'''
  This class implements a general tree object.
  
  Simon D. Levy             CSCI 315                    01 NOV 2007

'''

# helper class
class Branch:
	
    def __init__(self, tree, label):
       self.tree = tree
       self.label = label
	
class Tree:

    
    # Constructs a tree node with specified contents and no children.
    def __init__(self, contents):
	self.contents = contents
	self.children = []
    
    
    # Adds a labeled branch to this tree node.
    def addBranch(self, branch, label):
	self.children.append(Branch(branch, label))
   
    
    # Returns True if tree is terminal (has no children), False otherwise.
    def isTerminal(self):
	return len(self.children) == 0
   
    
    # Returns children of this tree.  Terminal tree returns null.
    def getChildren(self):
        return children    

    
    # Returns contents of this tree node.
    def getContents(self):
	return self.contents
    
    
    # Returns tree node on labeled branch.  For duplicate labels, 
    # returns first such node.  Throws an exception if node is terminal or 
    # label is bogus.
    def getBranch(self, label):

	if (self.isTerminal()):
	    raise Exception('Node is terminal')
	
	for child in self.children:
	    if child.label == label:
		return child.tree
	 
	raise Exception('Label not found')
    


    
    # Returns a string representation of this tree.  Each node in the
    # tree appears on a separate line, with depth indicated by indentation.
    def __str__(self, depth=0):

	s = str(self.contents) + '\n'

	for child in self.children:
	    s = s + self._pad(depth+1) + child.label + ' : ' + child.tree.__str__(depth+1)
	
	return s
    

    def _pad(self, n):
	s = ''
	for i in range(0, n):
	    s += '  '
	return s