{-| Functions related to parsing @Cache-Control@ header as defined in
https://tools.ietf.org/html/rfc7234#section-5.2

To get @max-age@/@s-maxage@ from @Cache-Control@ header, use 'parseMaxAge'. If you need to check
other directives use 'parseCacheControl'.

Rules which starts with @obs-@ is not required to implement because they are maked as "obsolete" as
per https://tools.ietf.org/html/rfc7230#section-1.2
-}

module Data.Parser.CacheControl
  ( CacheControl
  , CacheControlDirective (..)
  , parseCacheControl
  , parseMaxAge
  )
  where

import           Hasura.Prelude

import           Hasura.Server.Utils  (fmapL)

import qualified Data.Attoparsec.Text as AT
import qualified Data.Text            as T


type CacheControl = [CacheControlDirective]

data CacheControlDirective
  = CCDOnlyToken !Text
  | CCDTokenWithVal !Text !Text
  deriving (Show, Eq)

-- | Tries to parse the @max-age@ or @s-maxage@ present in the value of @Cache-Control@ header
parseMaxAge :: Integral a => Text -> Either String a
parseMaxAge t = do
  cc <- parseCacheControl t
  case find checkMaxAgeToken cc of
    Nothing -> Left parseErr
    Just d -> case d of
      CCDOnlyToken _        -> Left parseErr
      CCDTokenWithVal _ val -> fmapL (const parseErr) $ AT.parseOnly AT.decimal val
  where
    parseErr = "could not find max-age/s-maxage"
    checkMaxAgeToken = \case
      CCDOnlyToken token      -> token == "max-age" || token == "s-maxage"
      CCDTokenWithVal token _ -> token == "max-age" || token == "s-maxage"


-- | Parses a @Cache-Control@ header and returns a list of directives
parseCacheControl :: Text -> Either String CacheControl
parseCacheControl = AT.parseOnly cacheControlParser

-- ABNF: cache-control = *( "," OWS) cache-directive *( OWS "," [OWS cache-directive])
-- https://tools.ietf.org/html/rfc7234#appendix-C
cacheControlParser :: AT.Parser CacheControl
cacheControlParser = do
  void $ AT.many' ("," *> optionalWhitespaceParser)
  cd <- cacheDirectiveParser
  cds <- AT.many' $ optionalWhitespaceParser *> "," *> AT.option Nothing (pure <$> optionalDirective)
  return $ cd : catMaybes cds
  where
    optionalDirective = optionalWhitespaceParser *> cacheDirectiveParser

-- ABNF: OWS = *( SP / HTAB ) ; optional whitespace
-- https://tools.ietf.org/html/rfc7230#section-3.2.3
optionalWhitespaceParser :: AT.Parser (Maybe Char)
optionalWhitespaceParser = AT.option Nothing (pure <$> AT.space)

-- ABNF: cache-directive = token [ "=" ( token / quoted-string ) ]
-- https://tools.ietf.org/html/rfc7230#section-3.2.6
cacheDirectiveParser :: AT.Parser CacheControlDirective
cacheDirectiveParser = tokenWithValue <|> onlyToken
  where
    onlyToken = CCDOnlyToken <$> tokenParser
    tokenWithValue = do
      tok <- tokenParser
      void $ AT.char '='
      val <- tokenParser <|> quotedStringParser
      return $ CCDTokenWithVal tok val

-- ABNF: 1*tchar
-- https://tools.ietf.org/html/rfc7230#section-3.2.6
tokenParser :: AT.Parser Text
tokenParser = T.pack <$> AT.many1 tcharParser

-- ABNF: tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." / "^" / "_" / "`" / "|"
--               / "~" / DIGIT / ALPHA ; any VCHAR, except delimiters
-- https://tools.ietf.org/html/rfc7230#section-3.2.6
tcharParser :: AT.Parser Char
tcharParser = AT.char '!'
          <|> AT.char '#'
          <|> AT.char '$'
          <|> AT.char '%'
          <|> AT.char '&'
          <|> AT.char '\''
          <|> AT.char '*'
          <|> AT.char '+'
          <|> AT.char '-'
          <|> AT.char '.'
          <|> AT.char '^'
          <|> AT.char '_'
          <|> AT.char '`'
          <|> AT.char '|'
          <|> AT.char '~'
          <|> AT.digit
          <|> AT.letter

-- ABNF: DQUOTE = %x22 ; " (Double Quote)
-- https://tools.ietf.org/html/rfc5234#appendix-B.1
dquoteParser :: AT.Parser Char
dquoteParser = AT.char '"'

-- ABNF: VCHAR = %x21-7E ; visible (printing) characters
-- https://tools.ietf.org/html/rfc5234#appendix-B.1
vcharParser :: AT.Parser Char
vcharParser = AT.anyChar

-- ABNF: quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
-- https://tools.ietf.org/html/rfc7230#section-3.2.6
quotedStringParser :: AT.Parser Text
quotedStringParser =
  dquoteParser *> fmap T.pack (AT.many' (qdTextParser <|> quotedPairParser)) <* dquoteParser

-- ABNF: quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
-- https://tools.ietf.org/html/rfc7230#section-3.2.6
quotedPairParser :: AT.Parser Char
quotedPairParser =
  AT.string "\\" *> (AT.space <|> vcharParser)

-- ABNF: qdtext = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text
-- https://tools.ietf.org/html/rfc7230#section-3.2.6
qdTextParser :: AT.Parser Char
qdTextParser = AT.space
           <|> AT.char '!'  -- %x21
           -- skip %x22 as it is '"'
           <|> AT.char '#'  -- %x23
           <|> AT.char '$'  -- %x24
           <|> AT.char '%'  -- %x25
           <|> AT.char '&'  -- %x26
           <|> AT.char '\'' -- %x27 single quote
           <|> AT.char '('  -- %x28
           <|> AT.char ')'  -- %x29
           <|> AT.char '*'  -- %x2A
           <|> AT.char '+'  -- %x2B
           <|> AT.char ','  -- %x2C
           <|> AT.char '-'  -- %x2D
           <|> AT.char '.'  -- %x2E
           <|> AT.char '/'  -- %x2F
           <|> AT.digit     -- %x30-39
           <|> AT.char ':'  -- %x3A
           <|> AT.char ';'  -- %x3B
           <|> AT.char '<'  -- %x3C
           <|> AT.char '='  -- %x3D
           <|> AT.char '>'  -- %x3E
           <|> AT.char '?'  -- %x3F
           <|> AT.char '@'  -- %x40
           <|> AT.letter    -- %x41-5A / %x61-7A
           <|> AT.char '['  -- %x5B
           -- skip %x5C as it is '\'
           <|> AT.char ']'  -- %x5D
           <|> AT.char '^'  -- %x5E
           <|> AT.char '_'  -- %x5F
           <|> AT.char '`'  -- %x60
           <|> AT.char '{'  -- %x7B
           <|> AT.char '|'  -- %x7C
           <|> AT.char '}'  -- %x7D
           <|> AT.char '~'  -- %x7E