Merge pull request #434 from Thorium/niceName-opt

Faster and less-memory-consuming niceName

Author: gdziadkiewicz

src/FSharpx.Extras/Strings.fs

@@ -91,31 +91,54 @@ module Strings =
     let (|Upper|_|) = satisfies Char.IsUpper
     let (|Lower|_|) = satisfies Char.IsLower
 
+    /// Faster patterns with ValueOption instead of Option
+    [<RequireQualifiedAccess>]
+    module ValuePatterns =
+        // Active patterns & operators for parsing strings
+        let inline take (s:string) i = if i >= s.Length then ValueNone else ValueSome s.[i]
+
+        let inline internal satisfies predicate (charOption:voption<char>) = 
+            match charOption with 
+            | ValueSome c when predicate c -> charOption 
+            | _ -> ValueNone
+
+        [<return: Struct>]
+        let (|EOF|_|) = function 
+            | ValueSome _ -> ValueNone
+            | _ -> ValueSome ()
+        [<return: Struct>]
+        let (|LetterDigit|_|) = satisfies Char.IsLetterOrDigit
+        [<return: Struct>]
+        let (|Upper|_|) = satisfies Char.IsUpper
+        [<return: Struct>]
+        let (|Lower|_|) = satisfies Char.IsLower
+    
     /// Turns a string into a nice PascalCase identifier
     [<CompiledName("NiceName")>]
     let niceName (s:string) = 
         if s = s.ToUpper() then s else
         // Starting to parse a new segment 
-        let rec restart i = seq {
-          match s @? i with 
-          | EOF -> ()
-          | LetterDigit _ & Upper _ -> yield! upperStart i (i + 1)
-          | LetterDigit _ -> yield! consume i false (i + 1)
-          | _ -> yield! restart (i + 1) }
+        let rec restart i = 
+          match ValuePatterns.take s i with 
+          | ValuePatterns.EOF -> Seq.empty
+          | ValuePatterns.LetterDigit _ & ValuePatterns.Upper _ -> upperStart i (i + 1)
+          | ValuePatterns.LetterDigit _ -> consume i false (i + 1)
+          | _ -> restart (i + 1)
 
         // Parsed first upper case letter, continue either all lower or all upper
-        and upperStart from i = seq {
-          match s @? i with 
-          | Upper _ -> yield! consume from true (i + 1) 
-          | Lower _ -> yield! consume from false (i + 1) 
-          | _ -> yield! restart (i + 1) }
+        and upperStart from i = 
+          match ValuePatterns.take s i with 
+          | ValuePatterns.Upper _ -> consume from true (i + 1) 
+          | ValuePatterns.Lower _ -> consume from false (i + 1) 
+          | _ -> restart (i + 1)
         // Consume are letters of the same kind (either all lower or all upper)
-        and consume from takeUpper i = seq {
-          match s @? i with
-          | Lower _ when not takeUpper -> yield! consume from takeUpper (i + 1)
-          | Upper _ when takeUpper -> yield! consume from takeUpper (i + 1)
+        and consume from takeUpper i = 
+          match ValuePatterns.take s i with
+          | ValuePatterns.Lower _ when not takeUpper -> consume from takeUpper (i + 1)
+          | ValuePatterns.Upper _ when takeUpper -> consume from takeUpper (i + 1)
           | _ -> 
-              yield from, i
+            seq {
+              yield struct(from, i)
               yield! restart i }
 
         // Split string into segments and turn them to PascalCase