{-# LANGUAGE CPP #-} {-# LANGUAGE BangPatterns #-} #include "containers.h" ----------------------------------------------------------------------------- -- | -- Module : Utils.Containers.Internal.BitQueue -- Copyright : (c) David Feuer 2016 -- License : BSD-style -- Maintainer : libraries@haskell.org -- Portability : portable -- -- = WARNING -- -- This module is considered __internal__. -- -- The Package Versioning Policy __does not apply__. -- -- This contents of this module may change __in any way whatsoever__ -- and __without any warning__ between minor versions of this package. -- -- Authors importing this module are expected to track development -- closely. -- -- = Description -- -- An extremely light-weight, fast, and limited representation of a string of -- up to (2*WORDSIZE - 2) bits. In fact, there are two representations, -- misleadingly named bit queue builder and bit queue. The builder supports -- only `emptyQB`, creating an empty builder, and `snocQB`, enqueueing a bit. -- The bit queue builder is then turned into a bit queue using `buildQ`, after -- which bits can be removed one by one using `unconsQ`. If the size limit is -- exceeded, further operations will silently produce nonsense. ----------------------------------------------------------------------------- module Utils.Containers.Internal.BitQueue ( BitQueue , BitQueueB , emptyQB , snocQB , buildQ , unconsQ , toListQ ) where #if !MIN_VERSION_base(4,8,0) import Data.Word (Word) #endif import Utils.Containers.Internal.BitUtil (shiftLL, shiftRL, wordSize) import Data.Bits ((.|.), (.&.), testBit) #if MIN_VERSION_base(4,8,0) import Data.Bits (countTrailingZeros) #elif MIN_VERSION_base(4,5,0) import Data.Bits (popCount) #endif #if !MIN_VERSION_base(4,5,0) -- We could almost certainly improve this fall-back (copied straight from the -- default definition in Data.Bits), but it hardly seems worth the trouble -- to speed things up on GHC 7.4 and below. countTrailingZeros :: Word -> Int countTrailingZeros x = go 0 where go i | i >= wordSize = i | testBit x i = i | otherwise = go (i+1) #elif !MIN_VERSION_base(4,8,0) countTrailingZeros :: Word -> Int countTrailingZeros x = popCount ((x .&. (-x)) - 1) {-# INLINE countTrailingZeros #-} #endif -- A bit queue builder. We represent a double word using two words -- because we don't currently have access to proper double words. data BitQueueB = BQB {-# UNPACK #-} !Word {-# UNPACK #-} !Word newtype BitQueue = BQ BitQueueB deriving Show -- Intended for debugging. instance Show BitQueueB where show (BQB hi lo) = "BQ"++ show (map (testBit hi) [(wordSize - 1),(wordSize - 2)..0] ++ map (testBit lo) [(wordSize - 1),(wordSize - 2)..0]) -- | Create an empty bit queue builder. This is represented as a single guard -- bit in the most significant position. emptyQB :: BitQueueB emptyQB = BQB (1 `shiftLL` (wordSize - 1)) 0 {-# INLINE emptyQB #-} -- Shift the double word to the right by one bit. shiftQBR1 :: BitQueueB -> BitQueueB shiftQBR1 (BQB hi lo) = BQB hi' lo' where lo' = (lo `shiftRL` 1) .|. (hi `shiftLL` (wordSize - 1)) hi' = hi `shiftRL` 1 {-# INLINE shiftQBR1 #-} -- | Enqueue a bit. This works by shifting the queue right one bit, -- then setting the most significant bit as requested. {-# INLINE snocQB #-} snocQB :: BitQueueB -> Bool -> BitQueueB snocQB bq b = case shiftQBR1 bq of BQB hi lo -> BQB (hi .|. (fromIntegral (fromEnum b) `shiftLL` (wordSize - 1))) lo -- | Convert a bit queue builder to a bit queue. This shifts in a new -- guard bit on the left, and shifts right until the old guard bit falls -- off. {-# INLINE buildQ #-} buildQ :: BitQueueB -> BitQueue buildQ (BQB hi 0) = BQ (BQB 0 lo') where zeros = countTrailingZeros hi lo' = ((hi `shiftRL` 1) .|. (1 `shiftLL` (wordSize - 1))) `shiftRL` zeros buildQ (BQB hi lo) = BQ (BQB hi' lo') where zeros = countTrailingZeros lo lo1 = (lo `shiftRL` 1) .|. (hi `shiftLL` (wordSize - 1)) hi1 = (hi `shiftRL` 1) .|. (1 `shiftLL` (wordSize - 1)) lo' = (lo1 `shiftRL` zeros) .|. (hi1 `shiftLL` (wordSize - zeros)) hi' = hi1 `shiftRL` zeros -- Test if the queue is empty, which occurs when theres -- nothing left but a guard bit in the least significant -- place. nullQ :: BitQueue -> Bool nullQ (BQ (BQB 0 1)) = True nullQ _ = False {-# INLINE nullQ #-} -- | Dequeue an element, or discover the queue is empty. unconsQ :: BitQueue -> Maybe (Bool, BitQueue) unconsQ q | nullQ q = Nothing unconsQ (BQ bq@(BQB _ lo)) = Just (hd, BQ tl) where !hd = (lo .&. 1) /= 0 !tl = shiftQBR1 bq {-# INLINE unconsQ #-} -- | Convert a bit queue to a list of bits by unconsing. -- This is used to test that the queue functions properly. toListQ :: BitQueue -> [Bool] toListQ bq = case unconsQ bq of Nothing -> [] Just (hd, tl) -> hd : toListQ tl