Use cryptohash rather than SHA for hashing.

This is a massive win on OSX, which doesn't have a sha256sum normally.

Only use external hash commands when the file is > 1 mb,
since cryptohash is quite close to them in speed.

SHA is still used to calculate HMACs. I don't quite understand
cryptohash's API for those.

Used the following benchmark to arrive at the 1 mb number.

1 mb file:

benchmarking sha256/internal
mean: 13.86696 ms, lb 13.83010 ms, ub 13.93453 ms, ci 0.950
std dev: 249.3235 us, lb 162.0448 us, ub 458.1744 us, ci 0.950
found 5 outliers among 100 samples (5.0%)
  4 (4.0%) high mild
  1 (1.0%) high severe
variance introduced by outliers: 10.415%
variance is moderately inflated by outliers

benchmarking sha256/external
mean: 14.20670 ms, lb 14.17237 ms, ub 14.27004 ms, ci 0.950
std dev: 230.5448 us, lb 150.7310 us, ub 427.6068 us, ci 0.950
found 3 outliers among 100 samples (3.0%)
  2 (2.0%) high mild
  1 (1.0%) high severe

2 mb file:

benchmarking sha256/internal
mean: 26.44270 ms, lb 26.23701 ms, ub 26.63414 ms, ci 0.950
std dev: 1.012303 ms, lb 925.8921 us, ub 1.122267 ms, ci 0.950
variance introduced by outliers: 35.540%
variance is moderately inflated by outliers

benchmarking sha256/external
mean: 26.84521 ms, lb 26.77644 ms, ub 26.91433 ms, ci 0.950
std dev: 347.7867 us, lb 210.6283 us, ub 571.3351 us, ci 0.950
found 6 outliers among 100 samples (6.0%)

import Crypto.Hash
import Data.ByteString.Lazy as L
import Criterion.Main
import Common

testfile :: FilePath
testfile = "/run/shm/data" -- on ram disk

main = defaultMain
        [ bgroup "sha256"
                [ bench "internal" $ whnfIO internal
                , bench "external" $ whnfIO external
                ]
        ]

sha256 :: L.ByteString -> Digest SHA256
sha256 = hashlazy

internal :: IO String
internal = show . sha256 <$> L.readFile testfile

external :: IO String
external = do
	s <- readProcess "sha256sum" [testfile]
        return $ fst $ separate (== ' ') s

Backend/SHA.hs

@@ -1,6 +1,6 @@
 {- git-annex SHA backends
  -
- - Copyright 2011,2012 Joey Hess <joey@kitenet.net>
+ - Copyright 2011-2013 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -12,10 +12,10 @@ import qualified Annex
 import Types.Backend
 import Types.Key
 import Types.KeySource
+import Utility.Hash
 import Utility.ExternalSHA
 
 import qualified Build.SysConfig as SysConfig
-import Data.Digest.Pure.SHA
 import qualified Data.ByteString.Lazy as L
 import Data.Char
 
@@ -70,12 +70,14 @@ shaCommand shasize filesize
 	| shasize == 512 = use SysConfig.sha512 sha512
 	| otherwise = error $ "bad sha size " ++ show shasize
   where
-	use Nothing sha = Left $ showDigest . sha
-	use (Just c) sha
-		{- use builtin, but slower sha for small files
-		 - benchmarking indicates it's faster up to
-		 - and slightly beyond 50 kb files -}
-		| filesize < 51200 = use Nothing sha
+	use Nothing hasher = Left $ show . hasher
+	use (Just c) hasher
+		{- Use builtin, but slightly slower hashing for
+		 - smallish files. Cryptohash benchmarks 90 to 101%
+		 - faster than external hashers, depending on the hash
+		 - and system. So there is no point forking an external
+		 - process unless the file is large. -}
+		| filesize < 1048576 = use Nothing hasher
 		| otherwise = Right c
 
 {- A key is a checksum of its contents. -}