Factor out nextScalarIndex for matchBitsetScalar

Author: rctcwyvrn

Sources/_StringProcessing/Engine/Processor.swift

@@ -231,11 +231,18 @@ extension Processor {
     currentPosition < end ? input.unicodeScalars[currentPosition] : nil
   }
 
+  func nextScalarIndex(offsetBy n: Int, boundaryCheck: Bool) -> Input.Index? {
+    if let idx = input.unicodeScalars.index(currentPosition, offsetBy: 1, limitedBy: end),
+       (!boundaryCheck || input.isOnGraphemeClusterBoundary(idx)) {
+      return idx
+    }
+    return nil
+  }
+  
   mutating func matchScalar(_ s: Unicode.Scalar, boundaryCheck: Bool) -> Bool {
     guard let curScalar = loadScalar(),
           curScalar == s,
-          let idx = input.unicodeScalars.index(currentPosition, offsetBy: 1, limitedBy: end),
-          (!boundaryCheck || input.isOnGraphemeClusterBoundary(idx))
+          let idx = nextScalarIndex(offsetBy: 1, boundaryCheck: boundaryCheck)
     else {
       signalFailure()
       return false