optimise(sut): Make dumblog with journal use a better in-memory repre…

…sentation

src/sut/dumblog/src/Dumblog/Journal/StateMachine.hs

@@ -6,25 +6,26 @@ import Data.Binary (Binary)
 import Data.ByteString (ByteString)
 import qualified Data.ByteString.Lazy as LBS
 import qualified Data.ByteString.Lazy.Char8 as LBS8
+import Data.Sequence
 import GHC.Generics (Generic)
 
 import Dumblog.Journal.Types
 
 -- This is the main state of Dumblog, which is the result of applying all commands in the log
 data InMemoryDumblog = InMemoryDumblog
-  { theLog :: [ByteString] -- not very memory efficient, but not the point
+  { theLog :: Seq ByteString -- not very memory efficient, but not the point
   , nextIx :: Int
   } deriving Generic
 
 instance Binary InMemoryDumblog where
 
 initState :: InMemoryDumblog
-initState = InMemoryDumblog [] 0
+initState = InMemoryDumblog empty 0
 
 -- this could be pure?
 runCommand :: InMemoryDumblog -> Command -> IO (InMemoryDumblog, Response)
 runCommand state@(InMemoryDumblog appLog ix) cmd = case cmd of
-  Write bs -> pure (InMemoryDumblog (bs:appLog) (ix+1), LBS8.pack (show ix))
+  Write bs -> pure (InMemoryDumblog (appLog |> bs) (ix+1), LBS8.pack (show ix))
   Read i
-    | i < ix -> pure (state, LBS.fromStrict $ appLog !! (ix - 1 - i))
+    | i < ix -> pure (state, LBS.fromStrict $ index appLog i)
     | otherwise -> pure (state, "Transaction not in the store!") -- we probably should really signal failure