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

1 files changed, 2 insertions, 2 deletions

diff --git a/Git/CatFile.hs b/Git/CatFile.hs
index bd86ff326..f7438b410 100644
--- a/Git/CatFile.hs
+++ b/Git/CatFile.hs
@@ -18,7 +18,6 @@ module Git.CatFile (
 import System.IO
 import qualified Data.ByteString as S
 import qualified Data.ByteString.Lazy as L
-import Data.Digest.Pure.SHA
 import Data.Char
 import System.Process (std_out, std_err)
 import Numeric
@@ -31,6 +30,7 @@ import Git.Command
 import Git.Types
 import Git.FilePath
 import qualified Utility.CoProcess as CoProcess
+import Utility.Hash
 
 data CatFileHandle = CatFileHandle CoProcess.CoProcessHandle Repo
 
@@ -103,7 +103,7 @@ catObjectDetails (CatFileHandle hdl repo) object = CoProcess.query hdl send rece
 				}
 		fileEncoding h
 		content <- L.hGetContents h
-		let sha = (\s -> length s `seq` s) (showDigest $ sha1 content)
+		let sha = (\s -> length s `seq` s) (show $ sha1 content)
 		ok <- checkSuccessProcess pid
 		return $ if ok
 			then Just (content, Ref sha)