--- This module scans an SQL-Statement, which is passed as a string.
---@author Julia Krone
---@version 0.1
-- -------------------------------------------------------------------

module SQLScanner(scan) where

import Data.Char (isDigit, isAlpha, isAlphaNum, toLower)
import Data.List (splitOn)
import Data.Time (CalendarTime (..))

import qualified Data.Map as Map

import SQLToken

---Scans an SQL-statement - returning it as list of Token.
---@param chs - the statement as string
---@return the statement as list of token
scan :: String -> [Token]
scan chs =
   case chs of
      ""            -> []
      ('*':cs)      -> Asterix : scan cs
      ('(':cs)      -> LParen : scan cs
      (')':cs)      -> RParen : scan cs
      (',':cs)      -> Comma : scan cs
      (';':cs)      -> Semi : scan cs
      ('.':cs)      -> Stop : scan cs
      ('!':'=': cs) -> (BinOp Uneq) : scan cs
      ('=':cs)      -> (BinOp Equal) : scan cs
      ('{':cs)      -> readEmbedExp cs
      ('<':cs)      -> readLessOp cs
      ('>':cs)      -> readGreaterOp cs
      ('"' :cs)     -> readVarString cs
      ('\'':cs)     -> readVarChar cs
      (' ':cs)      -> scan cs
      ('\n':cs)     -> scan cs
      _             -> if (isDigit (head chs) || (head chs) == '-')
                         then (getNumberOrDate nStr) : scan nRs
                         else if (isAlpha (head chs))
                                 then (specialOrIdent s) : scan rs
                                 else (Unexpected (head chs)) : scan (tail chs)
  where (nStr, nRs) = span isDigitOrNChar chs
        (s,rs)      = span isAlphaNumUnderScore chs

-- auxiliary function to check if a character is digit or one of the
--following: '-', '.', ':'
isDigitOrNChar :: Char -> Bool
isDigitOrNChar c = isDigit c || (c == '-') || (c == '.') || (c == ':')

--auxiliary function which checks if a given character is alphanumerical
-- or '_' (used in relation names)
isAlphaNumUnderScore :: Char -> Bool
isAlphaNumUnderScore c = isAlphaNum c || (c == '_')

-- auxiliary function to determine whether a given string is a keyword
-- or an identifier
specialOrIdent:: String -> Token
specialOrIdent str =
  case Map.lookup lstr keywords of
          Just tk -> tk
          Nothing -> case Map.lookup lstr (Map.union operators constants) of
                       Just tk -> tk
                       Nothing -> Ident str
  where lstr = map (toLower) str

-- auxiliary function to determine whether a given string is a number
-- or a date and to sample the date
getNumberOrDate :: String -> Token
getNumberOrDate str = if (':' `elem` str)
                           then (Constant
                                  (Date
                                   (CalendarTime (read y)
                                                 (read m)
                                                 (read d)
                                                 (read h)
                                                 (read mi)
                                                 (read s) 0)))
                           else getNumber str
  where dateLs = splitOn [':'] str
        (y, dLs1) = getNext dateLs
        (m, dLs2) = getNext dLs1
        (d, dLs3) = getNext dLs2
        (h, dLs4) = getNext dLs3
        (mi, dLs5) = getNext dLs4
        (s, _) = getNext dLs5
        getNext [] = ("0" , [])
        getNext (num:ls) = (num, ls)

-- auxiliary function to read float and int values
getNumber :: String -> Token
getNumber str = if ('.' `elem` str) then Constant (NumFloat f)
                                    else Constant (NumInt (read str))
 where ((f,_):_) = reads str

-- reader for embedded curry expression surrounded by {}
readEmbedExp :: String -> [Token]
readEmbedExp str = (EmbedExp embed):(scan rrest)
  where (embed,rest) = break (\c -> c == '}') str
        rrest = if (null rest) then rest else tail rest

-- reader for string values surrounded by ""
readVarString :: String -> [Token]
readVarString str = (Constant (VarStr var)):(scan rrest)
  where (var,rest) = break (\c -> c == '"' ) str
        rrest = if (null rest) then rest else (tail rest)

-- reader for char values or string values surrounded by ' '
readVarChar :: String -> [Token]
readVarChar str = (Constant var):(scan rrest)
  where (svar,rest) = break (\c -> c== '\'') str
        var = if (length svar) == 1
                 then (VarChar (head svar))
                 else if (length svar) == 0
                       then (VarChar ' ')
                       else (VarStr svar)
        rrest = if (null rest) then rest else (tail rest)

-- reader for operations < and <=
readLessOp :: String -> [Token]
readLessOp [] = [(BinOp Lth)]
readLessOp (c:cs) = if ( c == '=') then (BinOp Lte) : scan cs
                                   else (BinOp Lth) : scan (c:cs)

-- reader for operations > and >=
readGreaterOp :: String -> [Token]
readGreaterOp [] = [(BinOp Gth)]
readGreaterOp (c:cs) = if (c == '=') then (BinOp Gte) : scan cs
                                     else (BinOp Gth) : scan (c:cs)

-- constructor of a finite map containing all keywords
keywords :: Map.Map String Token
keywords = Map.fromList [("select", KW_Select),
                         ("from", KW_From),
                         ("where", KW_Where),
                         ("order", KW_Order),
                         ("group", KW_Group),
                         ("by", KW_By),
                         ("having", KW_Having),
                         ("insert", KW_Insert),
                         ("into", KW_Into),
                         ("values", KW_Values),
                         ("update", KW_Update),
                         ("set", KW_Set),
                         ("delete", KW_Delete),
                         ("transaction", KW_Transaction),
                         ("commit", KW_Commit),
                         ("rollback", KW_Rollback),
                         ("begin",KW_Begin),
                         ("inner", KW_Inner),
                         ("cross", KW_Cross),
                         ("join", KW_Join),
                         ("on", KW_On),
                         ("as", KW_As),
                         ("satisfies", KW_Satisfies),
                         ("distinct", KW_Distinct),
                         ("all", KW_All),
                         ("case", KW_Case),
                         ("when", KW_When),
                         ("then", KW_Then),
                         ("else", KW_Else),
                         ("end", KW_End),
                         ("asc", KW_Asc),
                         ("desc", KW_Desc),
                         ("limit", KW_Limit),
                         ("exists", KW_Exists),
                         ("not", KW_Not),
                         ("table", KW_Table)]

-- constructor of a finite map containing all constants
constants :: Map.Map String Token
constants = Map.fromList [("true", Constant (Boolean True)),
                          ("false", Constant (Boolean False)),
                          ("null",Constant Null)]

-- constructor of a finite map containing all (literal) operators and functions
operators :: Map.Map String Token
operators = Map.fromList [("like", BinOp Like),
                          ("between", Between),
                          ( "and", LogOp And ),
                          ( "or", LogOp Or),
                          ( "in", In),
                          ( "is", Is),
                          ("count", Fun Count),
                          ("avg", Fun Avg),
                          ("min", Fun Min),
                          ("max", Fun Max),
                          ("sum", Fun Sum),
                          ("union", SetOp Union),
                          ("intersect", SetOp Intersect),
                          ("except", SetOp Except)]