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Talk:Toy compression implementations

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Revision as of 13:34, 9 March 2007 by MathematicalOrchid (Talk | contribs)

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Much kudos for fixing the underflow error. The new LZW implementation is much smaller, but... how in the name of God does it actually work? o_O MathematicalOrchid 11:36, 9 March 2007 (UTC)

To understand it I rewrote it a bit:

encode_LZW :: (Eq t) => [t] -> [t] -> [Int]
encode_LZW alphabet = work (map (:[]) alphabet) where
  work table []  = []
  work table lst = index : work table' rest
    where (tok, rest) = last . takeWhile ((`elem` table) . fst) . tail $ zip (inits lst) (tails lst)
          index = fromJust (elemIndex tok table)
          table' = table ++ [tok']
          tok' = tok ++ [head rest]

The idea of the the table, which is the 1st argument to 'work', is that some prefix of the input is already in the table.

(encode_LZW chars) uses 'chars' to make the initial table for the 'work' function by turning the list of characters into a list of length 1 strings.

The
where (tok,rst)
definition can be read right to left:
  • The
    zip (inits lst) (tails lst)
    computes every possible way to split
    lst
    input into a prefix and suffix, in increasing length of prefix.
  • The
    tail
    function just drops the head because it doesn't want to consider the length 0 prefix
  • takeWhile
    applies the predicate
    (`elem` table)
    to the prefix. This will always succeed on the length 1 prefix, and may find longer prefixes in the table.
  • The
    last
    function take the last prefix in the table, which will always be the longest such prefix
  • tok
    is this prefix, and
    rest
    is the remaining suffix to process.


Wow... a most ingenious (and inefficient) approach! Well, now it makes sense anyway. MathematicalOrchid 13:34, 9 March 2007 (UTC)