import sys
from Lexer import *

next_token = None
l = None

# expr
# Parses strings in the language generated by the rules:
#   <expr> -> <expr> + <term>
#   <expr> -> <expr> - <term>
#   <expr> -> <term>
def expr():
  global next_token
  global l
  print("Enter <expr>")
  term()
  while next_token.get_token().value == TokenTypes.ADD.value or \
        next_token.get_token().value == TokenTypes.SUB.value:
    next_token = l.lex()
    term()
  print("Exit <expr>")

# term
# Parses strings in the language generated by the rules:
#   <term> -> <term> * <factor>
#   <term> -> <term> / <factor>
#   <term> -> <factor>
def term():
  global next_token
  global l
  print("Enter <term>")
  factor()
  while next_token.get_token().value == TokenTypes.MUL.value or \
        next_token.get_token().value == TokenTypes.DIV.value:
    next_token = l.lex()
    factor()
  print("Exit <term>")

# factor
# Parses strings in the language generated by the rules:
#   <factor> -> id
#   <factor> -> int_constant
#   <factor> -> ( <expr> )
def factor():
  global next_token
  global l
  print("Enter <factor>")
  if next_token.get_token().value == TokenTypes.ID.value or \
     next_token.get_token().value == TokenTypes.INT.value:
    next_token = l.lex()
  else: # if the RHS is ( <expr> ), pass over (, call expr, check for )
    if next_token.get_token().value == TokenTypes.LPAREN.value:
      next_token = l.lex()
      expr()
      if next_token.get_token().value == TokenTypes.RPAREN.value:
        next_token = l.lex()
      else:
        error("Expecting RPAREN")
        sys.exit(-1)
    else:
      error("Expecting LPAREN") 
      sys.exit(-1)
  print("Exit <factor>")

def error(s):
  print("SYNTAX ERROR: "+s)

def main():
  global next_token
  global l
  l = Lexer(sys.argv[1])
  next_token = l.lex()
  expr()
  if next_token.get_token().value == TokenTypes.EOF.value:
    print("PARSE SUCCEEDED")
  else:
    print("PARSE FAILED")

main()