shithub: MicroHs

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Make it a little safer.

--- a/Tools/Compress.hs

+++ b/Tools/Compress.hs

@@ -5,8 +5,19 @@

 import System.IO

 --import Debug.Trace

+--

+-- A simple LZW compressor

+-- It uses a dictionary of maximum size 4096.

+-- The 12 bit code words are encoded with 2 code words in 3 bytes.

+-- It can only compress input of printable ASCII + '\n'

+

 type Table = M.Map [Char] Int

+-- Don't change this lightly.

+-- It needs to be coordinated with the decompressor transducer in eval.c

+maxDict :: Int

+maxDict = 4096

+

 toChar :: Int -> Char

 toChar i = chr (i + 32)

@@ -21,7 +32,7 @@

 compress t (c:cs) p =

   let p' = p ++ [c]

       s = M.size t

-      t' = if s < 4096 then M.insertBy compareString p' s t else t

+      t' = if s < maxDict then M.insertBy compareString p' s t else t

   in

 --      trace ("compress " ++ showString p') $

 --      trace (showList (showPair showString showInt) (M.toList t)) $

@@ -47,9 +58,14 @@

       b3 = i `quot` (256*256)

   in  b1 : b2 : b3 : toBytes is

+bad :: Char -> Bool

+bad c = not (isPrint c || eqChar c '\n')

+

 main :: IO ()

 main = do

   f <- hGetContents stdin

+  when (any bad f) $

+    error "Non-printable ASCII in input"

   let bs = compress initTable f []

   hSetBinaryMode stdout True

   putStr $ 'Z' : (map chr $ toBytes bs)

-- 

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