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2011-12-03T19:28:54.740
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Following the suggestion of acfoltzer (cereal source code), and Daniel Fischer (unsafeCreate), I wrote the code below that works well for my use case, and is fast too: <pre><code>{-#LANGUAGE MagicHash #-} import Data.ByteString (pack, unpack) import Data.ByteString.Internal (unsafeCreate,ByteString) import Data.Bits (shiftR) import GHC.Int (Int64) import GHC.Prim import GHC.Types import GHC.Word import Unsafe.Coerce import Criterion.Main import Foreign -- | Write a Word64 in little endian format putWord64le :: Word64 -> Ptr Word8 -> IO() putWord64le w p = do poke p (fromIntegral (w) :: Word8) poke (p `plusPtr` 1) (fromIntegral (shiftR w 8) :: Word8) poke (p `plusPtr` 2) (fromIntegral (shiftR w 16) :: Word8) poke (p `plusPtr` 3) (fromIntegral (shiftR w 24) :: Word8) poke (p `plusPtr` 4) (fromIntegral (shiftR w 32) :: Word8) poke (p `plusPtr` 5) (fromIntegral (shiftR w 40) :: Word8) poke (p `plusPtr` 6) (fromIntegral (shiftR w 48) :: Word8) poke (p `plusPtr` 7) (fromIntegral (shiftR w 56) :: Word8) {-# INLINE putWord64le #-} encodeDouble :: Double -> ByteString encodeDouble x = unsafeCreate 8 (putWord64le $ unsafeCoerce x) main :: IO () main = defaultMain [ bgroup "encodeDouble" [ bench "78901.234" $ whnf encodeDouble 78901.234 , bench "789.01" $ whnf encodeDouble 789.01 ] ] </code></pre> Criterion output (truncated): <pre><code>estimating cost of a clock call... mean is 46.80361 ns (35 iterations) found 5 outliers among 35 samples (14.3%) 3 (8.6%) high mild 2 (5.7%) high severe benchmarking encodeDouble/78901.234 mean: 18.80689 ns, lb 18.73805 ns, ub 18.97247 ns, ci 0.950 std dev: 516.7499 ps, lb 244.8588 ps, ub 1.043685 ns, ci 0.950 benchmarking encodeDouble/789.01 mean: 18.96963 ns, lb 18.90986 ns, ub 19.06127 ns, ci 0.950 std dev: 374.2191 ps, lb 275.3313 ps, ub 614.4281 ps, ci 0.950 </code></pre> From ~220ns down to ~19ns, nice! I didn't do anything fancy in compilation. Just -O flag will do in GHC7 (Mac, x86_64). Now, trying to figure out how to do it fast with list of doubles!
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