Optimize "." and ".." filtering

core/src/Streamly/Internal/FileSystem/Dir.hs

@@ -335,22 +335,15 @@ reader =
 eitherReader :: (MonadIO m, MonadCatch m) =>
     Unfold m OsPath (Either OsPath OsPath)
 eitherReader =
-    -- XXX bracketIO is expensive
+    -- XXX The measured overhead of bracketIO is not noticeable, if it turns
+    -- out to be a problems for small filenames we can use getdents64 to use
+    -- chunked read to avoid the overhead.
       UF.bracketIO openDirStream closeDirStream streamEitherReader
-    & UF.filter f
-
-    where
-
-    dot = OsPath.unsafeFromChar '.'
-    f p =
-        case p of
-            -- XXX This check could be made more efficient
-            Left x -> x /= OsPath.pack [dot] && x /= OsPath.pack [dot, dot]
-            Right _ -> True
 
 {-# INLINE eitherReaderPaths #-}
 eitherReaderPaths ::(MonadIO m, MonadCatch m) => Unfold m OsPath (Either OsPath OsPath)
 eitherReaderPaths =
+    -- XXX Do not resolve the children again
     UF.mapM2 (\dir -> return . bimap (dir </>) (dir </>)) eitherReader
 
 --

core/src/Streamly/Internal/FileSystem/ReadDir.hsc

@@ -12,7 +12,8 @@ module Streamly.Internal.FileSystem.ReadDir
     )
 where
 
-import Foreign (Ptr, Word8, nullPtr, peekByteOff)
+import Data.Char (ord)
+import Foreign (Ptr, Word8, nullPtr, peek, peekByteOff)
 import Foreign.C (resetErrno, Errno(..), getErrno, eINTR, throwErrno, CString)
 import System.OsPath.Posix (PosixPath)
 import System.Posix.Directory.Internals (DirStream(..), CDir, CDirent)
@@ -40,10 +41,24 @@ readDirStreamEither (DirStream dirp) = loop
         dname <- d_name ptr
         dtype :: #{type unsigned char} <- #{peek struct dirent, d_type} ptr
         name <- peekFilePath dname
-        return $
-            if (dtype == #const DT_DIR)
-            then (Left name)
-            else (Right name)
+        if (dtype == #const DT_DIR)
+        then do
+            -- XXX Assuming UTF8 encoding
+            c1 <- peek dname
+            if (c1 /= fromIntegral (ord '.'))
+            then return (Left name)
+            else do
+                c2 :: Word8 <- peekByteOff dname 1
+                if (c2 == 0)
+                then loop
+                else if (c2 /= fromIntegral (ord '.'))
+                then return (Left name)
+                else do
+                    c3 :: Word8 <- peekByteOff dname 2
+                    if (c3 == 0)
+                    then loop
+                    else return (Left name)
+        else return (Right name)
     else do
         errno <- getErrno
         if (errno == eINTR)