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An overly-commented implementation of a Palindrome Tree in Python
debug_prints = False
class PalindromeTreeNode:
# Important to note: we don't use start_index and end_index much
# Because if this node is 'bb' and our string is 'abbacabbac' we'll see 'bb' twice
# So we use the length to calculate where in the full string the start index would be
# Each time we run across this node
# The start and end indices are useful for printing out this node though
start_index = 0
end_index = 0
length = 0
suffix_index = 0
node_index = 0
# Any palindrome can have up to 26 child palindromes
# e.g. aPALINDROMEa, bPALINDROMEb, cPALINDROMEc, ..., zPALINDROMEz
# So we store the node index, in the palindromic tree, where each one lives
# (If it doesn't exist, its letter isnt in the dict)
child_palindrome_node_index = {}
def __init__(self, **kwargs):
if "start_index" in kwargs:
self.start_index = kwargs.pop("start_index")
if "end_index" in kwargs:
self.end_index = kwargs.pop("end_index")
if "length" in kwargs:
self.length = kwargs.pop("length")
if "suffix_index" in kwargs:
self.suffix_index = kwargs.pop("suffix_index")
self.child_palindrome_node_index = {}
class PalindromeTree:
nodes = []
prev_node_index = 0
magic_imaginary_node_index = 0
magic_null_string_node_index = 1
# Push on the 2 base nodes
def __init__(self):
# First node is for single-letter palindromes
# It starts with length -1 because after you add on the letter to each side
# It becomes length 1 - kind of weird but okay
# Its longest suffix (not itself) is set to itself, because what else would it be??
# Note that start_index needs to be -1 for our insert to work
self.nodes.append(PalindromeTreeNode(start_index=-1, end_index=-1, length=-1, suffix_index=self.magic_imaginary_node_index))
self.nodes[0].node_index = 0
# Second node is the base for 2-letter palindromes
# It's a null string
# Its length is 0, and then when you add on the letter to each side
# It becomes length 2 - totally normal
# Its longest suffix (not itself) is the imaginary node 0
self.nodes.append(PalindromeTreeNode(start_index=-1, end_index=-1, length=0, suffix_index=self.magic_imaginary_node_index))
self.nodes[1].node_index = 1
def find_parent_and_create_child(self, s, index):
new_letter = s[index]
# Go back through the train of parent node connections until we find the right one
# Might end up being our first magic node
parent_palindrome_node = self.nodes[self.prev_node_index]
while True:
parent_palindrome_start_index = index - parent_palindrome_node.length
# The parent_palindrome_start_index needs to be at least 1, so that we can check the cell before it
# If the cell before the suffix is equal to our new letter then it can be a palindrome
# In the event where the parent_palindrome_node is our magic first node:
# parent_palindrome_start_index will be index+1
# so it'll be >= 1, and then it'll compare new_letter to itself
# In the event where parent_palindrome_node is our magic second node:
# parent_palindrome_start_index will be index
# so for the first letter it won't be >= 1
# which is okay - the second node is for palindromes like "bb" not "b"
# but for all letters afterwards it will be >= 1
# so then it'll check our new_letter with the previous one, to see if they're the same
if parent_palindrome_start_index >= 1 and new_letter == s[parent_palindrome_start_index - 1]:
break
parent_palindrome_node = self.nodes[parent_palindrome_node.suffix_index]
# Now that we have our suffix (could be a magic node, could be a real palindrome)
# Check to see if the palindrome (new_letter suffix new_letter) is already in the tree
# i.e. check to see if the suffix already has an edge index for new_letter
# If it is already in the tree, we don't insert anything new
# Just update our prev_node_index counter to point to the node that has the new palindrome
# So that next insert we can try to build on it
# this might be the case in e.g.
# bbabba
# we would already have 'bb' when we get to it the 2nd time
if new_letter in parent_palindrome_node.child_palindrome_node_index:
self.prev_node_index = parent_palindrome_node.child_palindrome_node_index[new_letter]
if debug_prints:
print ("Parent node index is", parent_palindrome_node.node_index, "This node already exists")
return None
# Create a new node for this new entry
new_palindrome_length = parent_palindrome_node.length + 2
new_node_start = index-new_palindrome_length + 1
new_node = PalindromeTreeNode(start_index=new_node_start, end_index=index, length=new_palindrome_length)
# Append the node to our nodes array and update its index accordingly
self.nodes.append(new_node)
new_node.node_index = len(self.nodes) - 1
# Update the suffix node to have its directed edge for the new letter point to our new node
parent_palindrome_node.child_palindrome_node_index[new_letter] = new_node.node_index
self.prev_node_index = new_node.node_index
return parent_palindrome_node
def set_child_suffix_node(self, s, index, parent_node):
new_letter = s[index]
child_node = self.nodes[len(self.nodes) - 1]
# Magic base case: if the palindrome is length 1, its suffix is the magic null node
if child_node.length == 1:
child_node.suffix_index = self.magic_null_string_node_index
return
# Similarly to how we did in find_parent_and_create_child
# We trace back through the suffix edges until we find the right one
# But instead of trying to find the palindrome parent
# We're trying to find the suffix palindrome
# So we go all the way back until we find a match
# And then go forward to their child palindrome
# Kind of weird but okay
suffix_node = self.nodes[parent_node.suffix_index]
while True:
suffix_start_index = index - suffix_node.length
if suffix_start_index >= 1 and new_letter == s[suffix_start_index - 1]:
break
suffix_node = self.nodes[suffix_node.suffix_index]
child_node.suffix_index = suffix_node.child_palindrome_node_index[new_letter]
def insert(self, s, index):
if debug_prints:
print("Inserting letter" , s[index], "at index", index)
parent_node = self.find_parent_and_create_child(s, index)
if parent_node is not None:
self.set_child_suffix_node(s, index, parent_node)
tree = PalindromeTree()
s = "abbalrcabbac"
longest_palindrome = ""
longest_palindrome_node = None
# Build the tree and cache off the longest palindrome
for i in range(0, len(s)):
tree.insert(s, i)
latest_node = tree.nodes[tree.prev_node_index]
if latest_node.length > len(longest_palindrome):
longest_palindrome = s[latest_node.start_index:latest_node.end_index + 1]
longest_palindrome_node = latest_node
# Print all nodes in the tree
if debug_prints:
for i in range(2, len(tree.nodes)):
cur_node = tree.nodes[i]
print (i-2, "= ",s[cur_node.start_index:cur_node.end_index + 1])
print (longest_palindrome)
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