From 3d1deaacc1d90762d771a8620d19d1618df24dc5 Mon Sep 17 00:00:00 2001
From: Jack Wines <jack@winesj.com>
Date: Mon, 15 Apr 2024 01:51:35 -0700
Subject: [PATCH] decompression up to Maybe [Word]

not yet to ByteString though
---
 README.md   |  2 +-
 src/Main.hs | 33 +++++++++++++++++++++++++++------
 2 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/README.md b/README.md
index c6866cb..7bfdd5f 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-Compresses files using [Huffman coding](https://en.wikipedia.org/wiki/Huffman_coding). Please note that decompression isn't implemented yet.
+Compresses files using [Huffman coding](https://en.wikipedia.org/wiki/Huffman_coding). Please note that decompression isn't fully implemented yet. It is however implemented to the point where we know compression works correctly.
 
 To run with [Nix](nixos.org):
 ``` sh
diff --git a/src/Main.hs b/src/Main.hs
index 291a35a..9b20368 100644
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -39,6 +39,23 @@ findTreeDirections (Node{..}) = M.union (rec' L left) (rec' R right)
   rec' :: TreeDir -> HuffmanTree a -> M.Map a [TreeDir]
   rec' dir = M.map (dir :) . findTreeDirections
 
+decompress :: forall a. (Ord a, Integral a, B.FiniteBits a) => Maybe ([TreeDir], HuffmanTree a) -> Maybe [a]
+decompress Nothing = Just []
+decompress (Just (treeDirs, tree)) = decompress' treeDirs
+  where
+
+    decompress' :: [TreeDir] -> Maybe [a]
+    decompress' [] = Just []
+    decompress' xs = case nextLeaf xs tree of
+      Nothing -> Nothing
+      Just (x, remainingDirs) -> (x :) <$> decompress' remainingDirs
+
+    nextLeaf :: [TreeDir] -> HuffmanTree a -> Maybe (a, [TreeDir])
+    nextLeaf xs (Leaf a) = Just (a, xs)
+    nextLeaf [] _ = Nothing
+    nextLeaf (L:xs) (Node {..}) = nextLeaf xs left
+    nextLeaf (R:xs) (Node {..}) = nextLeaf xs right
+
 compress :: forall a. (Ord a, Integral a, B.FiniteBits a) => BS.ByteString -> Maybe ([TreeDir], HuffmanTree a)
 compress bs =
   liftA2 (,) treeDirections mergedHuffmanTrees
@@ -57,7 +74,10 @@ compress bs =
 
   dividedByteString = toBitsList bs
 
-encodeCompressed :: (C.Serialize a) => Maybe ([TreeDir], HuffmanTree a) -> BS.ByteString
+testCompression :: forall a. (Ord a, Eq a, Integral a, B.FiniteBits a) => D.Proxy a -> BS.ByteString -> Bool
+testCompression _ bs = ((Just . toBitsList $ bs) :: Maybe [a]) == (decompress . compress $ bs)
+
+encodeCompressed :: C.Serialize a => Maybe ([TreeDir], HuffmanTree a) -> BS.ByteString
 encodeCompressed = C.encode . fmap (Bi.first encodeTreeDirs)
  where
   encodeTreeDirs = BV.cloneToByteString . V.fromList . map (BV.Bit . (== R))
@@ -120,8 +140,9 @@ main :: IO ()
 main = do
   [filePath] <- SE.getArgs
   f <- BS.readFile filePath
-  TIO.putStrLn "compression ratios:"
-  print . ("Word64",) . compressionRatioFor (D.Proxy :: D.Proxy Word64) $ f
-  print . ("Word32",) . compressionRatioFor (D.Proxy :: D.Proxy Word32) $ f
-  print . ("Word16",) . compressionRatioFor (D.Proxy :: D.Proxy Word16) $ f
-  print . ("Word8",) . compressionRatioFor (D.Proxy :: D.Proxy Word8) $ f
+  -- TIO.putStrLn "compression ratios:"
+  print . ("Word64",) . testCompression (D.Proxy :: D.Proxy Word16) $ f
+  -- print . ("Word64",) . compressionRatioFor (D.Proxy :: D.Proxy Word64) $ f
+  -- print . ("Word32",) . compressionRatioFor (D.Proxy :: D.Proxy Word32) $ f
+  -- print . ("Word16",) . compressionRatioFor (D.Proxy :: D.Proxy Word16) $ f
+  -- print . ("Word8",) . compressionRatioFor (D.Proxy :: D.Proxy Word8) $ f