From 427caabb1b3d9bcdcbfc2c7dd87fd7c0eb496fb4 Mon Sep 17 00:00:00 2001
From: James Bowes <jbowes@repl.ca>
Date: Fri, 27 Jul 2012 16:42:02 -0300
Subject: [PATCH] add huffman implementation

---
 huffman.rb | 129 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 129 insertions(+)
 create mode 100644 huffman.rb

diff --git a/huffman.rb b/huffman.rb
new file mode 100644
index 0000000..c1681b1
--- /dev/null
+++ b/huffman.rb
@@ -0,0 +1,129 @@
+class HuffNode
+  attr_accessor :weight, :symbol, :left, :right, :parent
+
+  def initialize(params = {})
+    @weight   = params[:weight]  || 0
+    @symbol   = params[:symbol]  || ''
+    @left     = params[:left]    || nil
+    @right    = params[:right]   || nil
+    @parent   = params[:parent]  || nil
+  end
+
+  def walk(&block)
+    walk_node('', &block)
+  end
+
+  def walk_node(code, &block)
+    yield(self, code)
+    @left.walk_node(code + '0', &block) unless @left.nil?
+    @right.walk_node(code + '1', &block) unless @right.nil?
+  end
+
+  def leaf?
+    @symbol != ''
+  end
+
+  def internal?
+    @symbol == ''
+  end
+
+  def root?
+    internal? and @parent.nil?
+  end
+
+end
+
+class NodeQueue
+  attr_accessor :nodes, :huffman_root
+
+  def initialize(list)
+    frequencies = {}
+    list.each do |c|
+      frequencies[c] ||= 0
+      frequencies[c] += 1
+    end
+    @nodes = []
+    frequencies.each do |c, w|
+      @nodes << HuffNode.new(:symbol => c, :weight => w)
+    end
+    generate_tree
+  end
+
+  def generate_tree
+    while @nodes.size > 1
+      sorted = @nodes.sort { |a,b| a.weight <=> b.weight }
+      to_merge = []
+      2.times { to_merge << sorted.shift }
+      sorted << merge_nodes(to_merge[0], to_merge[1])
+      @nodes = sorted
+    end
+    @huffman_root = @nodes.first
+  end
+
+  def merge_nodes(node1, node2)
+    left = node1.weight > node2.weight ? node2 : node1
+    right = left == node1 ? node2 : node1
+    node = HuffNode.new(:weight => left.weight + right.weight, :left => left, :right => right)
+    left.parent = right.parent = node
+    node
+  end
+
+end
+
+class HuffmanEncoding
+  attr_accessor :root, :lookup, :input, :output
+
+  def initialize(input)
+    @input = input
+    @root = NodeQueue.new(input).huffman_root
+#    @output = encode_list(input)
+  end
+
+  def lookup
+    return @lookup if @lookup
+    @lookup = {}
+    @root.walk do |node, code|
+      @lookup[code] = node.symbol if node.leaf?
+    end
+    @lookup
+  end
+
+  def encode(entry)
+    self.lookup.invert[entry]
+  end
+
+  def decode(code)
+    self.lookup[code]
+  end
+
+  def encode_list(list)
+    code = ''
+    list.each do |c|
+      code += encode(c) 
+    end
+    code
+  end
+
+  def decode_string(code)
+    code = code.to_s
+    string = ''
+    subcode = ''
+    code.each_char do |bit|
+      subcode += bit
+      unless decode(subcode).nil?
+        string += decode(subcode)
+        subcode = ''
+      end
+    end
+    string
+  end
+
+  def to_s
+    @output
+  end
+
+  def [](char)
+    encode(char)
+  end
+
+end