"""
 stack.py

 Showing how "pointers" and a singly-linked list
 can be used to implement a 'stack' data structure
 in python.

 ... with help from the folks in the Algorithms class

 Jim Mahoney | Feb 2013 | MIT license
"""

class Node(object):

  def __init__(self, data=None, down=None):
    self.data = data
    self.down = down

class Stack(object):

    def __init__(self):
        self.top = None

    def push(self, data):
        """ add data to the 'top' of the stack"""
        new_node = Node(data)
        new_node.down = self.top
        self.top = new_node

    def pop(self):
        """ return and remove data from 'top' of the stack"""
        old_top = self.top
        self.top = old_top.down
        return old_top.data
        ## ... and then we hope old_top is garbage collected

    def __str__(self):
        """ return string version of stack and nodes """
        result = "<Stack: "
        node = self.top
        while node != None:
            result += str(node.data) + " - "
            node = node.down
        result += ">"
        return result

def main():
    """ test this stuff ... """
    s = Stack()
    s.push("alpha")
    s.push("beta")
    s.push("gamma")
    print s
    print " pop : " + s.pop()
    print " pop : " + s.pop()
    print s
    print "So there."

main()