) array copies.

UPDATE

I've benchmarked OP's code (removing the ... lines) using:

function gentokens() {
        $ret = [];
        for ( $i=0; $i< 3; $i++)
        {
                $str = "";
                for ( $x = rand(0,3); $x < 10; $x ++ )
                        $str .= chr( rand(0,25) + ord('a') );
                $ret[] = $str;
        }
        return $ret;
}


$start = microtime(true);

$array = [];    // this will hold the TestTokenGroup instances
$dummy = "";    // this will hold the tokens, space-separated and newline-separated
$dummy2= [];    // this will hold the space-concatenated strings

for ( $i=0; $i < 40000; $i++)
{
        $array[] = TestTokenGroup::create( $t = gentokens() );

        $dummy   .= implode(' ', $t ) . "\n";
        $dummy2[] = implode(' ', $t );
}

// write a test file to benchmark GNU sort:
file_put_contents("sort-data.txt", $dummy);

$inited = microtime(true);
printf("init: %f s\n", ($inited-$start));

usort( $array, [ 'TestTokenGroup', 'compare'] );

$sorted = microtime(true);
printf("sort: %f s\n", ($sorted-$inited));

usort( $dummy2, 'strcmp' );

$sorted2 = microtime(true);
printf("sort: %f s\n", ($sorted2-$sorted));

With the following results:

init: 0.359329 s    // for generating 40000 * 3 random strings and setup
sort: 1.012096 s    // for the TestTokenGroup::compare
sort: 0.120583 s    // for the 'strcmp' compare

And, running time sort sort-data.txt > /dev/null yields

.052 u  (user-time, in seconds).

optimisation 1: remove array copies

replacing ->getTokens() with ->tokens yields (I'll only list the TestTokenGroup::compare results):

sort: 0.832794 s

Optimisation 2: remove redundant array() in min

Changing the $minlength line to:

$minLength = min(count($tokenGroup1->tokens), count($tokenGroup2->tokens));

gives

sort: 0.779134 s

Optimisation 3: Only call count once for each tokenGroup

    $count1 = count($tokenGroup1->tokens);
    $count2 = count($tokenGroup2->tokens);
    $minLength = min($count1, $count2);
    $equalLengths = ($count1 == $count2);

gives

sort: 0.679649 s

Alternative approach

The fastest sort so far is strcmp( $stringarray, 'strcmp' ) : 0.12s - still twice as slow as GNU sort, but the latter only does one thing, and does it well.

So, to sort the TokenGroups efficiently we need to construct sort key consisting of a simple string. We can use \\0 as a delimiter for the tokens, and we don't have to worry about them being equal length, because as soon as one character is different, the compare aborts.

Here's the implementation:

$arr2 = [];
foreach ( $array as $o )
  $arr2[ implode("\0", $o->getTokens() ) ] = $o;

$init2 = microtime(true);
printf("init2: %f s\n", ($init2-$sorted2));

uksort( $arr2, 'strcmp' );

$sorted3 = microtime(true);
printf("sort: %f s\n", ($sorted3-$init2));

and here the results:

init2: 0.125939 s
sort: 0.104717 s