{-# 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__.
--
-- The 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)
#else
import Data.Bits (popCount)
#endif

#if !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 Int -> BitQueue -> ShowS
[BitQueue] -> ShowS
BitQueue -> String
forall a.
(Int -> a -> ShowS) -> (a -> String) -> ([a] -> ShowS) -> Show a
showList :: [BitQueue] -> ShowS
$cshowList :: [BitQueue] -> ShowS
show :: BitQueue -> String
$cshow :: BitQueue -> String
showsPrec :: Int -> BitQueue -> ShowS
$cshowsPrec :: Int -> BitQueue -> ShowS
Show

-- Intended for debugging.
instance Show BitQueueB where
  show :: BitQueueB -> String
show (BQB Word
hi Word
lo) = String
"BQ"forall a. [a] -> [a] -> [a]
++
    forall a. Show a => a -> String
show (forall a b. (a -> b) -> [a] -> [b]
map (forall a. Bits a => a -> Int -> Bool
testBit Word
hi) [(Int
wordSize forall a. Num a => a -> a -> a
- Int
1),(Int
wordSize forall a. Num a => a -> a -> a
- Int
2)..Int
0]
            forall a. [a] -> [a] -> [a]
++ forall a b. (a -> b) -> [a] -> [b]
map (forall a. Bits a => a -> Int -> Bool
testBit Word
lo) [(Int
wordSize forall a. Num a => a -> a -> a
- Int
1),(Int
wordSize forall a. Num a => a -> a -> a
- Int
2)..Int
0])

-- | Create an empty bit queue builder. This is represented as a single guard
-- bit in the most significant position.
emptyQB :: BitQueueB
emptyQB :: BitQueueB
emptyQB = Word -> Word -> BitQueueB
BQB (Word
1 Word -> Int -> Word
`shiftLL` (Int
wordSize forall a. Num a => a -> a -> a
- Int
1)) Word
0
{-# INLINE emptyQB #-}

-- Shift the double word to the right by one bit.
shiftQBR1 :: BitQueueB -> BitQueueB
shiftQBR1 :: BitQueueB -> BitQueueB
shiftQBR1 (BQB Word
hi Word
lo) = Word -> Word -> BitQueueB
BQB Word
hi' Word
lo' where
  lo' :: Word
lo' = (Word
lo Word -> Int -> Word
`shiftRL` Int
1) forall a. Bits a => a -> a -> a
.|. (Word
hi Word -> Int -> Word
`shiftLL` (Int
wordSize forall a. Num a => a -> a -> a
- Int
1))
  hi' :: Word
hi' = Word
hi Word -> Int -> Word
`shiftRL` Int
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 :: BitQueueB -> Bool -> BitQueueB
snocQB BitQueueB
bq Bool
b = case BitQueueB -> BitQueueB
shiftQBR1 BitQueueB
bq of
  BQB Word
hi Word
lo -> Word -> Word -> BitQueueB
BQB (Word
hi forall a. Bits a => a -> a -> a
.|. (forall a b. (Integral a, Num b) => a -> b
fromIntegral (forall a. Enum a => a -> Int
fromEnum Bool
b) Word -> Int -> Word
`shiftLL` (Int
wordSize forall a. Num a => a -> a -> a
- Int
1))) Word
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 :: BitQueueB -> BitQueue
buildQ (BQB Word
hi Word
0) = BitQueueB -> BitQueue
BQ (Word -> Word -> BitQueueB
BQB Word
0 Word
lo') where
  zeros :: Int
zeros = forall b. FiniteBits b => b -> Int
countTrailingZeros Word
hi
  lo' :: Word
lo' = ((Word
hi Word -> Int -> Word
`shiftRL` Int
1) forall a. Bits a => a -> a -> a
.|. (Word
1 Word -> Int -> Word
`shiftLL` (Int
wordSize forall a. Num a => a -> a -> a
- Int
1))) Word -> Int -> Word
`shiftRL` Int
zeros
buildQ (BQB Word
hi Word
lo) = BitQueueB -> BitQueue
BQ (Word -> Word -> BitQueueB
BQB Word
hi' Word
lo') where
  zeros :: Int
zeros = forall b. FiniteBits b => b -> Int
countTrailingZeros Word
lo
  lo1 :: Word
lo1 = (Word
lo Word -> Int -> Word
`shiftRL` Int
1) forall a. Bits a => a -> a -> a
.|. (Word
hi Word -> Int -> Word
`shiftLL` (Int
wordSize forall a. Num a => a -> a -> a
- Int
1))
  hi1 :: Word
hi1 = (Word
hi Word -> Int -> Word
`shiftRL` Int
1) forall a. Bits a => a -> a -> a
.|. (Word
1 Word -> Int -> Word
`shiftLL` (Int
wordSize forall a. Num a => a -> a -> a
- Int
1))
  lo' :: Word
lo' = (Word
lo1 Word -> Int -> Word
`shiftRL` Int
zeros) forall a. Bits a => a -> a -> a
.|. (Word
hi1 Word -> Int -> Word
`shiftLL` (Int
wordSize forall a. Num a => a -> a -> a
- Int
zeros))
  hi' :: Word
hi' = Word
hi1 Word -> Int -> Word
`shiftRL` Int
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 :: BitQueue -> Bool
nullQ (BQ (BQB Word
0 Word
1)) = Bool
True
nullQ BitQueue
_ = Bool
False
{-# INLINE nullQ #-}

-- | Dequeue an element, or discover the queue is empty.
unconsQ :: BitQueue -> Maybe (Bool, BitQueue)
unconsQ :: BitQueue -> Maybe (Bool, BitQueue)
unconsQ BitQueue
q | BitQueue -> Bool
nullQ BitQueue
q = forall a. Maybe a
Nothing
unconsQ (BQ bq :: BitQueueB
bq@(BQB Word
_ Word
lo)) = forall a. a -> Maybe a
Just (Bool
hd, BitQueueB -> BitQueue
BQ BitQueueB
tl)
  where
    !hd :: Bool
hd = (Word
lo forall a. Bits a => a -> a -> a
.&. Word
1) forall a. Eq a => a -> a -> Bool
/= Word
0
    !tl :: BitQueueB
tl = BitQueueB -> BitQueueB
shiftQBR1 BitQueueB
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 :: BitQueue -> [Bool]
toListQ BitQueue
bq = case BitQueue -> Maybe (Bool, BitQueue)
unconsQ BitQueue
bq of
      Maybe (Bool, BitQueue)
Nothing -> []
      Just (Bool
hd, BitQueue
tl) -> Bool
hd forall a. a -> [a] -> [a]
: BitQueue -> [Bool]
toListQ BitQueue
tl