# A general recursive descent parser

import re

import term

class SyntaxError(Exception): pass

class Parser:
    def __init__(self, str_):
        self._str = str_
        self._pos = 0
    
    def end(self):
        if not self._pos == len(self._str): self.syntax_err('End of string')
    
    def str_(self, str_, required=False):
        end_pos = self._pos + len(str_)
        if self._str[self._pos:end_pos] == str_:
            self._pos = end_pos
            return True
        elif required: self.syntax_err(str_)
        else: return False
    
    def re(self, pattern, required=False):
        matcher = re.compile(pattern).match(self._str, self._pos)
        if matcher:
            self._pos = matcher.end(0)
            return matcher.group(0)
        elif required: self.syntax_err(pattern)
        else: return None
    
    def syntax_err(self, token):
        after = self._str[self._pos:]
        if after == '': after += '<END>'
        raise SyntaxError(self.__class__.__name__+' syntax error: '+token
            +' expected in '+ term.as_style('90', self._str[:self._pos]) +after)