http://d.puremagic.com/issues/show_bug.cgi?id=6023

           Summary: std.random.uniform and std.bigint.BigInt compilation
                    error
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: dpx.infinity@gmail.com


--- Comment #0 from Vladimir Matveev <dpx.infinity@gmail.com> 2011-05-16 22:00:21 PDT ---
uniform function from standard library doesn't work correctly with standard
BigInt. There are no indications anywhere that it shouldn't work, so I think it
is a bug.
Code sample:

import std.stdio;
import std.bigint;
import std.random;

void main() {
    BigInt x = uniform!"[]"(BigInt(1), BigInt(1000));
    writefln("%s", x);
}

Compilation error:
/usr/include/d/dmd/phobos/std/conv.d(322): Error: function
std.bigint.BigInt.toString (void delegate(const(char)[]) sink, string
formatString) const is not callable using argument types ()
/usr/include/d/dmd/phobos/std/conv.d(322): Error: expected 2 function
arguments, not 0

Linux i686, dmd 2.052

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Paul D. Anderson <paul.d.anderson@comcast.net> changed:

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--- Comment #1 from Paul D. Anderson <paul.d.anderson@comcast.net> 2011-06-10 13:21:45 PDT ---
The error message is the one returned when std.conv.to!string(BigInt) is
called.

std.bigint doesn't support std.conv.to!string. (You know all about this from 5231.) This has been reported several times already.

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--- Comment #2 from Vladimir Matveev <dpx.infinity@gmail.com> 2011-06-10 23:48:18 PDT ---
(In reply to comment #1)
> The error message is the one returned when std.conv.to!string(BigInt) is
> called.
> 
> std.bigint doesn't support std.conv.to!string. (You know all about this from 5231.) This has been reported several times already.

Yes, I know. But isn't it strange that the standard library is inconsistent with itself? Shouldn't decisions like "remove a function which other parts of library expect to be" be more deliberate? I haven't seen such problems in any of the languages I used, and I think it is not good for the language itself to have them.

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Don <clugdbug@yahoo.com.au> changed:

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--- Comment #3 from Don <clugdbug@yahoo.com.au> 2011-06-11 00:49:41 PDT ---
(In reply to comment #1)
> The error message is the one returned when std.conv.to!string(BigInt) is
> called.
> 
> std.bigint doesn't support std.conv.to!string. (You know all about this from 5231.) This has been reported several times already.

More accurately, to!string doesn't support std.bigint.

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--- Comment #4 from Kenji Hara <k.hara.pg@gmail.com> 2012-05-19 18:38:45 PDT ---
Original sample code cannot yet compile, but error messages are different.

C:\dmd2\src\phobos\std\random.d(1045): Error: template std.random.uniform does
not match any function template declaration
C:\dmd2\src\phobos\std\random.d(1043): Error: template std.random.uniform
cannot deduce template function from argument types
!("[]",igInt,BigInt,MersenneTwisterEngine!(uint,32,624,397,31,-1727483681u,11,7,-1658038656u,15,-272236544u,18))(BigInt,BigInt,MersenneTwistrEngine!(uint,32,624,397,31,-1727483681u,11,7,-1658038656u,15,-272236544u,18))
C:\dmd2\src\phobos\std\random.d(1045): Error: template instance
uniform!("[]",BigInt,BigInt,MersenneTwisterEngine!(uint,32,624,397,31-1727483681u,11,7,-1658038656u,15,-272236544u,18))
errors instantiating template
test.d(8): Error: template instance std.random.uniform!("[]",BigInt,BigInt)
error instantiating
test.d(8): Error: template std.bigint.BigInt.__ctor does not match any function
template declaration
C:\dmd2\src\phobos\std\bigint.d(82): Error: template std.bigint.BigInt.__ctor
cannot deduce template function from argument types !()_error_)

Because BigInt type is not integral.

  import std.traits;
  pragma(msg, isIntegral!BigInt);  // pritns false

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--- Comment #6 from Don <clugdbug@yahoo.com.au> 2012-06-21 02:54:00 PDT ---
(In reply to comment #5)
> (In reply to comment #4)
> 
> > Because BigInt type is not integral.
> > 
> >   import std.traits;
> >   pragma(msg, isIntegral!BigInt);  // pritns false
> 
> Maybe we'll need to add to std.traits a new trait that's true for all integral values, BigInts too.

It's a bit difficult. If BigInt used the same algorithm for random number generation that built-in ints use, the performance would be horrible.

For an n-byte number, you'd have O(2n) memory allocations on average, with an unbounded worst case, instead of the single allocation you'd have with a dedicated algorithm.

And generating a string of 1 million random bits is the sort of thing that somebody is likely to do.

Supporting it with terrible performance isn't any better than not supporting it at all.

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Joseph Rushton Wakeling <joseph.wakeling@webdrake.net> changed:

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                   |                            |t


--- Comment #7 from Joseph Rushton Wakeling <joseph.wakeling@webdrake.net> 2013-08-30 04:41:06 PDT ---
(In reply to comment #0)
> uniform function from standard library doesn't work correctly with standard
> BigInt. There are no indications anywhere that it shouldn't work, so I think it is a bug.

Typical existing RNGs may be expected to have an underlying range of 0 .. uint.max, with others perhaps extending that to ulong.max.  They inherently can't cover the range of possible values that a BigInt can be expected to take, so there's a strong potential for statistical problems (albeit that those maxima are Very Big Indeed).

It might be possible to do some customization so that BigInts falling within the appropriate bounds can be used -- probably your best bet would be to convert to uints, call the integral-type uniform() and convert back -- but in general generating random BigInts would require a BigInt-dedicated RNG.

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