{-# LANGUAGE CPP               #-}
#if __GLASGOW_HASKELL__ >= 800
{-# LANGUAGE ApplicativeDo     #-}
#endif
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TupleSections     #-}
{-# LANGUAGE DeriveGeneric #-}

module Data.RESP
  ( RespReply(..)
  , RespExpr(..)
  , parseReply
  , parseExpression
  ) where

import qualified Data.ByteString       as BS
import qualified Data.ByteString.Char8 as BS8
import qualified Data.ByteString.Lazy  as BSL
import qualified Scanner               as Scanner

#if !MIN_VERSION_base(4,8,0)
import Data.Functor
import Control.Applicative
#endif

#if !MIN_VERSION_base(4,11,0)
# if MIN_VERSION_base(4,9,0)
import Data.Semigroup
import Data.Monoid (mempty)
# else
import Data.Monoid ((<>), mempty)
# endif
#endif

import Data.ByteString      (ByteString)
import Data.Char            (digitToInt)
import Data.Int             (Int64)
import Scanner              (Scanner)
import Control.Monad        (when, replicateM)
import GHC.Generics         (Generic)

-- This type synonym was introduced in bytestring 0.11.2.0
type LazyByteString = BSL.ByteString

#if MIN_VERSION_bytestring(0,10,0)
lazyBsToStrict :: LazyByteString -> ByteString
lazyBsToStrict = BSL.toStrict
#else
lazyBsToStrict :: LazyByteString -> ByteString
lazyBsToStrict = BS.concat . BSL.toChunks
#endif

-- | Top-level resp reply.
-- Cannot be nested.
data RespReply
  = RespPush !ByteString ![RespExpr]
  | RespExpr !RespExpr
  deriving (Show, Eq, Ord, Generic)

-- | RESP3 Expression.
--
-- This descriminates the difference between RespString and RespBlob,
-- even though both contain bytestrings, in order to not throw away
-- information. A caller might care whether the response was delivered
-- with "+", or "$".
--
-- We do not, however descriminate between the different encodings of
-- null. As far as I can tell, these are considered a mistake in the
-- previous versions of the RESP spec, and clients should treat the
-- different encodings the same.
--
-- Why don't we parse `RespString` into `Text`? Well, the caller might
-- not actually need to decode it into text, and so we let the caller
-- decide. This way, we don't have to deal with encoding errors.
--
-- Similarly, we don't parse a `RespMap` into a `HashMap`, because
-- that would involve imposing our choice of data structure on the caller.
-- They might want to use `HashMap`, `Map`, or just use the `lookup`
-- function.
--
-- Given these choices, our purview is simple: Parse the text protocol
-- into a Haskell datatype, maintaining all useful information, and not
-- imposing our taste onto the caller.
data RespExpr
  = RespString !ByteString
  | RespBlob !ByteString
  | RespStreamingBlob !LazyByteString
  | RespStringError !ByteString
  | RespBlobError !ByteString
  | RespArray ![RespExpr]
  | RespInteger !Int64
  | RespNull
  | RespBool !Bool
  | RespDouble !Double
  | RespVerbatimString !ByteString
  | RespVerbatimMarkdown !ByteString
  | RespBigInteger !Integer
  | RespMap ![(RespExpr, RespExpr)]
  | RespSet ![RespExpr]
  | RespAttribute ![(RespExpr, RespExpr)] RespExpr
  deriving (Show, Eq, Ord, Generic)

data MessageSize
  = MSVariable
  | MSFixed Int

data NullableMessageSize
  = NMSVariable
  | NMSMinusOne
  | NMSFixed Int

-- Top level RESP item
parseReply :: Scanner RespReply
parseReply = do
  c <- Scanner.anyChar8
  case c of
    '>' -> parsePush
    _ -> RespExpr <$> parseExpression' c

-- Non-top-level resp item
parseExpression :: Scanner RespExpr
parseExpression = Scanner.anyChar8 >>= parseExpression'

-- Non-top-level resp item, taking its first char as a parameter
parseExpression' :: Char -> Scanner RespExpr
parseExpression' c = case c of
  '$' -> parseBlob
  '+' -> parseString
  '-' -> parseStringError
  ':' -> RespInteger <$> parseInteger
  '*' -> parseArray RespArray
  '_' -> RespNull <$ parseEol 
  '#' -> RespBool . (== 't') <$> Scanner.anyChar8 <* parseEol
  ',' -> parseDouble
  '!' -> parseBlobError
  '=' -> parseVerbatimString
  '(' -> RespBigInteger <$> parseInteger
  '%' -> RespMap <$> parseMap
  '~' -> parseArray RespSet
  '|' -> RespAttribute <$> parseMap <*> parseExpression
  _ -> fail $ "Unknown expression prefix: " <> show c

parsePush :: Scanner RespReply
parsePush = do
  len <- parseMessageSize
  RespPush <$> parsePushType <*> replicateM (pred len) parseExpression

parsePushType :: Scanner ByteString
parsePushType = do
  c <- Scanner.anyChar8
  -- No idea whether this can be a simple string or not,
  -- the spec isn't specific enough.
  --
  -- The spec doesn't say that the push type *can't* be a
  -- streamed blob string (or null), but let's face it, only a sadist would
  -- return one of those. I'll try to get these possibilities excluded from
  -- the spec, but in the meantime, we're going to have to parse all the
  -- blobstrings.
  case c of
    '$' -> parseBlob' id lazyBsToStrict $ fail "Push message type can't be null"
    '+' -> parseLine
    _ -> fail "Invalid push message type"

parseMap :: Scanner [(RespExpr, RespExpr)]
parseMap = do
  len <- parseComplexMessageSize
  case len of
    MSFixed n -> replicateM n parseTwoEls
    MSVariable -> parseVarMapPairs

-- See https://github.com/redis/redis-specifications/blob/master/protocol/RESP3.md#streamed-aggregated-data-types
parseVarMapPairs :: Scanner [(RespExpr, RespExpr)]
parseVarMapPairs = do
  c <- Scanner.anyChar8
  case c of
    '.' -> [] <$ parseEol
    _ -> (:) <$> ((,) <$> parseExpression' c <*> parseExpression) <*> parseVarMapPairs

parseTwoEls :: Scanner (RespExpr, RespExpr)
parseTwoEls = (,) <$> parseExpression <*> parseExpression

-- See: https://github.com/redis/redis-specifications/issues/25
--    , https://github.com/redis/redis-specifications/issues/23
parseVerbatimString :: Scanner RespExpr
parseVerbatimString = do
  len <- parseMessageSize
  entireBlob <- Scanner.take len
  let body = BS8.drop 4 entireBlob
  parseEol
  case BS8.take 3 entireBlob of
    "txt" -> pure $ RespVerbatimString body
    "mkd" -> pure $ RespVerbatimMarkdown body
    _ -> fail "Unknown verbatim string type"

-- I suspect that this can't be streamed, or null
-- See: https://github.com/redis/redis-specifications/issues/23
parseBlobError :: Scanner RespExpr
parseBlobError = do
  len <- parseMessageSize
  RespBlobError <$> Scanner.take len <* parseEol

bsContains :: Char -> ByteString -> Bool
bsContains c = BS8.any (== c)

-- Scanning to NaN is a function so that we don't
-- feel guilty about inlining the patterns
parseLineAsNaN :: Scanner Double
parseLineAsNaN = (0 / 0) <$ parseLine

parseLineAsInf :: Scanner Double
parseLineAsInf = (1 / 0) <$ parseLine

-- (inf|-inf|nan|(+|-)?\d+(\.\d+)?([eE](+|-)?\d+))
--
-- Due to Redis bugs prior to 7.2, we also have to deal with
-- /(-)?nan(\(.*\))?/i, even though they're not part of the
-- RESP spec...
parseDouble :: Scanner RespExpr
parseDouble = do
  c <- Scanner.anyChar8
  RespDouble <$> case c of
    '+' -> go1 =<< Scanner.anyChar8
    '-' -> fmap negate $ go1 =<< Scanner.anyChar8
    'i' -> do
      -- Note: We're not validating that the rest of the line
      -- is actually "nf", because `,i` uniquely determines the
      -- set of valid responses.
      parseLineAsInf
    'n' -> parseLineAsNaN
    'N' -> parseLineAsNaN
    _ -> go1 c

  where
    -- takes first non-sign char of the significand
    go1 :: Char -> Scanner Double
    go1 'i' = parseLineAsInf
    go1 'n' = parseLineAsNaN
    go1 'N' = parseLineAsNaN
    go1 c1 = fromRational <$> do
      decStr <- Scanner.takeWhileChar8 $ not . (`bsContains` ".\reE")
      let dec = parseNatural1 c1 decStr :: Integer
      c2 <- Scanner.anyChar8
      case c2 of
        '\r' -> fromIntegral dec <$ expectChar '\n' 
        '.' -> do
          decStr1 <- Scanner.takeWhileChar8 $ not . (`bsContains` "\reE")
          let dec1 = fromIntegral (parseNatural' dec decStr1) / (10 ^ BS.length decStr1) :: Rational
          c3 <- Scanner.anyChar8
          case c3 of
            '\r' -> dec1 <$ expectChar '\n'
            _ {- c3 `elem` "eE" -} -> go2 dec1
        _ {- c3 `elem` "eE" -} -> go2 $ fromIntegral dec

    -- from first char of exponent (after [eE])
    go2 :: Rational -> Scanner Rational
    go2 n = do
      c <- Scanner.anyChar8
      (negExp, exponent') <- case c of
        '-' -> (True,) . parseNatural <$> parseLine
        '+' -> (False,) . parseNatural <$> parseLine
        _ {- isDigit c -} -> (False,) . parseNatural1 c <$> parseLine
      let expMul = fromIntegral (10 ^ (exponent' :: Integer) :: Integer) :: Rational
      pure $ if negExp then n / expMul else n * expMul

parseNatural :: Integral a => ByteString -> a
parseNatural = parseNatural' 0

parseNatural' :: Integral a => a -> ByteString -> a
parseNatural' = BS8.foldl' (\a b -> a * 10 + fromIntegral (digitToInt b))

parseNatural1 :: Integral a => Char -> ByteString -> a
parseNatural1 = parseNatural' . fromIntegral . digitToInt

-- RESP2 calls these 'multi bulk'
-- RESP3 calls it an 'array'
--
-- This is used to parse arrays and sets, meaning that we parse
-- "~-1\r\n" as RespNull, although this isn't a valid form in the spec.
parseArray :: ([RespExpr] -> RespExpr) -> Scanner RespExpr
parseArray construct = do
  messageSize <- parseComplexNullableMessageSize
  case messageSize of
    NMSFixed n -> construct <$> replicateM n parseExpression
    NMSMinusOne -> pure RespNull
    NMSVariable -> construct <$> parseVarArrayItems

-- See https://github.com/redis/redis-specifications/blob/master/protocol/RESP3.md#streamed-aggregated-data-types
parseVarArrayItems :: Scanner [RespExpr]
parseVarArrayItems = do
  c <- Scanner.anyChar8
  case c of
    '.' -> [] <$ parseEol
    _ -> (:) <$> parseExpression' c <*> parseVarArrayItems

-- RESP2 calls these 'bulk strings'
-- RESP3 calls them 'blob strings' (in the markdown, on the website they're still 'bulk strings')
parseBlob :: Scanner RespExpr
parseBlob = parseBlob' RespBlob RespStreamingBlob $ pure RespNull

-- general case for something that's pretty blobstring-like
parseBlob'
  :: (ByteString -> a)
  -> (LazyByteString -> a)
  -> Scanner a
  -> Scanner a
parseBlob' strictConstr lazyConstr nullConstr = do
  ms <- parseComplexNullableMessageSize
  case ms of
    NMSFixed n -> strictConstr <$> Scanner.take n <* parseEol
    NMSVariable -> lazyConstr . BSL.fromChunks <$> streamingBlobParts
    NMSMinusOne -> nullConstr

parseMessageSize :: Scanner Int
parseMessageSize = parseNatural <$> parseLine

-- Used for blobs and arrays
parseComplexNullableMessageSize :: Scanner NullableMessageSize
parseComplexNullableMessageSize = do
  line <- parseLine
  case line of
    "?" -> pure NMSVariable
    "-1" -> pure NMSMinusOne
    _ -> pure $ NMSFixed $ parseNatural line

-- Used for maps, attributes, sets
parseComplexMessageSize :: Scanner MessageSize
parseComplexMessageSize = do
  line <- parseLine
  case line of
    "?" -> pure MSVariable
    _ -> pure $ MSFixed $ parseNatural line

streamingBlobParts :: Scanner [ByteString]
streamingBlobParts = do
  expectChar ';'
  ms <- parseMessageSize
  case ms of
    0 -> pure mempty
    n -> (:) <$> Scanner.take n <* parseEol <*> streamingBlobParts

parseString :: Scanner RespExpr
parseString = RespString <$> parseLine

-- Cautious interpretation, until we can clarify that the
-- error tag is mandatory.
-- https://github.com/redis/redis-specifications/issues/24
parseStringError :: Scanner RespExpr
parseStringError = RespStringError <$> parseLine

parseInteger :: Integral a => Scanner a
parseInteger = do
  c <- Scanner.anyChar8
  case c of
    '+' -> parseNatural <$> parseLine
    '-' -> negate . parseNatural <$> parseLine
    _ -> parseNatural1 c <$> parseLine

parseLine :: Scanner ByteString
parseLine = Scanner.takeWhileChar8 (/= '\r') <* parseEol

expectChar :: Char -> Scanner ()
expectChar c = do
  d <- Scanner.anyChar8
  when (c /= d) $ fail $ "Expected " <> show c <> ", but got " <> show d

parseEol :: Scanner ()
parseEol = do
  expectChar '\r'
  expectChar '\n'