Functional Programming Lecture 15 - Case Study: Huffman Codes

The Problem Design a coding/decoding scheme and implement in Haskell. This requires: - an algorithm to encode a message, - an algorithm to decode a message, - an implementation.

Fixed and Variable Length Codes A fixed length code assigns the same number of bits to each code word. E.g. ASCII letter -> 7 bits (up to 128 code words) So to encode the string at we need 14 bits. A variable length code assigns a different number of bits to each code word, depending on the frequency of the code word. Frequent words are assigned short codes; infrequent words are assigned long codes. E.g. a at encoded by for go left b t 1 for go right tree to encode and decode

Coding 0 1 a 0 1 b t a is encoded by 1 bit, 0 b is encoded by 2 bits, 10 t is encoded by 2 bits, 11 An important property of a Huffman code is that the codes are prefix codes: no code of a letter (code word) is the prefix of the code of another letter (code word). E.g. 0 is not a prefix of 10 or is not a prefix of 0 or is not a prefix of 0 or 10 So, aa is encoded by 00. ba is encoded by 100.

Decoding 0 1 a 0 1 b t The encoded message is decoded as: 10 - b 0 - a 11 - t 0 - a 11 - t In view of the frequency of t, this is probably not a good code. t should be encoded by 1 bit! ps. Morse code is a type of Huffman code.

A Haskell Implementation Types -- codes -- data Bit = L | R deriving (Eq, Show) type Hcode = [Bit] -- Huffman coding tree characters at leaf nodes, plus frequencies frequencies as well at internal nodes -- data Tree = Leaf Char Int | Node Int Tree Tree Assume that codes are kept in table (rather than read off a tree). -- table of codes -- type Table = [(Char, Hcode)]

Encoding -- encode a message according to code table encode each character and concatenate -- codeMessage :: Table -> [Char] -> Hcode codeMessage tbl = concat. map (lookupTable tbl) -- lookup the code for a character in code table -- lookupTable :: Table -> Char -> Hcode lookupTable [] c = error lookupTable lookupTable ((ch,code):tbl) c | ch == c = code | otherwise = lookupTable tbl c

Decoding -- decode a message according to code tree if at a leaf node, then character is decoded, start again at root if at an internal node, then follow sub-tree according to next code bit -- decode :: Tree -> Hcode -> [Char] decode tr = decodetree tr where decodetree (Node f t1 t2) (L:rest) = decodetree t1 rest decodetree (Node f t1 t2) (R:rest) = decodetree t2 rest decodetree (Leaf ch f) rest = ch:(decodetree tr rest)

Example codetree = Node 3 (Leaf a 0) (Node 3 (Leaf b 1) (Leaf t 2)) -- assume a is most frequent, denoted by smallest number -- message = [R,L,L,R,R,R,R,L,R,R] decode codetree message => decodetree Node 3 t1 (Node 3..) R: [L,L,R,R,R,R,L,R,R] => decodetree (Node 3 (Leaf b 1) (Leaf t 2)) L: [L,R,R,R,R,L,R,R] => decodetree ( Leaf b 1) L:[R,R,R,R,L,R,R] => b : decodetree Node 3 (Leaf a 0) (Node 3..) L: [R,R,R,R,L,R,R] => b: decodetree (Leaf a 0)) [R,R,R,R,L,R,R] => b : a: decodetree Node 3 (Leaf a 0) (Node 3..) [R,R,R,R,L,R,R]

We still have to make: the code tree the code table (Next lecture!)