Added Huffman Coding Algorithm (#798)

compression/huffman.py

@@ -0,0 +1,87 @@
+import sys
+
+class Letter:
+    def __init__(self, letter, freq):
+        self.letter = letter
+        self.freq = freq
+        self.bitstring = ""
+
+    def __repr__(self):
+        return f'{self.letter}:{self.freq}'
+
+
+class TreeNode:
+    def __init__(self, freq, left, right):
+        self.freq = freq
+        self.left = left
+        self.right = right
+
+
+def parse_file(file_path):
+    """ 
+    Read the file and build a dict of all letters and their
+    frequences, then convert the dict into a list of Letters.
+    """
+    chars = {}
+    with open(file_path) as f:
+        while True:
+            c = f.read(1)
+            if not c:
+                break
+            chars[c] = chars[c] + 1 if c in chars.keys() else 1
+    letters = []
+    for char, freq in chars.items():
+        letter = Letter(char, freq)
+        letters.append(letter)
+    letters.sort(key=lambda l: l.freq)
+    return letters
+
+def build_tree(letters):
+    """ 
+    Run through the list of Letters and build the min heap
+    for the Huffman Tree.
+    """
+    while len(letters) > 1:
+        left = letters.pop(0)
+        right = letters.pop(0)
+        total_freq = left.freq + right.freq
+        node = TreeNode(total_freq, left, right)
+        letters.append(node)
+        letters.sort(key=lambda l: l.freq)
+    return letters[0]
+
+def traverse_tree(root, bitstring):
+    """ 
+    Recursively traverse the Huffman Tree to set each
+    Letter's bitstring, and return the list of Letters
+    """
+    if type(root) is Letter:
+        root.bitstring = bitstring
+        return [root]
+    letters = []
+    letters += traverse_tree(root.left, bitstring + "0")
+    letters += traverse_tree(root.right, bitstring + "1")
+    return letters
+
+def huffman(file_path):
+    """ 
+    Parse the file, build the tree, then run through the file
+    again, using the list of Letters to find and print out the 
+    bitstring for each letter.
+    """
+    letters_list = parse_file(file_path)
+    root = build_tree(letters_list)
+    letters = traverse_tree(root, "")
+    print(f'Huffman Coding of {file_path}: ')
+    with open(file_path) as f:
+        while True:
+            c = f.read(1)
+            if not c:
+                break
+            le = list(filter(lambda l: l.letter == c, letters))[0]
+            print(le.bitstring, end=" ")
+    print()
+
+if __name__ == "__main__":
+    # pass the file path to the huffman function
+    huffman(sys.argv[1])