[GRASS5] r.random broken

[Glynn Clements]

Hamish wrote:

> > > > lrand48() is supposed to return random numbers between 0 and 2^31.
> > > > The above lrand48() macro will end up returning values which are far
> > > > too small, so, the random test will return true too often, resulting
> > > > in the desired number of random cells being reached far too early.
> > ..
> > 
> > just replace 32767.0 with 2^31. Maybe try-
> 
> 
> sorry, ignore that.
> 
> better:
> 
> #define lrand48() rand()*65536.

Still wrong.

lrand48() returns a value between 0 and 2^31.

rand() returns a value between 0 and RAND_MAX, where RAND_MAX is an
implementation-defined constant.

To emulate lrand48() using rand(), you would need something like:

	#define lrand48() (rand() * (1<<31) / RAND_MAX)

Except that the above will overflow. To prevent that, convert to
double then back to long, e.g.:

	#define lrand48() ((long)((double) rand() * (1<<31) / RAND_MAX))

Or, if your compiler supports it, use "long long":

	#define lrand48() ((long)(rand() * (1LL<<31) / RAND_MAX))

-- 
Glynn Clements <glynn at gclements.plus.com>