On changing rand(3) to random(3) in awk(1)

[Chenguang Li]

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Vitaly Magerya <vmagerya at gmail.com> wrote:

> On 08/28/14 16:28, Chenguang Li wrote:
>> I am using RANDOM_MAX here to maintain the original code structure.
>> I've updated my patch, please check the gist. Thanks!
>> 
>> jmp_buf env;
>> extern int     pairstack[];
>> @@ -1522,7 +1522,7 @@
>>                break;
>>        case FRAND:
>>                /* in principle, rand() returns something in 0..RAND_MAX */
>> -               u = (Awkfloat) (rand() % RAND_MAX) / RAND_MAX;
>> +               u = (Awkfloat) (random() % RANDOM_MAX) / RANDOM_MAX;
>>                break;
>>        case FSRAND:
>>                if (isrec(x))   /* no argument provided */
> 
> OK, so this part is wrong (and it is wrong in the original sources
> too): this construction generates 0.0 twice as often as it generates
> other numbers -- both when random() returns 0 and when it returns
> RANDOM_MAX. The correct construction is this:

Yes, I've noticed there's a fairness problem, 0 comes out more often. Since
the original code is buggy too, do we need to be compatible with it?

>>        case FRAND:
>> -               /* in principle, rand() returns something in 0..RAND_MAX */
>> -               u = (Awkfloat) (rand() % RAND_MAX) / RAND_MAX;
>> +               /* random() returns values in [0, RANDOM_MAX] */
>> +               u = (Awkfloat) random() / (RANDOM_MAX + 1ul);
>>                break;
> 
> Note that the "ul" part in "1ul" is vitally important: otherwise there
> will be an overflow on systems where long is 32 bit wide. Alternatively
> "1.0" can be used here instead.
> 
> (Because this overflow may not be obvious in a casual reading, I'm not
> a fan of defining RANDOM_MAX separately; I personally would insert the
> actual number here directly).

Thanks for pointing that out, I've modified the gist as your suggestion.

Chenguang Li

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