I needed to be able to give the hex representation of a SHA512 hash. Maybe I just didn't look hard enough, but I could find any functions on Hackage to do it. So I wrote an implementation using unfoldrN. It's definitely fast enough for my purposes, but I'm wondering if anyone knows of a faster approach.
I've put my implementation up on Github as a gist: https://gist.github.com/2356925 . The file also includes a simple implementation based on Numeric.showHex, a QuickCheck test and a criterion benchmark. My current results of the simple version versus the unfoldrN version are:
benchmarking simple
mean: 4.677296 ms, lb 4.656011 ms, ub 4.696684 ms, ci 0.950
std dev: 104.2791 us, lb 87.77023 us, ub 128.1627 us, ci 0.950
found 5 outliers among 100 samples (5.0%)
4 (4.0%) low mild
variance introduced by outliers: 15.195%
variance is moderately inflated by outliers
benchmarking unfoldrN_MS1
mean: 370.0101 us, lb 365.9819 us, ub 373.8619 us, ci 0.950
std dev: 20.17016 us, lb 16.92772 us, ub 24.08982 us, ci 0.950
found 14 outliers among 100 samples (14.0%)
7 (7.0%) low mild
7 (7.0%) high mild
variance introduced by outliers: 52.467%
variance is severely inflated by outliers
Anyone want to take a stab at improving it?
Going lower-level,
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)
I could reduce the conversion time on my box by another factor of about 3.5. Making sample a bit longer (25000), I got
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
For the original 500 long sample, it was
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