#14818: Provide highestOneBit function in Data.Bits module -------------------------------------+------------------------------------- Reporter: kostmo | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nomeata): JFTR: The `containers` library needs this function, and defines it in its `Utils` module: http://hackage.haskell.org/package/containers-0.5.11.0/docs/Utils- Containers-Internal-BitUtil.html: {{{ -- 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 #-} }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14818#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler