module Lexer where

data Token = Plus
           | Hyphen
           | Asterisk
           | ForwardSlash
           | Integer Int
           | LBrace
           | RBrace
           deriving (Eq, Show)

lex :: String -> [Token]
lex source = reverse (lex' source [] (length source))

lex' :: String -> [Token] -> Int -> [Token]
lex' "" tokens _ = tokens
lex' (' ':xs) tokens orgLength = lex' xs tokens orgLength
lex' (x:xs) tokens orgLength = case x of
    '+' -> lex' xs (Plus:tokens) orgLength
    '-' -> lex' xs (Hyphen:tokens) orgLength
    '*' -> lex' xs (Asterisk:tokens) orgLength
    '/' -> lex' xs (ForwardSlash:tokens) orgLength
    '(' -> lex' xs (LBrace:tokens) orgLength
    ')' -> lex' xs (RBrace:tokens) orgLength
    _ -> lexNumbers (x:xs) tokens orgLength

lexNumbers :: String -> [Token] -> Int -> [Token]
lexNumbers (x:xs) tokens orgLength
    | isDigit x = lex' rest ((Integer(read digits::Int)):tokens) orgLength
    | otherwise = syntaxError x (orgLength - (length (x:xs)) + 1)
    where
        digits = takeWhile isDigit (x:xs)
        rest = dropWhile isDigit (x:xs)
        isDigit c = c `elem` "0123456789"

syntaxError char pos = error $ "Syntax error: " ++ [char] ++ " (" ++ show pos ++ ")"