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|
-- |
-- Module : Data.Double.Conversion.ByteString
-- Copyright : (c) 2011 MailRank, Inc.
--
-- License : BSD-style
-- Maintainer : bos@serpentine.com
-- Stability : experimental
-- Portability : GHC
--
-- Fast, efficient support for converting between double precision
-- floating point values and text.
--
-- Although about 15 times faster than plain 'show', these functions
-- are /slower/ than their 'Text' counterparts, at roughly half the
-- speed. (This seems to be due to the cost of allocating
-- 'ByteString' values via @malloc@.)
module Data.Double.Conversion.ByteString
(
toExponential
, toFixed
, toPrecision
, toShortest
) where
import Control.Monad (when)
import Foreign.ForeignPtr (withForeignPtr)
import Data.Double.Conversion.FFI
import Data.Word (Word8)
import Data.ByteString.Internal (ByteString(..), mallocByteString)
import Foreign.C.Types (CDouble, CInt)
import Foreign.Ptr (Ptr)
import System.IO.Unsafe (unsafePerformIO)
-- | Compute a representation in exponential format with the requested
-- number of digits after the decimal point. The last emitted digit is
-- rounded. If -1 digits are requested, then the shortest exponential
-- representation is computed.
toExponential :: Int -> Double -> ByteString
toExponential ndigits = convert "toExponential" len $ \val mba ->
c_ToExponential val mba (fromIntegral ndigits)
where len = c_ToExponentialLength
{-# NOINLINE len #-}
-- | Compute a decimal representation with a fixed number of digits
-- after the decimal point. The last emitted digit is rounded.
toFixed :: Int -> Double -> ByteString
toFixed ndigits = convert "toFixed" len $ \val mba ->
c_ToFixed val mba (fromIntegral ndigits)
where len = c_ToFixedLength
{-# NOINLINE len #-}
-- | Compute the shortest string of digits that correctly represent
-- the input number.
toShortest :: Double -> ByteString
toShortest = convert "toShortest" len c_ToShortest
where len = c_ToShortestLength
{-# NOINLINE len #-}
-- | Compute @precision@ leading digits of the given value either in
-- exponential or decimal format. The last computed digit is rounded.
toPrecision :: Int -> Double -> ByteString
toPrecision ndigits = convert "toPrecision" len $ \val mba ->
c_ToPrecision val mba (fromIntegral ndigits)
where len = c_ToPrecisionLength
{-# NOINLINE len #-}
convert :: String -> CInt -> (CDouble -> Ptr Word8 -> IO CInt)
-> Double -> ByteString
convert func len act val = unsafePerformIO $ do
fp <- mallocByteString (fromIntegral len)
size <- withForeignPtr fp $ act (realToFrac val)
when (size == -1) .
fail $ "Data.Double.Conversion.ByteString." ++ func ++
": conversion failed (invalid precision requested)"
return $ PS fp 0 (fromIntegral size)
|
