Randomness

Eris allow multiple sources of randomness, with the requirements that they must accept a seed for reproducibility. Therefore, sequence Pseudo Random Number Generators (PRNG) are used instead of Cryptographically Secure PRNG, which would provide no additional value in generating test cases but make impossible to run the same test twice.

The supported random number generators are:

  • the rand PHP function: this is the default, and simpler, choice.
  • the mt_rand PHP function: this is a faster PRNG.
  • the PHP code implementation purePhpMtRand() is equivalent to mt_rand.

Being implemented inside a PHP object, purePhpMtRand() allows to isolate its state while the first two implementations modify the global state of the PHP process. Use purePhpMtRand() when your code calls rand() or mt_rand() and you don’t want it to interact with the testing framework.

Configuration

<?php
use Eris\Generator;
use Eris\Random;
use Eris\TestTrait;

class RandConfigurationTest extends PHPUnit_Framework_TestCase
{
    use TestTrait;

    public function testUsingTheDefaultRandFunction()
    {
        $this
            ->withRand('rand')
            ->forAll(
                Generator\int()
            )
            ->withMaxSize(1000 * 1000* 1000)
            ->then($this->isInteger());
    }

    public function testUsingTheDefaultMtRandFunction()
    {
        $this
            ->withRand('mt_rand')
            ->forAll(
                Generator\int()
            )
            ->then($this->isInteger());
    }

    public function testUsingThePurePhpMtRandFunction()
    {
        if (defined('HHVM_VERSION')) {
            $this->markTestSkipped('MersenneTwister class does not support HHVM');
        }

        $this
            ->withRand(Random\purePhpMtRand())
            ->forAll(
                Generator\int()
            )
            ->then($this->isInteger());
    }

    private function isInteger()
    {
        return function ($number) {
            $this->assertInternalType('integer', $number);
        };
    }
}

testUsingTheDefaultRandFunction specifies the rand variant, but is equivalent to not calling withRand() at all. srand is the corresponding seed function.

testUsingTheDefaultMtRandFunction configured mt_rand and mt_srand as its seed function.

testUsingThePurePhpMtRandFunction configures purePhpMtRand().

Maximum sizes

The size that can be set and actually reached with withMaxSize() is limited by the chosen PRNG.

  • For rand the maximum values is the result of getrandmax(), which is platform dependent but usually 2^31-1.
  • For mt_rand the maximum value is the result of mt_getrandmax(), which is also platform dependent but usually 2^31-1.
  • For purePhpMtRand(), being implemented in PHP code, the maximum value is 2^32-1.

The limitation on size depends not only on the processor architecture but also on the parameters of the algorithm. Both mt_rand and purePhpMtRand()` implement MT19937, which generates 32-bit integers that can be scaled on any smaller interval.

However, according to the PHP source code, mt_rand implementations uses a lower limit for backward compatibility with rand. purePhpMtRand() has no need for backward compatibility and chooses to allow numbers up to 2^32-1, which is the maximum unsigned number representable with 32 bit.

Seeding

The PRNGS are seeded using the microtime() of the system, or with the ERIS_SEED environment variable for test reproducibility.

Comparison

Variant Portability Speed Global state
rand PHP core Slow Yes
mt_rand PHP core Fast Yes
purePhpMtRand() Eris source code Medium No