Builder for Data.Fixed?

[matthewbauer]

I wonder if there is a fast ByteString Builder for numbers from Data.Fixed?

I've written one myself, but don't think it's quite as fast as it could be:

-- | Encode a 'Fixed' resolution number using ASCII digits.
fixedDec :: HasResolution a => Fixed a -> BB.Builder
fixedDec fa@(MkFixed a) | a < 0 = P.primFixed P.char8 '-' <> fixedDec (asTypeOf (MkFixed (negate a)) fa)
fixedDec fa@(MkFixed a) = integerDec i <> P.primFixed P.char8 '.' <> stimesMonoid zeros (P.primFixed P.char8 '0') <> integerDec d
  where
    (i, d) = a `quotRem` resolution fa
    zeros :: Integer = if d == 0 then 0 else ceiling (logBase 10 (fromIntegral (resolution fa) / fromIntegral d / 10) :: Double)

Calculating logBase 10 gets expensive, so it doesn't seem to perform well with lots of Fixed numbers.

Ideally if there was an efficient builder of fixed, it could be included in bytestring. Anyone have ideas of how to make it more efficient?

[matthewbauer]

I was able to get some pretty good results (sub 1ms for 10,000 calls based on benchmarks) with this version:

-- | Encode a 'Fixed' resolution number using ASCII digits.
fixedDec :: HasResolution a => Fixed a -> Builder
fixedDec fa@(MkFixed a) | a < 0 = P.primFixed P.char8 '-' <> fixedDec (asTypeOf (MkFixed (negate a)) fa)
fixedDec fa@(MkFixed a) = integerDec i <> P.primFixed P.char8 '.' <> P.primMapListFixed P.char8 (List.genericReplicate zeros '0') <> integerDec d
  where
    !(i, d) = a `quotRem` resolution fa
    zeros :: Integer = if d == 0 then 0 else ceiling (logBase 10 (fromIntegral (resolution fa) / fromIntegral d / 10) :: Double)

You can about half (like 500μs) that by approximating Fixed to "Word64" instead of Integer:

-- | Encode a 'Fixed' resolution number using ASCII digits, approximated to Word64.
fixedDecWord64 :: HasResolution a => Fixed a -> Builder
fixedDecWord64 fa@(MkFixed a) | a < 0 = P.primFixed P.char8 '-' <> fixedDecWord64 (asTypeOf (MkFixed (negate a)) fa)
fixedDecWord64 fa@(MkFixed a) = word64Dec i <> P.primFixed P.char8 '.' <> P.primMapListFixed P.char8 (List.replicate zeros '0') <> word64Dec d
  where
    !(i, d) = fromIntegral a `quotRem` fromIntegral (resolution fa)
    zeros :: Int = if d == 0 then 0 else ceiling (logBase 10 (fromIntegral (resolution fa) / fromIntegral d / 10) :: Double)

I have a branch available here: master...matthewbauer:bytestring:fixedDec