Editing Incubator escapee wiki:FAQ/Technical (section)

== Is the "random article" feature really random? {{anchor|random}} ==
: No, although it's random enough to provide a small sample of articles reliably.

: In the Wikipedia [[database]], each page is assigned a "random index", which is a random floating point number uniformly distributed between 0 (inclusive) and 1 (exclusive). The "random article" feature (Special:Random) chooses a random [[double-precision|double-precision floating-point number]], and returns the next article whose random index is greater than the selected random number. Some articles will have a larger gap before them, in the random index space, and so will be more likely to be selected. So the actual probability of any given article being selected is in fact itself random.

: The random index value for new articles, and the random value used by Special:Random, is selected by reading two 31-bit words from a [[Mersenne twister]], which is seeded at each request by PHP's initialisation code using a high-resolution timer and the PID. The words are combined using:

::(mt_rand() * $max + mt_rand()) / $max / $max

:Some old articles had their page_random value reset using MySQL's RAND():

:: rand_st->seed1=(rand_st->seed1*3+rand_st->seed2) % rand_st->max_value;
:: rand_st->seed2=(rand_st->seed1+rand_st->seed2+33) % rand_st->max_value;
:: return (((double) rand_st->seed1)/rand_st->max_value_dbl);

:Due to [[phab:T208909|a bug]], many pages created in 2004 and 2005 used to have non-random page_random values; this was corrected in 2018.