问题 有效地将ByteString转换为十六进制表示

走低级，

import Data.ByteString.Internal
import Foreign.Ptr
import Foreign.Storable
import qualified Data.ByteString as B
import Data.ByteString.Unsafe
import Data.Bits
import Data.Word

maxLen :: Int
maxLen = maxBound `quot` 2

hexDig :: Word8 -> Word8
hexDig d
    | d < 10    = d + 48
    | otherwise = d + 87

toHex :: ByteString -> ByteString
toHex bs
    | len > maxLen = error "too long to convert"
    | otherwise    = unsafeCreate nl (go 0)
      where
        len = B.length bs
        nl  = 2*len
        go i p
            | i == len  = return ()
            | otherwise = case unsafeIndex bs i of
                            w -> do poke p (hexDig $ w `shiftR` 4)
                                    poke (p `plusPtr` 1) (hexDig $ w .&. 0xF)
                                    go (i+1) (p `plusPtr` 2)

我可以将我的盒子上的转换时间减少大约3.5倍。制造 sample 我得到了更长的时间（25000）

benchmarking simple
mean: 13.76532 ms, lb 13.64184 ms, ub 13.88680 ms, ci 0.950
std dev: 633.2413 us, lb 582.6342 us, ub 687.9701 us, ci 0.950
variance introduced by outliers: 44.438%
variance is moderately inflated by outliers

benchmarking unfoldrN_MS1
mean: 430.5705 us, lb 424.9206 us, ub 438.5689 us, ci 0.950
std dev: 33.85429 us, lb 26.25623 us, ub 45.74915 us, ci 0.950
found 4 outliers among 100 samples (4.0%)
  3 (3.0%) high mild
  1 (1.0%) high severe
variance introduced by outliers: 69.726%
variance is severely inflated by outliers

benchmarking LowHex
mean: 123.6000 us, lb 123.0551 us, ub 124.7084 us, ci 0.950
std dev: 3.837497 us, lb 1.869370 us, ub 6.470112 us, ci 0.950
found 6 outliers among 100 samples (6.0%)
  4 (4.0%) high mild
  2 (2.0%) high severe
variance introduced by outliers: 25.818%
variance is moderately inflated by outliers

对于原来的500长 sample， 它是

benchmarking simple
mean: 2.603306 ms, lb 2.583054 ms, ub 2.629212 ms, ci 0.950
std dev: 116.5341 us, lb 81.61409 us, ub 191.3293 us, ci 0.950
found 7 outliers among 100 samples (7.0%)
  2 (2.0%) low severe
  3 (3.0%) low mild
  1 (1.0%) high severe
variance introduced by outliers: 42.490%
variance is moderately inflated by outliers

benchmarking unfoldrN_MS1
mean: 83.19349 us, lb 82.88474 us, ub 83.58283 us, ci 0.950
std dev: 1.771460 us, lb 1.486104 us, ub 2.174729 us, ci 0.950
found 14 outliers among 100 samples (14.0%)
  12 (12.0%) high mild
  2 (2.0%) high severe
variance introduced by outliers: 14.225%
variance is moderately inflated by outliers

benchmarking LowHex
mean: 24.50564 us, lb 24.41683 us, ub 24.61241 us, ci 0.950
std dev: 497.1908 ns, lb 415.6366 ns, ub 609.7594 ns, ci 0.950
found 5 outliers among 100 samples (5.0%)
  5 (5.0%) high mild
variance introduced by outliers: 13.256%
variance is moderately inflated by outliers