Optimize Filename.checkPathForIllegalChars

src/utils/filename.fs

@@ -15,17 +15,29 @@ let illegalPathChars =
     let chars = Path.GetInvalidPathChars ()
     chars
 
-let checkPathForIllegalChars (path:string) =
-    let len = path.Length
-    for i = 0 to len - 1 do
-        let c = path.[i]
-
-        // Determine if this character is disallowed within a path by
-        // attempting to find it in the array of illegal path characters.
-        for badChar in illegalPathChars do
-            if c = badChar then
-                raise(IllegalFileNameChar(path, c))
+type PathState =
+    | Legal
+    | Illegal of path: string * illegalChar: char
 
+let checkPathForIllegalChars =
+    let cache = System.Collections.Concurrent.ConcurrentDictionary<string, PathState>()
+    fun (path: string) ->
+        match cache.TryGetValue path with
+        | true, Legal -> () 
+        | true, Illegal (path, c) -> raise(IllegalFileNameChar(path, c))
+        | _ ->
+            let len = path.Length
+            for i = 0 to len - 1 do
+                let c = path.[i]
+
+                // Determine if this character is disallowed within a path by
+                // attempting to find it in the array of illegal path characters.
+                for badChar in illegalPathChars do
+                    if c = badChar then
+                        cache.[path] <- Illegal(path, c)
+                        raise(IllegalFileNameChar(path, c))
+            cache.[path] <- Legal
+
 // Case sensitive (original behaviour preserved).
 let checkSuffix (x:string) (y:string) = x.EndsWith(y,System.StringComparison.Ordinal)