feat: Extracting word types

java-classfile.cabal

@@ -28,6 +28,7 @@ library
       Language.Java.Classfile
       Language.Java.Classfile.Extract
       Language.Java.Classfile.Extractable
+      Language.Java.Classfile.FromBigEndian
       Language.Java.Classfile.Version
   other-modules:
       Paths_java_classfile

src/Language/Java/Classfile/Extract.hs

@@ -1,8 +1,9 @@
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE InstanceSigs #-}
-module Language.Java.Classfile.Extract (Extract()) where
+module Language.Java.Classfile.Extract (Extract(), bytes) where
 
 import Data.ByteString.Lazy (ByteString)
+import qualified Data.ByteString.Lazy as ByteString
 
 data Extract a = Extract (Continuation a)
 
@@ -28,6 +29,16 @@ instance Applicative Extract where
     Fail -> Fail
 
 
+-- | Get a specified count of bytes. Fail if there are not enough bytes available.
+
+bytes :: Word -> Extract ByteString
+bytes count = Extract $ \ input -> let
+  count' = fromIntegral count
+  (bs, rest) = ByteString.splitAt count' input
+  in if ByteString.length bs /= count'
+    then Fail
+    else Done rest bs
+
 
 {- It seems I cannot define a lawful monad instance
 instance Monad Extract where

src/Language/Java/Classfile/Extractable.hs

@@ -1,4 +1,11 @@
-module M () where
+{-# LANGUAGE InstanceSigs #-}
+module Language.Java.Classfile.Extractable () where
+import Language.Java.Classfile.Extract (Extract, bytes)
+import Data.Word (Word8, Word16, Word32)
+import qualified Data.ByteString.Lazy as ByteString
+import Data.ByteString.Lazy (ByteString)
+import Data.Bits (Bits(shiftL, (.|.)))
+import GHC.Generics (U1 (U1))
 
 
 class Extractable a where
@@ -6,4 +13,19 @@ class Extractable a where
 
 instance Extractable Word8 where
   extract :: Extract Word8
-  extract = _
+  extract = ByteString.head <$> bytes 1
+
+instance Extractable Word16 where
+  extract :: Extract Word16
+  extract = build <$> bytes 2
+
+instance Extractable Word32 where
+  extract :: Extract Word32
+  extract = build <$> bytes 4
+
+build :: (Bits a, Num a) => ByteString -> a
+build = ByteString.foldl' shiftOr 0
+
+shiftOr :: (Bits a1, Num a1) => a1 -> Word8 -> a1
+shiftOr word appendix = shiftL word 8 .|. fromIntegral appendix
+

src/Language/Java/Classfile/FromBigEndian.hs

@@ -0,0 +1,21 @@
+{-# LANGUAGE InstanceSigs #-}
+module Language.Java.Classfile.FromBigEndian () where
+import Data.Word (Word16, byteSwap16, Word32, byteSwap32)
+import GHC.ByteOrder (ByteOrder(..))
+import qualified GHC.ByteOrder as GHC
+
+class FromBigEndian a where
+  fromBigEndian :: a -> a
+
+instance FromBigEndian Word16 where
+  fromBigEndian :: Word16 -> Word16
+  fromBigEndian = case GHC.targetByteOrder of
+    LittleEndian -> byteSwap16
+    BigEndian    -> id
+
+instance FromBigEndian Word32 where
+  fromBigEndian :: Word32 -> Word32
+  fromBigEndian = case GHC.targetByteOrder of
+    LittleEndian -> byteSwap32
+    BigEndian    -> id
+