-- |
-- Module      : Unicode.Internal.Bits
-- Copyright   : (c) 2020 Andrew Lelechenko
--               (c) 2020 Composewell Technologies
-- License     : BSD-3-Clause
-- Maintainer  : streamly@composewell.com
-- Stability   : experimental
-- Portability : GHC
--
-- Fast, static bitmap lookup utilities

module Unicode.Internal.Bits
    (
      lookupBit64
    ) where

import Data.Bits (finiteBitSize, popCount)
import GHC.Exts
       (Addr#, Int(..), Word(..), indexWordOffAddr#, and#, andI#,
        uncheckedIShiftRL#, uncheckedShiftL#)

-- | @lookup64 addr index@ looks up the bit stored at bit index @index@ using a
-- bitmap starting at the address @addr@. Looks up the 64-bit word containing
-- the bit and then the bit in that word. The caller must make sure that the
-- 64-bit word at the byte address (addr + index / 64) * 8 is legally
-- accessible memory.
--
lookupBit64 :: Addr# -> Int -> Bool
lookupBit64 :: Addr# -> Int -> Bool
lookupBit64 Addr#
addr# (I# Int#
index#) = Word# -> Word
W# (Word#
word## Word# -> Word# -> Word#
`and#` Word#
bitMask##) Word -> Word -> Bool
forall a. Eq a => a -> a -> Bool
/= Word
0
  where
    !fbs :: Int
fbs@(I# Int#
fbs#) = Word -> Int
forall b. FiniteBits b => b -> Int
finiteBitSize (Word
0 :: Word) Int -> Int -> Int
forall a. Num a => a -> a -> a
- Int
1
    !(I# Int#
logFbs#) = case Int
fbs of
      Int
31 -> Int
5
      Int
63 -> Int
6
      Int
_  -> Int -> Int
forall a. Bits a => a -> Int
popCount Int
fbs -- this is a really weird architecture

    wordIndex# :: Int#
wordIndex# = Int#
index# Int# -> Int# -> Int#
`uncheckedIShiftRL#` Int#
logFbs#
    word## :: Word#
word## = Addr# -> Int# -> Word#
indexWordOffAddr# Addr#
addr# Int#
wordIndex#
    bitIndex# :: Int#
bitIndex# = Int#
index# Int# -> Int# -> Int#
`andI#` Int#
fbs#
    bitMask## :: Word#
bitMask## = Word#
1## Word# -> Int# -> Word#
`uncheckedShiftL#` Int#
bitIndex#