Hello all,
Here's an interesting little bug that arises when std.random.uniform is called
using dchar as the variable type:
/****************************************************************************/
import std.conv, std.random, std.stdio, std.string, std.typetuple;
void main()
{
foreach (C; TypeTuple!(char, wchar, dchar))
{
writefln("Testing with %s: [%s, %s]", C.stringof, to!ulong(C.min),
to!ulong(C.max));
foreach (immutable _; 0 .. 100)
{
auto u = uniform!"[]"(C.min, C.max);
assert(C.min <= u, format("%s.min = %s, u = %s", C.stringof,
to!ulong(C.min), to!ulong(u)));
assert(u <= C.max, format("%s.max = %s, u = %s", C.stringof,
to!ulong(C.max), to!ulong(u)));
}
}
}
/****************************************************************************/
When closed boundaries "[]" are used with uniform, and the min and max of the
distribution are equal to T.min and T.max (where T is the variable type), the
integral/char-type uniform() makes use of an optimization, and returns
std.random.uniform!ResultType(rng);
That is, it uses a specialization of uniform() for the case where one wants a
random number drawn from all the possible bits of a given integral type.
With char and wchar (8- and 16-bit) this works fine. However, dchar (32-bit)
has a .max value that is less than the corresponding number of bits used to
represent it: dchar.max is 1114111, while its 32 bits are theoretically capable
of handling values of up to 4294967295.
A second consequence is that uniform!dchar (the all-the-bits specialization)
will return invalid code points.
I take it this is a consequence of dchar being something of an oddity as a data
type -- while stored as an "integral-like" value, it doesn't actually make use
of the full range of values available in its 32 bits (unlike char and wchar
which make full use of their 8-bit and 16-bit range).
I think it should suffice to forbid uniform!T from accepting dchar parameters
and to tweak the integral-type uniform()'s internal check to avoid calling that
specialization with dchar.
Thoughts ... ?
Thanks & best wishes,
-- Joe
