agent: export decompressedSize (#1763)

Co-authored-by: Evgeny @ SimpleX Chat <259188159+evgeny-simplex@users.noreply.github.com>

src/Simplex/Messaging/Compression.hs

@@ -7,6 +7,7 @@ module Simplex.Messaging.Compression
     compressionLevel,
     compress1,
     decompress1,
+    decompressedSize,
   ) where
 
 import qualified Codec.Compression.Zstd as Z1
@@ -42,6 +43,11 @@ compress1 bs
   | B.length bs <= maxLengthPassthrough = Passthrough bs
   | otherwise = Compressed . Large $ Z1.compress compressionLevel bs
 
+decompressedSize :: Compressed -> Maybe Int
+decompressedSize = \case
+  Passthrough bs -> Just $ B.length bs
+  Compressed (Large bs) -> Z1.decompressedSize bs
+
 decompress1 :: Int -> Compressed -> Either String ByteString
 decompress1 limit = \case
   Passthrough bs -> Right bs