#3563: A couple of additions to Data.Bits
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Reporter: porges | Owner:
Type: proposal | Status: new
Priority: normal | Component: libraries/base
Version: 6.11 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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Population count is an often-needed function when bitfiddling, so I think
a version should be supplied. I also have included functions for
converting to and from lists of Booleans:
{{{
-- population count
popCount :: (Bits a, Num t) => a -> t
popCount x = count' (bitSize x) x 0
where
count' 0 _ acc = acc
count' n x acc = count' (n-1) (x `shiftR` 1) (acc + if x .&. 1 == 1 then
1 else 0)
-- this weird if/else is to preserve the nice type
signature :)
-- converts a list of bools to a number
fromBools :: (Bits a) => [Bool] -> a
fromBools = foldl' (\i b -> (i `shiftL` 1) .|. if b then 1 else 0) 0 --
likewise
-- converts a number to a list of bools
toBools :: (Bits a) => a -> [Bool]
toBools x = reverse (toBools' (bitSize x) x)
where
toBools' 0 _ = []
toBools' n x = (x .&. 1 == 1) : toBools' (n-1) (x `shiftR` 1)
}}}
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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3563>
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