#5501: randomR overflow
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Reporter: daniel.is.fischer | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries/random
Version: 7.2.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Incorrect result at runtime
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When given a large range, `randomR` overflows at `Double` etc.
`randomIvalDouble` has two problems: first, the calculation of the center,
`(l+h)/2` overflows if the range is located near `±Infinity`; second, and
that concerns also `randomRFloating`, the scaling factor `(h-l)` overflows
if the range is large enough.
Both problems can be fixed "well enough" by multiplying the bounds by 0.5
before the calculations and scaling up at the end,
{{{
0.5*l + 0.5*h instead of (l+h)/2
(0.5*h - 0.5*l)/(0.5*realToFrac int32Count) in randomIvalDouble
2.0*(0.5*l + coef*(0.5*h - 0.5*l)) in randomRFloating
}}}
These transformations can introduce a small error when a subnormal number
is involved, but I think we can ignore that (no sane person would have a
[nonzero] subnormal number as a bound, and a correct-for-all-cases
transformation would be somewhat convoluted).
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5501>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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