4 ns optimisation of repeated calls to hasDifference on the same Differences

I want this as fast as possible, so it can be added to code paths without
slowing them down.

Avoid the set lookup, and rely on laziness,
drops runtime from 14.37 ns to 11.03 ns according to this criterion benchmark:

import Criterion.Main
import qualified Types.Difference as New
import qualified Types.DifferenceOld as Old

main :: IO ()
main = defaultMain
	[ bgroup "hasDifference"
		[ bench "new" $ whnf (New.hasDifference New.OneLevelObjectHash) new
		, bench "old" $ whnf (Old.hasDifference Old.OneLevelObjectHash) old
		]
	]
  where
	s = "fromList [ObjectHashLower, OneLevelObjectHash, OneLevelBranchHash]"
	new = New.readDifferences s
	old = Old.readDifferences s

A little bit of added boilerplate, but I suppose it's worth it to not
need to worry about set lookup overhead. Note that adding more differences
would slow down the old implementation; the new implementation will run
the same speed.

1 files changed, 2 insertions, 2 deletions

diff --git a/Logs/Difference.hs b/Logs/Difference.hs
index 8d37a09c4..fd93fc3cf 100644
--- a/Logs/Difference.hs
+++ b/Logs/Difference.hs
@@ -23,10 +23,10 @@ import Logs.UUIDBased
 import Logs.Difference.Pure
 
 recordDifferences :: Differences -> UUID -> Annex ()
-recordDifferences (Differences differences) uuid = do
+recordDifferences ds@(Differences {}) uuid = do
 	ts <- liftIO getPOSIXTime
 	Annex.Branch.change differenceLog $
-		showLog id . changeLog ts uuid (show differences) . parseLog Just
+		showLog id . changeLog ts uuid (showDifferences ds) . parseLog Just
 recordDifferences UnknownDifferences _ = return ()
 
 -- Map of UUIDs that have Differences recorded.