Data/BitUtil.hs

    {-# LANGUAGE CPP #-}
    #if __GLASGOW_HASKELL__
    {-# LANGUAGE MagicHash #-}
    #endif
    #if !defined(TESTING) && __GLASGOW_HASKELL__ >= 703
    {-# LANGUAGE Trustworthy #-}
    #endif
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.BitUtil
    -- Copyright   :  (c) Clark Gaebel 2012
    --                (c) Johan Tibel 2012
    -- License     :  BSD-style
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  provisional
    -- Portability :  portable
    -----------------------------------------------------------------------------

    module Data.BitUtil
        ( highestBitMask
        , shiftLL
        , shiftRL
        ) where

    -- On GHC, include MachDeps.h to get WORD_SIZE_IN_BITS macro.
    #if defined(__GLASGOW_HASKELL__)
    # include "MachDeps.h"
    #endif

    import Data.Bits ((.|.), xor)

    #if __GLASGOW_HASKELL__
    import GHC.Exts (Word(..), Int(..))
    import GHC.Prim (uncheckedShiftL#, uncheckedShiftRL#)
    #else
    import Data.Word (shiftL, shiftR)
    #endif

    -- The highestBitMask implementation is based on
    -- http://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2
    -- which has been put in the public domain.

    -- | Return a word where only the highest bit is set.
    highestBitMask :: Word -> Word
    highestBitMask x1 = let x2 = x1 .|. x1 `shiftRL` 1
                            x3 = x2 .|. x2 `shiftRL` 2
                            x4 = x3 .|. x3 `shiftRL` 4
                            x5 = x4 .|. x4 `shiftRL` 8
                            x6 = x5 .|. x5 `shiftRL` 16
    #if !(defined(__GLASGOW_HASKELL__) && WORD_SIZE_IN_BITS==32)
                            x7 = x6 .|. x6 `shiftRL` 32
                         in x7 `xor` (x7 `shiftRL` 1)
    #else
                         in x6 `xor` (x6 `shiftRL` 1)
    #endif
    {-# INLINE highestBitMask #-}

    -- Right and left logical shifts.
    shiftRL, shiftLL :: Word -> Int -> Word
    #if __GLASGOW_HASKELL__
    {--------------------------------------------------------------------
      GHC: use unboxing to get @shiftRL@ inlined.
    --------------------------------------------------------------------}
    shiftRL (W# x) (I# i) = W# (uncheckedShiftRL# x i)
    shiftLL (W# x) (I# i) = W# (uncheckedShiftL#  x i)
    #else
    shiftRL x i   = shiftR x i
    shiftLL x i   = shiftL x i
    #endif
    {-# INLINE shiftRL #-}
    {-# INLINE shiftLL #-}