Refactor getMempoolSize

ouroboros-consensus/src/Ouroboros/Consensus/Mempool/API.hs

@@ -286,6 +286,13 @@ data MempoolSize = MempoolSize
     -- ^ The summed byte size of all the transactions in the mempool.
   } deriving (Eq, Show)
 
+instance Semigroup MempoolSize where
+  MempoolSize xt xb <> MempoolSize yt yb = MempoolSize (xt + yt) (xb + yb)
+
+instance Monoid MempoolSize where
+  mempty = MempoolSize { msNumTxs = 0, msNumBytes = 0 }
+  mappend = (<>)
+
 -- | Events traced by the Mempool.
 data TraceEventMempool blk
   = TraceMempoolAddTxs

ouroboros-consensus/src/Ouroboros/Consensus/Mempool/Impl.hs

@@ -465,17 +465,16 @@ implGetSnapshot MempoolEnv{mpEnvStateVar} = do
 getMempoolSize :: (IOLike m, ApplyTx blk)
                => MempoolEnv m blk
                -> STM m MempoolSize
-getMempoolSize MempoolEnv{mpEnvStateVar} = do
-    IS { isTxs } <- readTVar mpEnvStateVar
-    pure $ Foldable.foldl'
-      (\MempoolSize { msNumTxs, msNumBytes } tx ->
-        MempoolSize
-          { msNumTxs = msNumTxs + 1
-          , msNumBytes = msNumBytes + txSize tx
-          }
-      )
-      MempoolSize { msNumTxs = 0, msNumBytes = 0 }
-      isTxs
+getMempoolSize MempoolEnv{mpEnvStateVar} =
+    txsToMempoolSize . isTxs <$> readTVar mpEnvStateVar
+
+-- | Given a 'Foldable' of transactions, calculate what the 'MempoolSize'
+-- would be if a mempool were to consist /only/ of those transactions.
+txsToMempoolSize :: (Foldable t, ApplyTx blk) => t (GenTx blk) -> MempoolSize
+txsToMempoolSize = foldMap toMempoolSize
+  where
+    toMempoolSize :: ApplyTx blk => GenTx blk -> MempoolSize
+    toMempoolSize tx = MempoolSize { msNumTxs = 1, msNumBytes = txSize tx }
 
 {-------------------------------------------------------------------------------
   MempoolSnapshot Implementation