Initial data commit

Task/LZW-compression/Haskell/lzw-compression.hs

@@ -0,0 +1,38 @@
+import Data.List (elemIndex, tails)
+import Data.Maybe (fromJust)
+
+doLZW :: Eq a => [a] -> [a] -> [Int]
+doLZW _ [] = []
+doLZW as (x:xs) = lzw (return <$> as) [x] xs
+  where
+    lzw a w [] = [fromJust $ elemIndex w a]
+    lzw a w (x:xs)
+      | w_ `elem` a = lzw a w_ xs
+      | otherwise = fromJust (elemIndex w a) : lzw (a ++ [w_]) [x] xs
+      where
+        w_ = w ++ [x]
+
+undoLZW :: [a] -> [Int] -> [a]
+undoLZW _ [] = []
+undoLZW a cs =
+  cs >>=
+  (!!)
+    (foldl
+       ((.) <$> (++) <*>
+        (\x xs -> return (((++) <$> head <*> take 1 . last) ((x !!) <$> xs))))
+       (return <$> a)
+       (take2 cs))
+
+take2 :: [a] -> [[a]]
+take2 xs = filter ((2 ==) . length) (take 2 <$> tails xs)
+
+main :: IO ()
+main = do
+  print $ doLZW ['\0' .. '\255'] "TOBEORNOTTOBEORTOBEORNOT"
+  print $
+    undoLZW
+      ['\0' .. '\255']
+      [84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]
+  print $
+    ((==) <*> ((.) <$> undoLZW <*> doLZW) ['\NUL' .. '\255'])
+      "TOBEORNOTTOBEORTOBEORNOT"