##
## Simple list-processor that does prefix math, but can be extended
## fairly easily to do more powerful stuff.
## I wrote this to help me learn python better, and exercise some of
## it's more (IMO) interesting and powerful features.
##
## Version: 0.1 alpha (i.e. something i hacked up late one night)
## Last Update: Sep 10, 2004
##
## By: Scott Hurring (scott @ hurring.com)
## License: GPL: http://www.opensource.org/licenses/gpl-license.php
##

class ListProcessor:
    def __init__(self):
        self.stack = []
        pass

    def parse(self, s):
        """
        Turn the string into a list of individual chars
        """
        chars = s.split()
        return chars
    
    def eval(self, chars):
        """
        Evaluates a list of characters and calls the necessary objects
        (see 'decode' for input->object mapping)
        """
        stack = []
        
        while len(chars):
            c = chars.pop(0)
            # What does this value mean
            value = self.decode(c)
            #print "[read]: ", c

            if c == "(":
                #print "[open paren] ", chars
                stack.append(self.eval(chars))
                
            elif c == ")":
                #print "[close paren] ", chars, stack
                try:
                    f = stack.pop(0)
                    v = f( stack )
                    #print "value = ", v.value
                    return v.value
                except:
                    raise Exception("There was an error with the stack ", stack)

            else:
                stack.append(value)
                #print "\t[appended to stack] ", value
    
        #print "\n[stack] = ", stack
        return stack.pop(0)
    
    def decode(self, s):
        """
        Maps input keyword to the appropriate Python class
        """
        try:
            return float(s)
        except:
            if s == "+": return Add
            elif s == "-": return Subtract
            elif s == "*": return Multiply
            elif s == "/": return Divide
            elif s == "FortyTwo": return FortyTwo
            elif s == "Print": return Print
            elif s == "List": return List
            else: return s
        
class Add:
    value = 0.0;
    def __init__(self, values):
        for x in values:
            self.value += x
        
class Subtract:
    value = 0.0;
    def __init__(self, values):
        self.value = values.pop(0)
        for x in values:
            self.value -= x

class Multiply:
    value = 1.0;
    def __init__(self, values):
        for x in values:
            self.value *= x
            
class Divide:
    value = 1.0;
    def __init__(self, values):
        for x in values:
            self.value /= x
            
# Returns "42" no matter what ;-)    
class FortyTwo:
    value = 42
    def __init__(self, values):
        pass

class Print:
    value = ""
    def __init__(self, values):
        self.value = values
        print "[[PRINT]]: ", values

class List:
    value = ""
    def __init__(self, values):
        self.value = values


c = ListProcessor()

## NOTE: Becuase i'm lazy, you must put spaces between *everything* 

chars = c.parse("( + 5 ( + 2 ( + 9 ( * 5 ( FortyTwo ) ) ) ) )")
#chars = c.parse("( + 5 ( - 5 4 ) )");
#chars = c.parse("( Print no ( List xxyz ( FortyTwo ) abc ( Print Hey there ) for this ) )");
try:
    value = c.eval(chars)
    print "\nValue of input string = ", value
except Exception, e:
    print e