from collections import deque

base_chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
end_of_token_chars = "() \t"
whitespace_chars = " \t"

class UnexpectedCharacterException(Exception): 
	def __init__(self, msg, offset, char, *args):
		Exception.__init__(self, *args)
		self._msg = msg
		self._offset = offset
		self._char = char


class TokenStream(object):
	def __init__(self, file_):
		self._file = file_
		self._file.seek(0, 0)
		self._offset = 0

	def _getc(self):
		c = self._file.read(1)
		if(c):
			self._offset += 1
		return c
	def _ungetc(self):
		self._file.seek(self._offset - 1, 0)
		self._offset -= 1

	def get_token(self):
		result = deque()
		while True:
			c = self._getc()
			if(not c):
				# EOF.
				break
			if(c in base_chars):
				result.append(c)
				continue
			if(c in end_of_token_chars):
				if(not result):
					# We are not inside a token.
					if(c in whitespace_chars):
						# Some whitespace. Ignore it.
						continue
					# A parenthesis.
					return c

				# End of token.
				self._ungetc()
				break
			raise UnexpectedCharacterException("Unexpected character while tokenizing", self._offset, c)
		return "".join(result)