{-# LANGUAGE CPP #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE TypeFamilies #-}

{- |
Parse a TOML document.

References:

* https://toml.io/en/v1.0.0
* https://github.com/toml-lang/toml/blob/1.0.0/toml.abnf
-}
module TOML.Parser (
  parseTOML,
) where

import Control.Monad (guard, unless, void, when)
import Control.Monad.Combinators.NonEmpty (sepBy1)
import Data.Bifunctor (bimap)
import Data.Char (chr, isDigit, isSpace, ord)
import Data.Fixed (Fixed (..))
import Data.Foldable (foldl', foldlM)
import Data.Functor (($>))
import Data.List.NonEmpty (NonEmpty)
import qualified Data.List.NonEmpty as NonEmpty
import Data.Map.Strict (Map)
import qualified Data.Map.Strict as Map
import Data.Maybe (fromMaybe)
import Data.Text (Text)
import qualified Data.Text as Text
import Data.Time (Day, LocalTime, TimeOfDay, TimeZone)
import qualified Data.Time as Time
import Data.Void (Void)
import qualified Numeric
import Text.Megaparsec hiding (sepBy1)
import Text.Megaparsec.Char hiding (space, space1)
import qualified Text.Megaparsec.Char.Lexer as L

import TOML.Error (NormalizeError (..), TOMLError (..))
import TOML.Utils.Map (getPathLens)
import TOML.Value (Table, Value (..))

parseTOML ::
  -- | Name of file (for error messages)
  String ->
  -- | Input
  Text ->
  Either TOMLError Value
parseTOML filename input =
  case runParser parseTOMLDocument filename input of
    Left e -> Left $ ParseError $ Text.pack $ errorBundlePretty e
    Right result -> Table <$> normalize result

-- 'Value' generalized to allow for unnormalized + annotated Values.
data GenericValue map key tableMeta arrayMeta
  = GenericTable tableMeta (map key (GenericValue map key tableMeta arrayMeta))
  | GenericArray arrayMeta [GenericValue map key tableMeta arrayMeta]
  | GenericString Text
  | GenericInteger Integer
  | GenericFloat Double
  | GenericBoolean Bool
  | GenericOffsetDateTime (LocalTime, TimeZone)
  | GenericLocalDateTime LocalTime
  | GenericLocalDate Day
  | GenericLocalTime TimeOfDay

fromGenericValue ::
  (map key (GenericValue map key tableMeta arrayMeta) -> Table) ->
  GenericValue map key tableMeta arrayMeta ->
  Value
fromGenericValue fromGenericTable = \case
  GenericTable _ t -> Table $ fromGenericTable t
  GenericArray _ vs -> Array $ map (fromGenericValue fromGenericTable) vs
  GenericString x -> String x
  GenericInteger x -> Integer x
  GenericFloat x -> Float x
  GenericBoolean x -> Boolean x
  GenericOffsetDateTime x -> OffsetDateTime x
  GenericLocalDateTime x -> LocalDateTime x
  GenericLocalDate x -> LocalDate x
  GenericLocalTime x -> LocalTime x

{--- Parse raw document ---}

type Parser = Parsec Void Text

-- | An unannotated, unnormalized value.
type RawValue = GenericValue LookupMap Key () ()

type Key = NonEmpty Text
type RawTable = LookupMap Key RawValue
newtype LookupMap k v = LookupMap {unLookupMap :: [(k, v)]}

data TOMLDoc = TOMLDoc
  { rootTable :: RawTable
  , subTables :: [TableSection]
  }

data TableSection = TableSection
  { tableSectionHeader :: TableSectionHeader
  , tableSectionTable :: RawTable
  }

data TableSectionHeader = SectionTable Key | SectionTableArray Key

parseTOMLDocument :: Parser TOMLDoc
parseTOMLDocument = do
  emptyLines
  rootTable <- parseRawTable
  emptyLines
  subTables <- many parseTableSection
  emptyLines
  eof
  return TOMLDoc{..}

parseRawTable :: Parser RawTable
parseRawTable = fmap LookupMap $ many $ parseKeyValue <* endOfLine <* emptyLines

parseTableSection :: Parser TableSection
parseTableSection = do
  tableSectionHeader <-
    choice
      [ SectionTableArray <$> parseHeader "[[" "]]"
      , SectionTable <$> parseHeader "[" "]"
      ]
  endOfLine
  emptyLines
  tableSectionTable <- parseRawTable
  emptyLines
  return TableSection{..}
  where
    parseHeader brackStart brackEnd = hsymbol brackStart *> parseKey <* hsymbol brackEnd

parseKeyValue :: Parser (Key, RawValue)
parseKeyValue = do
  key <- parseKey
  hsymbol "="
  value <- parseValue
  pure (key, value)

parseKey :: Parser Key
parseKey =
  (`sepBy1` try (hsymbol ".")) . choice $
    [ parseBasicString
    , parseLiteralString
    , parseUnquotedKey
    ]
  where
    parseUnquotedKey =
      takeWhile1P
        (Just "[A-Za-z0-9_-]")
        (`elem` ['A' .. 'Z'] ++ ['a' .. 'z'] ++ ['0' .. '9'] ++ "-_")

parseValue :: Parser RawValue
parseValue =
  choice
    [ try $ GenericTable () <$> label "table" parseInlineTable
    , try $ GenericArray () <$> label "array" parseInlineArray
    , try $ GenericString <$> label "string" parseString
    , try $ GenericOffsetDateTime <$> label "offset-datetime" parseOffsetDateTime
    , try $ GenericLocalDateTime <$> label "local-datetime" parseLocalDateTime
    , try $ GenericLocalDate <$> label "local-date" parseLocalDate
    , try $ GenericLocalTime <$> label "local-time" parseLocalTime
    , try $ GenericFloat <$> label "float" parseFloat
    , try $ GenericInteger <$> label "integer" parseInteger
    , try $ GenericBoolean <$> label "boolean" parseBoolean
    ]

parseInlineTable :: Parser RawTable
parseInlineTable = do
  hsymbol "{"
  kvs <- parseKeyValue `sepBy` try (hsymbol ",")
  hsymbol "}"
  return $ LookupMap kvs

parseInlineArray :: Parser [RawValue]
parseInlineArray = do
  _ <- char '[' <* emptyLines
  vs <- (parseValue <* emptyLines) `sepEndBy` (char ',' <* emptyLines)
  _ <- char ']'
  return vs

parseString :: Parser Text
parseString =
  choice
    [ try parseMultilineBasicString
    , try parseMultilineLiteralString
    , try parseBasicString
    , parseLiteralString
    ]

-- | A string in double quotes.
parseBasicString :: Parser Text
parseBasicString =
  label "double-quoted string" $
    between (char '"') (char '"') $
      fmap Text.pack . many . choice $
        [ satisfy isBasicChar
        , parseEscaped
        ]

-- | A string in single quotes.
parseLiteralString :: Parser Text
parseLiteralString =
  label "single-quoted string" $
    between (char '\'') (char '\'') $
      takeWhileP (Just "literal-char") isLiteralChar

-- | A multiline string with three double quotes.
parseMultilineBasicString :: Parser Text
parseMultilineBasicString =
  label "double-quoted multiline string" $ do
    _ <- string "\"\"\"" *> optional eol
    lineContinuation
    Text.concat <$> manyTill (mlBasicContent <* lineContinuation) (exactly 3 '"')
  where
    mlBasicContent =
      choice
        [ Text.singleton <$> try parseEscaped
        , Text.singleton <$> satisfy isBasicChar
        , parseMultilineDelimiter '"'
        , eol
        ]
    lineContinuation = many (try $ char '\\' *> hspace *> eol *> space) *> pure ()

-- | A multiline string with three single quotes.
parseMultilineLiteralString :: Parser Text
parseMultilineLiteralString =
  label "single-quoted multiline string" $ do
    _ <- string "'''" *> optional eol
    Text.concat <$> manyTill mlLiteralContent (exactly 3 '\'')
  where
    mlLiteralContent =
      choice
        [ Text.singleton <$> satisfy isLiteralChar
        , parseMultilineDelimiter '\''
        , eol
        ]

parseEscaped :: Parser Char
parseEscaped = char '\\' *> parseEscapedChar
  where
    parseEscapedChar =
      choice
        [ char '"'
        , char '\\'
        , char 'b' $> '\b'
        , char 'f' $> '\f'
        , char 'n' $> '\n'
        , char 'r' $> '\r'
        , char 't' $> '\t'
        , char 'u' *> unicodeHex 4
        , char 'U' *> unicodeHex 8
        ]

    unicodeHex n = do
      code <- readHex . Text.pack <$> count n hexDigitChar
      guard $ isUnicodeScalar code
      pure $ chr code

{- |
Parse the multiline delimiter (" in """ quotes, or ' in ''' quotes), unless
the delimiter indicates the end of the multiline string.

i.e. parse 1 or 2 delimiters, or 4 or 5, which is 1 or 2 delimiters at the
end of a multiline string (then backtrack 3 to mark the end).
-}
parseMultilineDelimiter :: Char -> Parser Text
parseMultilineDelimiter delim =
  choice
    [ exactly 1 delim
    , exactly 2 delim
    , do
        _ <- lookAhead (exactly 4 delim)
        Text.pack <$> count 1 (char delim)
    , do
        _ <- lookAhead (exactly 5 delim)
        Text.pack <$> count 2 (char delim)
    ]

isBasicChar :: Char -> Bool
isBasicChar c =
  case c of
    ' ' -> True
    '\t' -> True
    _ | 0x21 <= code && code <= 0x7E -> c /= '"' && c /= '\\'
    _ | isNonAscii c -> True
    _ -> False
  where
    code = ord c

isLiteralChar :: Char -> Bool
isLiteralChar c =
  case c of
    ' ' -> True
    '\t' -> True
    _ | 0x21 <= code && code <= 0x7E -> c /= '\''
    _ | isNonAscii c -> True
    _ -> False
  where
    code = ord c

parseOffsetDateTime :: Parser (LocalTime, TimeZone)
parseOffsetDateTime = (,) <$> parseLocalDateTime <*> parseTimezone
  where
    parseTimezone =
      choice
        [ char' 'Z' $> Time.utc
        , do
            applySign <- parseSign
            h <- parseHours
            _ <- char ':'
            m <- parseMinutes
            return $ Time.minutesToTimeZone $ applySign $ h * 60 + m
        ]

parseLocalDateTime :: Parser LocalTime
parseLocalDateTime = do
  d <- parseLocalDate
  _ <- char' 'T' <|> char ' '
  t <- parseLocalTime
  return $ Time.LocalTime d t

parseLocalDate :: Parser Day
parseLocalDate = do
  y <- parseDecDigits 4
  _ <- char '-'
  m <- parseDecDigits 2
  _ <- char '-'
  d <- parseDecDigits 2
  maybe empty return $ Time.fromGregorianValid y m d

parseLocalTime :: Parser TimeOfDay
parseLocalTime = do
  h <- parseHours
  _ <- char ':'
  m <- parseMinutes
  _ <- char ':'
  sInt <- parseSeconds
  sFracRaw <- optional $ fmap Text.pack $ char '.' >> some digitChar
  let sFrac = MkFixed $ maybe 0 readPicoDigits sFracRaw
  return $ Time.TimeOfDay h m (fromIntegral sInt + sFrac)
  where
    readPicoDigits s = readDec $ Text.take 12 (s <> Text.replicate 12 "0")

parseHours :: Parser Int
parseHours = do
  h <- parseDecDigits 2
  guard $ 0 <= h && h < 24
  return h

parseMinutes :: Parser Int
parseMinutes = do
  m <- parseDecDigits 2
  guard $ 0 <= m && m < 60
  return m

parseSeconds :: Parser Int
parseSeconds = do
  s <- parseDecDigits 2
  guard $ 0 <= s && s <= 60 -- include 60 for leap seconds
  return s

parseFloat :: Parser Double
parseFloat = do
  applySign <- parseSign
  num <-
    choice
      [ try normalFloat
      , try $ string "inf" $> inf
      , try $ string "nan" $> nan
      ]
  pure $ applySign num
  where
    normalFloat = do
      intPart <- parseDecIntRaw
      (fracPart, expPart) <-
        choice
          [ try $ (,) <$> pure "" <*> parseExp
          , (,) <$> parseFrac <*> optionalOr "" parseExp
          ]

      -- guess if the exponent is too big to fit in a double precision float anyway.
      -- https://github.com/brandonchinn178/toml-reader/issues/8
      pure $
        if Text.length expPart > 7
          then inf
          else readFloat $ intPart <> fracPart <> expPart

    parseExp =
      fmap Text.concat . sequence $
        [ string' "e"
        , parseSignRaw
        , parseNumRaw digitChar digitChar
        ]
    parseFrac =
      fmap Text.concat . sequence $
        [ string "."
        , parseNumRaw digitChar digitChar
        ]

    inf = read "Infinity"
    nan = read "NaN"

parseInteger :: Parser Integer
parseInteger =
  choice
    [ try parseBinInt
    , try parseOctInt
    , try parseHexInt
    , parseSignedDecInt
    ]
  where
    parseSignedDecInt = do
      applySign <- parseSign
      num <- readDec <$> parseDecIntRaw
      pure $ applySign num
    parseHexInt =
      parsePrefixedInt readHex "0x" hexDigitChar
    parseOctInt =
      parsePrefixedInt readOct "0o" octDigitChar
    parseBinInt =
      parsePrefixedInt readBin "0b" binDigitChar

    parsePrefixedInt readInt prefix parseDigit = do
      _ <- string prefix
      readInt <$> parseNumRaw parseDigit parseDigit

parseBoolean :: Parser Bool
parseBoolean =
  choice
    [ True <$ string "true"
    , False <$ string "false"
    ]

{--- Normalize into Value ---}

-- | An annotated, normalized Value
type AnnValue = GenericValue Map Text TableMeta ArrayMeta

type AnnTable = Map Text AnnValue

unannotateTable :: AnnTable -> Table
unannotateTable = fmap unannotateValue

unannotateValue :: AnnValue -> Value
unannotateValue = fromGenericValue unannotateTable

data TableType
  = -- | An inline table, e.g. "a.b" in:
    --
    -- @
    -- a.b = { c = 1 }
    -- @
    InlineTable
  | -- | A table created implicitly from a nested key, e.g. "a" in:
    --
    -- @
    -- a.b = 1
    -- @
    ImplicitKey
  | -- | An explicitly named section, e.g. "a.b.c" and "a.b" but not "a" in:
    --
    -- @
    -- [a.b.c]
    -- [a.b]
    -- @
    ExplicitSection
  | -- | An implicitly created section, e.g. "a" in:
    --
    -- @
    -- [a.b]
    -- @
    --
    -- Can later be converted into an explicit section
    ImplicitSection
  deriving (Eq)

data TableMeta = TableMeta
  { tableType :: TableType
  }

data ArrayMeta = ArrayMeta
  { isStaticArray :: Bool
  }

newtype NormalizeM a = NormalizeM
  { runNormalizeM :: Either NormalizeError a
  }

instance Functor NormalizeM where
  fmap f = NormalizeM . fmap f . runNormalizeM
instance Applicative NormalizeM where
  pure = NormalizeM . pure
  NormalizeM f <*> NormalizeM x = NormalizeM (f <*> x)
instance Monad NormalizeM where
  m >>= f = NormalizeM $ runNormalizeM . f =<< runNormalizeM m

normalizeError :: NormalizeError -> NormalizeM a
normalizeError = NormalizeM . Left

normalize :: TOMLDoc -> Either TOMLError Table
normalize = bimap NormalizeError unannotateTable . runNormalizeM . normalize'

normalize' :: TOMLDoc -> NormalizeM AnnTable
normalize' TOMLDoc{..} = do
  root <- flattenTable rootTable
  foldlM mergeTableSection root subTables
  where
    mergeTableSection :: AnnTable -> TableSection -> NormalizeM AnnTable
    mergeTableSection baseTable TableSection{..} = do
      case tableSectionHeader of
        SectionTable key ->
          mergeTableSectionTable key tableSectionTable baseTable
        SectionTableArray key ->
          mergeTableSectionArray key tableSectionTable baseTable

mergeTableSectionTable :: Key -> RawTable -> AnnTable -> NormalizeM AnnTable
mergeTableSectionTable sectionKey table baseTable =
  setValueAtPath valueAtPathOptions sectionKey baseTable $ \mVal -> do
    tableToExtend <-
      case mVal of
        -- if a value doesn't already exist, initialize an empty Map
        Nothing -> pure Map.empty
        -- if a Table already exists at the path ...
        Just existingValue@(GenericTable meta existingTable) ->
          case tableType meta of
            -- ... and is an inline table, error
            InlineTable -> duplicateKeyError existingValue
            -- ... and was created as a nested key elsewhere, error
            ImplicitKey -> extendTableError
            -- ... and was created as a Table section explicitly defined elsewhere, error
            ExplicitSection -> duplicateSectionError
            -- ... otherwise, return the existing table
            _ -> pure existingTable
        -- if some other Value already exists at the path, error
        Just existingValue -> duplicateKeyError existingValue

    mergedTable <-
      mergeRawTable
        MergeOptions{recurseImplicitSections = False}
        tableToExtend
        table

    let newTableMeta = TableMeta{tableType = ExplicitSection}
    pure $ GenericTable newTableMeta mergedTable
  where
    valueAtPathOptions =
      ValueAtPathOptions
        { shouldRecurse = \case
            InlineTable -> False
            ImplicitKey -> False
            ExplicitSection -> True
            ImplicitSection -> True
        , implicitType = ImplicitSection
        , makeMidPathNotTableError = nonTableInNestedKeyError sectionKey table
        }
    duplicateKeyError existingValue =
      normalizeError
        DuplicateKeyError
          { _path = sectionKey
          , _existingValue = unannotateValue existingValue
          , _valueToSet = Table $ rawTableToApproxTable table
          }
    extendTableError =
      normalizeError
        ExtendTableError
          { _path = sectionKey
          , _originalKey = sectionKey
          }
    duplicateSectionError =
      normalizeError
        DuplicateSectionError
          { _sectionKey = sectionKey
          }

mergeTableSectionArray :: Key -> RawTable -> AnnTable -> NormalizeM AnnTable
mergeTableSectionArray sectionKey table baseTable = do
  setValueAtPath valueAtPathOptions sectionKey baseTable $ \mVal -> do
    (meta, currArray) <-
      case mVal of
        -- if nothing exists, initialize an empty array
        Nothing -> do
          let meta = ArrayMeta{isStaticArray = False}
          pure (meta, [])
        -- if an array exists, insert table to the end of the array
        Just (GenericArray meta existingArray)
          | not (isStaticArray meta) ->
              pure (meta, existingArray)
        -- otherwise, error
        Just existingValue ->
          normalizeError
            ImplicitArrayForDefinedKeyError
              { _path = sectionKey
              , _existingValue = unannotateValue existingValue
              , _tableSection = rawTableToApproxTable table
              }

    let newTableMeta = TableMeta{tableType = ExplicitSection}
    newTable <- GenericTable newTableMeta <$> flattenTable table
    pure $ GenericArray meta $ currArray <> [newTable]
  where
    valueAtPathOptions =
      ValueAtPathOptions
        { shouldRecurse = \case
            InlineTable -> False
            ImplicitKey -> False
            ExplicitSection -> True
            ImplicitSection -> True
        , implicitType = ImplicitSection
        , makeMidPathNotTableError = \history existingValue ->
            NonTableInNestedImplicitArrayError
              { _path = history
              , _existingValue = unannotateValue existingValue
              , _sectionKey = sectionKey
              , _tableSection = rawTableToApproxTable table
              }
        }

flattenTable :: RawTable -> NormalizeM AnnTable
flattenTable =
  mergeRawTable
    MergeOptions{recurseImplicitSections = True}
    Map.empty

data MergeOptions = MergeOptions
  { recurseImplicitSections :: Bool
  }

mergeRawTable :: MergeOptions -> AnnTable -> RawTable -> NormalizeM AnnTable
mergeRawTable MergeOptions{..} baseTable table = foldlM insertRawValue baseTable (unLookupMap table)
  where
    insertRawValue accTable (key, rawValue) = do
      let valueAtPathOptions =
            ValueAtPathOptions
              { shouldRecurse = \case
                  InlineTable -> False
                  ImplicitKey -> True
                  ExplicitSection -> True
                  ImplicitSection -> recurseImplicitSections
              , implicitType = ImplicitKey
              , makeMidPathNotTableError = nonTableInNestedKeyError key table
              }
      setValueAtPath valueAtPathOptions key accTable $ \case
        Nothing -> fromRawValue rawValue
        Just existingValue ->
          normalizeError
            DuplicateKeyError
              { _path = key
              , _existingValue = unannotateValue existingValue
              , _valueToSet = rawValueToApproxValue rawValue
              }

    fromRawValue = \case
      GenericTable _ rawTable -> do
        let meta = TableMeta{tableType = InlineTable}
        GenericTable meta <$> flattenTable rawTable
      GenericArray _ rawValues -> do
        let meta = ArrayMeta{isStaticArray = True}
        GenericArray meta <$> mapM fromRawValue rawValues
      GenericString x -> pure (GenericString x)
      GenericInteger x -> pure (GenericInteger x)
      GenericFloat x -> pure (GenericFloat x)
      GenericBoolean x -> pure (GenericBoolean x)
      GenericOffsetDateTime x -> pure (GenericOffsetDateTime x)
      GenericLocalDateTime x -> pure (GenericLocalDateTime x)
      GenericLocalDate x -> pure (GenericLocalDate x)
      GenericLocalTime x -> pure (GenericLocalTime x)

data ValueAtPathOptions = ValueAtPathOptions
  { shouldRecurse :: TableType -> Bool
  , implicitType :: TableType
  , makeMidPathNotTableError :: Key -> AnnValue -> NormalizeError
  }

-- | Implementation for makeMidPathNotTableError for NonTableInNestedKeyError
nonTableInNestedKeyError :: Key -> RawTable -> (Key -> AnnValue -> NormalizeError)
nonTableInNestedKeyError key table = \history existingValue ->
  NonTableInNestedKeyError
    { _path = history
    , _existingValue = unannotateValue existingValue
    , _originalKey = key
    , _originalValue = Table $ rawTableToApproxTable table
    }

setValueAtPath ::
  ValueAtPathOptions ->
  Key ->
  AnnTable ->
  (Maybe AnnValue -> NormalizeM AnnValue) ->
  NormalizeM AnnTable
setValueAtPath ValueAtPathOptions{..} fullKey initialTable f = do
  (mValue, setValue) <- getPathLens doRecurse fullKey initialTable
  setValue <$> f mValue
  where
    doRecurse history = \case
      -- If nothing exists, recurse into a new empty Map
      Nothing -> do
        let newTableMeta = TableMeta{tableType = implicitType}
        pure (Map.empty, GenericTable newTableMeta)
      -- If a Table exists, recurse into it
      Just (GenericTable meta subTable) -> do
        unless (shouldRecurse $ tableType meta) $
          normalizeError
            ExtendTableError
              { _path = history
              , _originalKey = fullKey
              }
        pure (subTable, GenericTable meta)
      -- If an Array exists, recurse into the last Table, per spec:
      --   Any reference to an array of tables points to the
      --   most recently defined table element of the array.
      Just (GenericArray aMeta vs)
        | Just vs' <- NonEmpty.nonEmpty vs
        , GenericTable tMeta subTable <- NonEmpty.last vs' -> do
            when (isStaticArray aMeta) $
              normalizeError $
                ExtendTableInInlineArrayError history fullKey
            pure (subTable, GenericArray aMeta . snoc (NonEmpty.init vs') . GenericTable tMeta)
      -- If something else exists, throw error with makeMidPathNotTableError
      Just v -> normalizeError $ makeMidPathNotTableError history v

    snoc xs x = xs <> [x]

-- | Convert a RawTable into a Table, for use in errors + debugging.
rawTableToApproxTable :: RawTable -> Table
rawTableToApproxTable =
  Map.fromList
    . map (\(k, v) -> (Text.intercalate "." $ NonEmpty.toList k, rawValueToApproxValue v))
    . unLookupMap

-- | Convert a RawValue into a Value, for use in errors + debugging.
rawValueToApproxValue :: RawValue -> Value
rawValueToApproxValue = fromGenericValue rawTableToApproxTable

{--- Parser Helpers ---}

-- | https://github.com/toml-lang/toml/blob/1.0.0/toml.abnf#L38
isNonAscii :: Char -> Bool
isNonAscii c = (0x80 <= code && code <= 0xD7FF) || (0xE000 <= code && code <= 0x10FFFF)
  where
    code = ord c

-- | https://unicode.org/glossary/#unicode_scalar_value
isUnicodeScalar :: Int -> Bool
isUnicodeScalar code = (0x0 <= code && code <= 0xD7FF) || (0xE000 <= code && code <= 0x10FFFF)

-- | Returns "", "-", or "+"
parseSignRaw :: Parser Text
parseSignRaw = optionalOr "" (string "-" <|> string "+")

parseSign :: Num a => Parser (a -> a)
parseSign = do
  sign <- parseSignRaw
  pure $ if sign == "-" then negate else id

parseDecIntRaw :: Parser Text
parseDecIntRaw =
  choice
    [ try $ parseNumRaw (satisfy $ \c -> isDigit c && c /= '0') digitChar
    , Text.singleton <$> digitChar
    ]

parseDecDigits :: (Show a, Num a, Eq a) => Int -> Parser a
parseDecDigits n = readDec . Text.pack <$> count n digitChar

parseNumRaw :: Parser Char -> Parser Char -> Parser Text
parseNumRaw parseLeadingDigit parseDigit = do
  leading <- parseLeadingDigit
  rest <- many $ optional (char '_') *> parseDigit
  pure $ Text.pack $ leading : rest

{--- Parser Utilities ---}

hsymbol :: Text -> Parser ()
hsymbol s = hspace >> string s >> hspace >> pure ()

-- | Parse trailing whitespace/trailing comments + newline
endOfLine :: Parser ()
endOfLine = L.space hspace1 skipComments empty >> (void eol <|> eof) >> pure ()

-- | Parse spaces, newlines, and comments
emptyLines :: Parser ()
emptyLines = L.space space1 skipComments empty

skipComments :: Parser ()
skipComments = do
  _ <- string "#"
  void . many $ do
    c <- satisfy (/= '\n')
    let code = ord c
    case c of
      '\r' -> void $ lookAhead (char '\n')
      _
        | (0x00 <= code && code <= 0x08) || (0x0A <= code && code <= 0x1F) || code == 0x7F ->
            fail $ "Comment has invalid character: \\" <> show code
      _ -> pure ()

space, space1 :: Parser ()
space = void $ many parseSpace
space1 = void $ some parseSpace

-- | TOML does not support bare '\r' without '\n'.
parseSpace :: Parser ()
parseSpace = void (satisfy (\c -> isSpace c && c /= '\r')) <|> void (string "\r\n")

#if !MIN_VERSION_megaparsec(9,0,0)
hspace :: Parser ()
hspace = void $ takeWhileP (Just "white space") isHSpace

hspace1 :: Parser ()
hspace1 = void $ takeWhile1P (Just "white space") isHSpace

isHSpace :: Char -> Bool
isHSpace x = isSpace x && x /= '\n' && x /= '\r'
#endif

optionalOr :: a -> Parser a -> Parser a
optionalOr def = fmap (fromMaybe def) . optional

exactly :: Int -> Char -> Parser Text
exactly n c = try $ Text.pack <$> count n (char c) <* notFollowedBy (char c)

{--- Read Helpers ---}

-- | Assumes string satisfies @all isDigit@.
readFloat :: (Show a, RealFrac a) => Text -> a
readFloat = runReader Numeric.readFloat

-- | Assumes string satisfies @all isDigit@.
readDec :: (Show a, Num a, Eq a) => Text -> a
readDec = runReader Numeric.readDec

-- | Assumes string satisfies @all isHexDigit@.
readHex :: (Show a, Num a, Eq a) => Text -> a
readHex = runReader Numeric.readHex

-- | Assumes string satisfies @all isOctDigit@.
readOct :: (Show a, Num a, Eq a) => Text -> a
readOct = runReader Numeric.readOct

-- | Assumes string satisfies @all (`elem` "01")@.
readBin :: (Show a, Num a) => Text -> a
readBin = foldl' go 0 . Text.unpack
  where
    go acc x =
      let digit
            | x == '0' = 0
            | x == '1' = 1
            | otherwise = error $ "readBin got unexpected digit: " <> show x
       in 2 * acc + digit

runReader :: Show a => ReadS a -> Text -> a
runReader rdr digits =
  case rdr $ Text.unpack digits of
    [(x, "")] -> x
    result -> error $ "Unexpectedly unable to parse " <> show digits <> ": " <> show result