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03-HuffmanCoding.py
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import sys
class Node:
def __init__(self, key, value):
self.key = key
self.value = value
self.left = None
self.right = None
class Tree:
def __init__(self, root):
self.root = root
# take a string (data) and determine the relevant frequencies of the characters
def character_frequencies(data):
character_frequency_dict = {}
for a in data:
if a not in character_frequency_dict:
character_frequency_dict[a] = 1
else:
character_frequency_dict[a] += 1
return(character_frequency_dict)
# build and sort a list of tuples from lowest to highest frequencies
def sort_character_frequencies(data):
items = list(data.items())
items.sort(key=lambda x : x[1])
return items
def encode_tree(root, string, encoded_characters):
if(root.right is None and root.left is None):
encoded_characters[root.key] = string
else:
if root.left is not None:
encode_tree(root.left, string + "0", encoded_characters)
if root.right is not None:
encode_tree(root.right, string + "1", encoded_characters)
def insert_element_into_list(node, mapped_sorted_frequencies):
for index, element in enumerate(mapped_sorted_frequencies):
if node.value < element.value:
mapped_sorted_frequencies.insert(index, node)
break
elif index == len(mapped_sorted_frequencies) - 1:
mapped_sorted_frequencies.append(node)
break
# build the Huffman Tree by assigning a binary code to each letter
def huffman_encoding(data):
if not data:
return data, None
else:
frequencies = character_frequencies(data)
sorted_frequencies = sort_character_frequencies(frequencies)
# Create a Node object that includes a key value pair, but make it a list so we maintain the order
mapped_sorted_frequencies = list(
map(lambda x: Node(x[0], x[1]), sorted_frequencies))
tree = None
while len(mapped_sorted_frequencies) > 1:
# trim the Huffman Tree, i.e. remove the frequencies from the previously built tree
first_item = mapped_sorted_frequencies.pop(0)
second_item = mapped_sorted_frequencies.pop(0)
root_node = Node(first_item.value + second_item.value, first_item.value + second_item.value)
root_node.left = first_item
root_node.right = second_item
# insert the root node into the mapped_sorted_frequencies list
insert_element_into_list(root_node, mapped_sorted_frequencies)
if len(mapped_sorted_frequencies) == 0:
tree = Tree(root_node)
if tree is None:
if len(mapped_sorted_frequencies) == 1:
first_item = mapped_sorted_frequencies.pop(0)
tree = Tree(Node(first_item.value, first_item.value))
tree.root.left = Node(first_item.key, first_item.value)
encoded_characters = dict()
encoded_string = ""
encode_tree(tree.root, "", encoded_characters)
for character in data:
encoded_string += encoded_characters[character]
return tree, encoded_string
def huffman_decoding(data, root):
if root is None:
return data, None
def decode(data, root, index, decode_string):
if root.left is None and root.right is None:
decode_string += root.key
return index, decode_string
elif data[index] == "0":
return decode(data, root.left, index + 1, decode_string)
else:
return decode(data, root.right, index + 1, decode_string)
index = 0
decode_string = ""
while(index <= len(data) - 1):
index, decode_string = decode(data, root, index, decode_string)
return decode_string
def run_tests(string):
print ("The content of the data is: {}\n".format(string))
print ("The size of the data is: {}\n".format(sys.getsizeof(string)))
tree, encoded_data = huffman_encoding(string)
print ("The content of the encoded data is: {}\n".format(encoded_data))
if encoded_data == None:
print("This is an empty string and cannot be encoded.")
else:
print ("The size of the encoded data is: {}\n".format(sys.getsizeof(int(encoded_data, base=2))))
decoded_data = huffman_decoding(encoded_data, tree.root)
print ("The content of the decoded data is: {}\n".format(decoded_data))
print ("The size of the decoded data is: {}\n".format(sys.getsizeof(decoded_data)))
if __name__ == "__main__":
codes = {}
# Test 1, one character
run_tests("a")
# The content of the data is: a
# The size of the data is: 50
# The content of the encoded data is: 0
# The size of the encoded data is: 24
# The content of the decoded data is: a
# The size of the decoded data is: 50
# Test 2, multiples of the same character
run_tests("aaa")
# The content of the data is: aaa
# The size of the data is: 52
# The content of the encoded data is: 000
# The size of the encoded data is: 24
# The content of the decoded data is: aaa
# The size of the decoded data is: 52
# Test 3, a simple string
run_tests("apple")
# The content of the data is: apple
# The size of the data is: 54
# The content of the encoded data is: 0011110110
# The size of the encoded data is: 28
# The content of the decoded data is: apple
# The size of the decoded data is: 54
# Test 4, an empty string
run_tests("")
# The content of the data is:
# The size of the data is: 49
# The content of the encoded data is: None
# This is an empty string and cannot be encoded.
# Test 5, a simple sentence
run_tests("The bird is the word.")
# The content of the data is: The bird is the word.
# The size of the data is: 70
# The content of the encoded data is: 0100101111101100101111100000111011110110110011110111110110100010010000011010
# The size of the encoded data is: 36
# The content of the decoded data is: The bird is the word.
# The size of the decoded data is: 70