cal

cal.py

#coding:utf-8


'''
expr ::= term { (+|-) term }*

term ::= factor { (*|/) factor }*

factor ::= ( expr )
    |   NUM
'''


class Token(object):
    def __init__(self, type, value):
        self.type = type
        self.value = value

    def __str__(self):
        return "type:%s, value:%s" % (self.type, self.value)


def gen_tokens(text):
    index = 0
    while index < len(text):
        c = text[index]
        index += 1
        if c in (" ", "\t", "\n"):
            continue
        elif c == "+":
            yield Token("+", c)
        elif c == "-":
            yield Token("-", c)
        elif c == "*":
            yield Token("*", c)
        elif c == "/":
            yield Token("/", c)
        elif c == "(":
            yield Token("(", c)
        elif c == ")":
            yield Token(")", c)
        elif str.isdigit(c):
            s = index - 1
            while index < len(text) and str.isdigit(text[index]):
                index += 1
            yield Token("NUM", text[s:index])


class Parse(object):

    def __init__(self):
        self.token = None
        self.next_token = None
        self.tokens = None

    def parse(self, text):
        self.tokens = gen_tokens(text)
        self.advance()
        return self.expr()

    def advance(self):
        self.token, self.next_token = self.next_token, next(self.tokens, None)

    def accept(self, type):
        if self.next_token and self.next_token.type == type:
            self.advance()
            return True
        return False

    def expr(self):
        exp = self.term()
        while self.accept("+") or self.accept("-"):
            op = self.token.type
            r = self.term()
            if op == "+":
                exp += r
            else:
                exp -= r

        return exp

    def term(self):
        exp = self.factor()
        while self.accept("*") or self.accept("/"):
            op = self.token.type
            r = self.factor()
            if op == "*":
                exp *= r
            else:
                exp /= r

        return exp

    def factor(self):

        if self.accept("NUM"):
            return int(self.token.value)
        elif self.accept("("):
            exp = self.expr()
            self.accept(")")
            return exp
        else:
            raise


if __name__ == "__main__":
    p = Parse()
    print p.parse("2")
    print p.parse("1+2")
    print p.parse("2+1*3")
    print p.parse("2*((1+7) - 5))")