Maciej Sieczka wrote:
> $ r.mapcalc 'map=rand(-2147483648,2147483647)'
>
> $ r.stats -1 map | sort -n | uniq
> 100%
> -2147483647
> -2147483646
>
> Why only 2 values from 4294967295 possible? Region is big enough to
> accomodate more than 2 values from the range:
res[i] = (lo == hi) ? lo : lo + x % (hi - lo);
The expression (hi - lo) is overflowing the range of a signed integer.
Also, x will be restricted to 0 <= x < 2^31, possibly less if the
system doesn't have lrand48().
The wrapping can be fixed by adding casts, i.e.:
res[i] = (lo == hi) ? lo : lo + (unsigned) x % (unsigned) (hi - lo);
The 31-bit limitation can be fixed by replacing lrand48() with
mrand48(), which covers the full (signed) 32-bit range.
The attached patch appears to work.
One final point: -2^31 (= 0x80000000 = -2147483648) is the null value
for the CELL type, so you'll never see that value in a map.
--
Glynn Clements <[EMAIL PROTECTED]>
Index: raster/r.mapcalc/xrand.c
===================================================================
--- raster/r.mapcalc/xrand.c (revision 30316)
+++ raster/r.mapcalc/xrand.c (working copy)
@@ -13,7 +13,7 @@
#if !defined(HAVE_DRAND48)
#define drand48() ((double)rand()/((double)RAND_MAX + 1))
-#define lrand48() ((long)rand())
+#define mrand48() ((long)rand())
#endif
int
@@ -35,11 +35,11 @@
CELL *arg2 = args[2];
for (i = 0; i < columns; i++)
{
- long x = lrand48();
+ unsigned long x = (unsigned long) mrand48();
int lo = arg1[i];
int hi = arg2[i];
if (lo > hi) { int tmp = lo; lo = hi; hi = tmp; }
- res[i] = (lo == hi) ? lo : lo + x % (hi - lo);
+ res[i] = (lo == hi) ? lo : lo + x % (unsigned long) (hi
- lo);
}
return 0;
}
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