Hi
On 12/20/22 09:15, Hans Henrik Bergan wrote:
btw while we're on the topic, does anyone know if this function gives
biased results/is-safe or not? i honestly don't know:
function random_float(float $min, float $max): float
{
if ($min > $max) throw new \InvalidArgumentException("min must be
less than max");
if ($min === $max) return $min;
return $min + random_int(0, PHP_INT_MAX) / PHP_INT_MAX * ($max - $min);
}
To give a clear answer on this one:
For a full evaluation the reader lacks information: What is this
function supposed to do? For floats, the definition of the boundaries
matter (that's why it's a parameter for Randomizer::getFloat()). Your
function is implemented on the closed interval [0, 1] (i.e. both
including 0 and 1). As mentioned in my email a few minutes ago, I
consider the ClosedClosed behavior to not be the most useful in the
general case, but in your case it might be the correct behavior.
Nonetheless, the function is definitely broken:
1. A IEEE 754 double precision float as used by PHP only has 53 Bits of
precision, but you generate a random 64 Bit integer. Thus rounding will
occur and might cause some values to be returned more often than other.
To improve the function you should only generate 53 Bits of randomness
(i.e. use 2**53 as the divisor and either 2**53-1 or 2**53 as the dividend).
2. Using the (min + rand * (max - min)) construction will generate
biased results (see also my other email).
PHP 8.3's Randomizer::getFloat() method uses the γ-section algorithm
proposed by Prof. Goualard to generate unbiased results for arbitrary
ranges. This algorithm cannot reasonably be implemented in userland,
because the necessary "building blocks" (obtaining the next/previous
float for a given float) are not available for userland code. The best
you can do in PHP 8.2 userland is what Randomizer::nextFloat() does.
TLDR: Yes, the function is broken.
Best regards
Tim Düsterhus
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