Issue 4125 - [emscripten] Convert filter classes to C++

compiled/StringScanner.h

+#pragma once
+
+#include "String.h"
+
+class StringScanner
+{
+private:
+  const DependentString mStr;
+  String::size_type mPos;
+  String::size_type mEnd;
+  String::value_type mTerminator;
+public:
+  explicit StringScanner(const String& str, String::size_type pos = 0,
+        String::value_type terminator = u'\0')
+      : mStr(str), mPos(pos), mEnd(str.length()), mTerminator(terminator) {}
+
+  bool done() const
+  {
+    return mPos >= mEnd;
+  }
+
+  String::size_type position() const
+  {
+    return mPos - 1;
+  }
+
+  String::value_type next()
+  {
+    String::value_type result = done() ? mTerminator : mStr[mPos];
+    mPos++;
+    return result;
+  }
+
+  bool skipOne(String::value_type ch)
+  {
+    if (!done() && mStr[mPos] == ch)
+    {
+      mPos++;
+      return true;
+    }
+
+    return false;
+  }
+
+  bool skip(String::value_type ch)
+  {
+    bool skipped = false;
+    while ((skipped = skipOne(ch)));
+    return skipped;
+  }
+};