Question

I want to produce a natural number (a positive integer between 0 and x) by using lcg_value(). This is what I can see from the actual source code for the function:

/*
 * combinedLCG() returns a pseudo random number in the range of (0, 1).
 * The function combines two CGs with periods of
 * 2^31 - 85 and 2^31 - 249. The period of this function
 * is equal to the product of both primes.
 */

Now, how would I turn that float into a natural number so that I do not lose information/entropy. Currently I just do it like this:

$number = pow(2,31) * lcg_value();

However, I have a feeling in my gut that it’s not perfect. How about 2^62? Or perhaps 2^31-334? Or 2^62-334? I don’t know.

Answer 1

either type float or int are bounded by platform

Integer The size of an integer is platform-dependent, although a maximum value of about two billion is the usual value (that’s 32 bits signed). 64-bit platforms usually have a maximum value of about 9E18. PHP does not support unsigned integers. Integer size can be determined using the constant PHP_INT_SIZE, and maximum value using the constant PHP_INT_MAX since PHP 4.4.0 and PHP 5.0.5.

float The size of a float is platform-dependent, although a maximum of ~1.8e308 with a precision of roughly 14 decimal digits is a common value (the 64 bit IEEE format).

to maintain highest precision of float, u can refer http://www.php.net/manual/en/function.bcmul.php