-- |
-- Module      : Data.UUID.V7
-- License     : MIT
-- Maintainer  : mmzk1526@outlook.com
-- Portability : GHC
--
-- UUIDv7 implementation.
--
-- UUIDv7 is not currently present in the uuid package, therefore I have to
-- make a quick patch of my own.
--
-- Note that since the specification for v7 is not yet finalised, this module's
-- implementation may change in the future according to the potential
-- adjustments in the specification.
--
module Data.UUID.V7
  (
  -- * Data type
    UUID
  -- * 'UUID'v7 generation
  , genUUID
  , genUUID'
  , genUUIDs
  -- * Validation
  , validate
  , validateWithTime
  -- * Miscellaneous helpers
  , getTime
  , getEpochMilli
  ) where

import           Control.Monad
import           Control.Monad.IO.Class
import           Data.Binary
import           Data.Binary.Get
import           Data.Binary.Put
import           Data.Bits
import qualified Data.ByteString.Lazy as BSL
import           Data.IORef
import           Data.Time.Clock.POSIX
import           Data.UUID.Types.Internal

import           Data.UUID.Versions
import           System.Entropy
import           System.IO.Unsafe (unsafePerformIO)

-- | Generate a 'UUID'v7.
genUUID :: MonadIO m => m UUID
genUUID = head <$> genUUIDs 1
{-# INLINE genUUID #-}

-- | Generate a stateless 'UUID'v7.
--
-- It is faster than 'genUUID' but it is not guaranteed to be monotonically
-- increasing if multiple 'UUID's are generated at the same timestamp.
--
-- In use cases where the ordering is not important, this function is could be
-- preferred.
genUUID' :: MonadIO m => m UUID
genUUID' = do
  timestamp <- getEpochMilli
  entropy16 <- getEntropyWord16
  entropy64 <- getEntropyWord64
  let bs = runPut do
        fillTime timestamp
        fillVerAndRandA entropy16
        fillVarAndRandB entropy16 entropy64
  pure . uncurry UUID $ runGet (join (liftM2 (,)) getWord64be) bs
{-# INLINE genUUID' #-}

-- | Generate a list of 'UUID'v7s.
--
-- It tries its best to generate 'UUID's at the same timestamp, but it may not
-- be possible if we are asking too many 'UUID's at the same time.
--
-- It is guaranteed that the first 32768 'UUID's are generated at the same
-- timestamp.
genUUIDs :: MonadIO m => Word16 -> m [UUID]
genUUIDs 0 = pure []
genUUIDs n = liftIO do
  timestamp <- getEpochMilli
  -- We set the first bit of the entropy to 0 to ensure that there's enough
  -- room for incrementing the sequence number.
  entropy16 <- (.&. 0x7FFF) <$> getEntropyWord16
  -- Calculate the maximum number of slots we can use for the current timestamp
  -- before the sequence number overflows.
  let getMaxSlots num seqNo = if 0xFFFF - seqNo < num
        then (0xFFFF - seqNo, 0xFFFF)
        else (num, seqNo + num)
  -- Get the sequence number corresponding to the current timestamp and the
  -- number of UUIDs we can generate.
  (n', seqNo)  <- atomicModifyIORef __state__ $ \(ts, seqNo) -> if
    | ts < timestamp -> let (n', entropy16') = getMaxSlots n entropy16
                        in  ((timestamp, entropy16'), (n', entropy16 + 1))
    | ts > timestamp -> ((ts, seqNo), (0, 0))
    | otherwise      -> let (n', entropy16') = getMaxSlots n seqNo
                        in  ((timestamp, entropy16'), (n', seqNo + 1))
  -- If we can't generate any UUIDs, we try again, hoping that the timestamp
  -- has changed.
  if n' == 0
    then genUUIDs n
    else do
      uuids <- forM [0..(n' - 1)] $ \curN -> do
        entropy64 <- getEntropyWord64
        let bs = runPut do
              fillTime timestamp
              fillVerAndRandA (seqNo + curN)
              fillVarAndRandB (seqNo + curN) entropy64
        pure . uncurry UUID $ runGet (join (liftM2 (,)) getWord64be) bs
      if n' == n then pure uuids else (uuids ++) <$> genUUIDs (n - n')

-- | Validate the version and variant of the 'UUID'v7.
validate :: UUID -> Bool
validate = flip validateWithVersion V7
{-# INLINE validate #-}

-- | Validate the version and variant of the 'UUID'v7 as well as its timestamp
-- is no greater than the current time.
validateWithTime :: MonadIO m => UUID -> m Bool
validateWithTime uuid = do
  curTime <- getEpochMilli
  pure $ validate uuid && (getTime uuid <= curTime)
{-# INLINE validateWithTime #-}
  
-- | Get the current time in milliseconds since the Unix epoch.
getEpochMilli :: MonadIO m => m Word64
getEpochMilli = liftIO do
  t <- getPOSIXTime
  pure . round $ t * 1000
{-# INLINE getEpochMilli #-}

-- | Get the time field (unix_ts_ms) of a 'UUID'v7.
getTime :: UUID -> Word64
getTime (UUID w1 _) = w1 `shiftR` 16
{-# INLINE getTime #-}

-- | The global mutable state of (timestamp, sequence number).
--
-- The "NOINLINE" pragma is IMPORTANT! The logic would be flawed if it is
-- is inlined by its definition.
__state__ :: IORef (Word64, Word16)
__state__ = unsafePerformIO (newIORef (0, 0))
{-# NOINLINE __state__ #-}

-- | Fill in the 48-bit time field (unix_ts_ms) of a 'UUID'v7 with the given
-- time.
fillTime :: Word64 -> Put
fillTime timestamp = do
  let (_, p2, p1, p0) = splitWord64ToWord16s timestamp
  mapM_ putWord16be [p2, p1, p0]
{-# INLINE fillTime #-}

-- | Fill in the version and rand_a part of a 'UUID'v7 with the given sequence
-- number.
--
-- The sequence number is a 16-bit integer, of which the first 12 bits are used
-- here in rand_a, and the last 4 bits are used in rand_b. The version is 7.
fillVerAndRandA :: Word16 -> Put
fillVerAndRandA seqNo = do
  let seqNoRandA   = seqNo `shiftR` 4
  let randAWithVer = seqNoRandA .|. (0x7 `shiftL` 12)
  putWord16be randAWithVer
{-# INLINE fillVerAndRandA #-}

-- | Fill in the variant and rand_b part of a 'UUID'v7 with the given sequence
-- number and random number. The variant is 2.
--
-- The sequence number is a 16-bit integer, of which the last 4 bits are used
-- here in rand_b while the first 12 bits are used in rand_a.
--
-- The random number is a 64-bit integer of which the last 58 bits are used
-- while the rest are replaced by the variant bits and the last 4 bits of the
-- sequence number.
fillVarAndRandB :: Word16 -> Word64 -> Put
fillVarAndRandB seqNo entropy = do
  let seqNoRandB   = seqNo .&. 0xF
  let randBWithVar = fromIntegral (seqNoRandB .|. (0x2 `shiftL` 4))
  putWord64be $ (entropy .&. 0x3FFFFFFFFFFFFFF) .|. (randBWithVar `shiftL` 58)
{-# INLINE fillVarAndRandB #-}

splitWord64ToWord16s :: Word64 -> (Word16, Word16, Word16, Word16)
splitWord64ToWord16s n =
  let b0 = fromIntegral (n .&. 0xFFFF)
      b1 = fromIntegral ((n `shiftR` 16) .&. 0xFFFF)
      b2 = fromIntegral ((n `shiftR` 32) .&. 0xFFFF)
      b3 = fromIntegral ((n `shiftR` 48) .&. 0xFFFF)
  in (b3, b2, b1, b0)
{-# INLINE splitWord64ToWord16s #-}

getEntropyWord16 :: MonadIO m => m Word16
getEntropyWord16 = liftIO do
  bs <- BSL.fromStrict <$> getEntropy 2
  pure $ runGet getWord16host bs
{-# INLINE getEntropyWord16 #-}

getEntropyWord64 :: MonadIO m => m Word64
getEntropyWord64 = liftIO do
  bs <- BSL.fromStrict <$> getEntropy 8
  pure $ runGet getWord64host bs
{-# INLINE getEntropyWord64 #-}