[erlang-bugs] Unstated limits of random:uniform/1

[Michael Truog]

27817185604309 == 30269 * 30307 * 30323 is the highest N should ever be, otherwise the result is garbage.


On 02/11/2014 06:47 AM, Kostis Sagonas wrote:
> The documentation of random:uniform/1 reads:
>
>   uniform(N) -> integer() >= 1
>
>   Types:
>
>     N = integer() >= 1
>
>   Given an integer N >= 1, uniform/1 returns a random integer uniformly distributed between 1 and N, updating the state in the process dictionary.
>
> and from it a (naive?) Erlang programmer would assume that it works for Erlang integers. However, apparently there is an (unstated) upper limit.
>
> ========================================================================
> Eshell V6.0  (abort with ^G)
> 1> random:uniform(1 bsl 42).
> 1950905779137
> 2> random:uniform(1 bsl 1023).
> 64990220693815292632299777453770053245701880982584490305757715776780176648584151835529728245903303858071465265235635364507930685677056366431569479144084789774752709050314473717035731429737215919311815680621634352115003928201262448305879457258028874562676857884269587024825648343920396535221283000212783104001
> 3> random:uniform(1 bsl 1024).
> ** exception error: an error occurred when evaluating an arithmetic expression
>      in function  random:uniform/1
>      in call from erl_eval:do_apply/6
>      in call from shell:exprs/7
>      in call from shell:eval_exprs/7
>      in call from shell:eval_loop/3
>      in call from prim_file:set_cwd/2
> ========================================================================
>
> Minimally, the published documentation (and the types of all functions of this module) has to be updated to explicitly mention this limit.
>
> Ideally, the implementation has to change to avoid use of floats when manipulating Erlang integers.  IMO, it does not really have to do this and result in crashes like that.
>
> Kostis
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