[GHC] #5280: System.Random commits (rand `mod` base) error.

[GHC]

#5280: System.Random commits (rand `mod` base) error.
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    Reporter:  rrnewton          |       Owner:  rrnewton                   
        Type:  bug               |      Status:  new                        
    Priority:  normal            |   Component:  libraries/random           
     Version:  7.0.3             |    Keywords:  random mod base            
    Testcase:                    |   Blockedby:                             
          Os:  Unknown/Multiple  |    Blocking:                             
Architecture:  Unknown/Multiple  |     Failure:  Incorrect result at runtime
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 You have probably at some point come across the C code "{{{rand() %
 base}}}"'.  It is very intuitive, but unfortunately creates non-uniform
 random numbers, which is easy to see if you imagine {{{rand()}}} producing
 numbers in say `[0,15)` and base being `10`.

 In the function `System.Random.randomIvalInteger` you can see the same
 thing happening.

 The only way I know how to deal with it and generate uniform integers
 within a range is to artificially restrict the range of the source of
 randomness to be a multiple of the desired base.  It can be done simply by
 throwing out some random results.

 This strategy appears to be used by GMP's mpz_urandomm function:

     http://gmplib.org/manual/Integer-Random-Numbers.html#Integer-Random-
 Numbers

 The file `urandomm.c` has the code.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5280>
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