Optimise list decoder (#7358)

Author: kwxm

plutus-core/flat/src/PlutusCore/Flat/Decoder/Strict.hs

@@ -55,15 +55,35 @@ import Numeric.Natural (Natural)
 import PlutusCore.Flat.Data.ZigZag
 #include "MachDeps.h"
 
-{-# INLINE decodeListWith #-}
 decodeListWith :: Get a -> Get [a]
-decodeListWith dec = go
-  where
-    go = do
-      b <- dBool
-      if b
-        then (:) <$> dec <*> go
-        else return []
+decodeListWith dec = do
+  b <- dBool
+  if b
+    then Get $
+         \end s -> do
+           GetResult s' h <- runGet dec end s
+           if s' /= s
+             then
+             let goNormal = do
+                   b <- dBool
+                   if b
+                     then (:) <$> dec <*> goNormal
+                     else pure []
+                 {-# INLINE goNormal #-}
+             in runGet ((h:) <$> goNormal) end s'
+             else
+             let goZero x =
+                   let goZero'= do
+                         b <- dBool
+                         if b
+                           then (x:) <$> goZero'
+                           else pure []
+                       {-# INLINE goZero' #-}
+                   in goZero'
+                 {-# INLINE goZero #-}
+             in runGet ((h:) <$> goZero h) end s'
+    else pure []
+{-# INLINE decodeListWith #-}
 
 decodeArrayWith :: Get a -> Get [a]
 decodeArrayWith dec = DL.toList <$> getAsL_ dec