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These schemes are useful for transmitting numerical data +in a more compact form than would be available from simple big- or little-endian +encoding of the values. +-} + +module Data.Attoparsec.VarWord + ( varWordBe, denseVarWordBe + , varWordLe + ) where + +import Data.Attoparsec.ByteString (Parser) +import Data.Bits (Bits) + +import Data.Attoparsec.VarWord.Internal (varWordBe', varWordLe') + + +-- | Decode a big-endian (most significant byte first), variable-length encoded +-- value. +-- +-- > +-- Keep reading +-- > | +-- Keep reading +-- > | | +-- Stop +-- > v v v +-- > 10000001 10010000 00100000 +-- > |<--->| |<--->| |<--->| +-- > Data(A) Data(B) Data(C) +-- > +-- > == 1 0010000 0100000 == 18464 +-- > (A) (B) (C) +-- +-- >>> parseOnly varWordBe "\129\144\32" +-- Right 18464 +-- +varWordBe :: (Bits a, Integral a) => Parser a +varWordBe = varWordBe' 0 0 + +-- | Decode a big-endian (least significant byte first), variable length +-- encoded value where the continuation byte is used to carry data also. +-- +-- > +-- Keep reading +-- > | +-- Keep reading +-- > | | +-- Stop +-- > v v v +-- > 10000001 10010000 00100000 +-- > |<--->| |<--->| |<--->| +-- > Data(A) Data(B) Data(C) +-- > +-- > == 10 0010001 0100000 == 34976 +-- > (A+1) (B+1) (C) +-- +-- >>> parseOnly denseVarWordBeP "\129\144\32" +-- Right 34976 +-- +denseVarWordBe :: (Bits a, Integral a) => Parser a +denseVarWordBe = varWordBe' 1 0 + +-- | Decode a little-endian (least significant byte first), variable length +-- encoded value. +-- +-- > +-- Keep reading +-- > | +-- Keep reading +-- > | | +-- Stop +-- > v v v +-- > 10000001 10010000 00100000 +-- > |<--->| |<--->| |<--->| +-- > Data(A) Data(B) Data(C) +-- > +-- > == 100000 0010000 0000001 == 526337 +-- > (C) (B) (A) +-- +-- >>> parseOnly varWordLe "\129\144\32" +-- Right 526337 +-- +varWordLe :: (Bits a, Integral a) => Parser a +varWordLe = varWordLe' 0 0 diff --git a/src/Data/Attoparsec/VarWord/Internal.hs b/src/Data/Attoparsec/VarWord/Internal.hs new file mode 100644 --- /dev/null +++ b/src/Data/Attoparsec/VarWord/Internal.hs @@ -0,0 +1,40 @@ +{- | +Internal implementation of variable-length decoding. This API is not guaranteed +to be stable. +-} +module Data.Attoparsec.VarWord.Internal + ( varWordBe', varWordLe' + ) where + +import Data.Attoparsec.ByteString (Parser, anyWord8) +import Data.Bits (Bits, shiftL, testBit, (.&.), (.|.)) +import Data.Word + + +msb :: Word8 -> Bool +msb = flip testBit 7 + +lsbs :: Word8 -> Word8 +lsbs = (.&. 0b01111111) + + +-- | Internal implementation of big-endian variable-length decoding, exposing +-- the accumulator and how much to increment each continuation chunk. +varWordBe' :: (Bits a, Integral a) => a -> a -> Parser a +varWordBe' contInc acc = do + byte <- anyWord8 + let acc' = shiftL acc 7 .|. fromIntegral (lsbs byte) + if msb byte + then varWordBe' contInc (acc' + contInc) + else return acc' + + +-- | Internal implementation of little-endian variable-length decoding, exposing +-- the accumulator and the shift for each chunk. +varWordLe' :: (Bits a, Integral a) => Int -> a -> Parser a +varWordLe' shiftBy acc = do + byte <- anyWord8 + let acc' = acc .|. shiftL (fromIntegral $ lsbs byte) shiftBy + if msb byte + then varWordLe' (shiftBy + 7) acc' + else return acc' diff --git a/test/Data/Attoparsec/VarWordSpec.hs b/test/Data/Attoparsec/VarWordSpec.hs new file mode 100644 --- /dev/null +++ b/test/Data/Attoparsec/VarWordSpec.hs @@ -0,0 +1,37 @@ +module Data.Attoparsec.VarWordSpec where + +import Test.Hspec +import Test.QuickCheck +import Data.Attoparsec.ByteString (Parser, parseOnly) +import qualified Data.ByteString as BS +import qualified Data.ByteString.Lazy as LBS +import Data.Word + +import Data.ByteString.Builder (Builder) +import qualified Data.ByteString.Builder as Builder +import qualified Data.ByteString.Builder.VarWord as Builder + +import Data.Attoparsec.VarWord (varWordBe, denseVarWordBe, varWordLe) + +spec :: Spec +spec = do + varWordSpec "Big-endian chunked number parser" + varWordBe Builder.varWordBe 18464 + varWordSpec "Big-endian dense chunked number parser" + denseVarWordBe Builder.denseVarWordBe 34976 + varWordSpec "Little-endian chunked number parser" + varWordLe Builder.varWordLe 526337 + + +varWordSpec :: String -> Parser Word32 -> (Word32 -> Builder) -> Word32 -> Spec +varWordSpec s p b expected = describe s $ do + it "should parse a known bytestring" $ do + parseOnly p (BS.pack [0b10000001, 0b10010000, 0b00100000]) + `shouldBe` Right expected + it "should parse any valid single byte" $ forAll (choose (0, 127)) $ \word8 -> + parseOnly p (BS.singleton word8) `shouldBe` Right (fromIntegral word8) + it "should roundtrip any valid bytestring" $ property $ \word32 -> + let + encoded = LBS.toStrict $ Builder.toLazyByteString $ b word32 + in + parseOnly p encoded `shouldBe` Right word32 diff --git a/test/Spec.hs b/test/Spec.hs new file mode 100644 --- /dev/null +++ b/test/Spec.hs @@ -0,0 +1,1 @@ +{-# OPTIONS_GHC -F -pgmF hspec-discover #-}