Treat identifiers containing ignorable characters as invalid; Add unit tests for ConverterHelper class.

src/org/jetbrains/java/decompiler/modules/renamer/ConverterHelper.java

@@ -1,6 +1,7 @@
 // Copyright 2000-2017 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 package org.jetbrains.java.decompiler.modules.renamer;
 
+import org.jetbrains.java.decompiler.code.CodeConstants;
 import org.jetbrains.java.decompiler.main.extern.IIdentifierRenamer;
 
 import java.util.Arrays;
@@ -29,13 +30,48 @@ public class ConverterHelper implements IIdentifierRenamer {
     String value = elementType == Type.ELEMENT_CLASS ? className : element;
     return value == null ||
            value.length() <= 2 ||
-           Character.isDigit(value.charAt(0)) ||
+           !isValidIdentifier(elementType == Type.ELEMENT_METHOD, value) ||
            KEYWORDS.contains(value) ||
            elementType == Type.ELEMENT_CLASS && (
              RESERVED_WINDOWS_NAMESPACE.contains(value.toLowerCase(Locale.US)) ||
              value.length() > 255 - ".class".length());
   }
 
+  /**
+   * Return {@code true} if, and only if identifier passed is compliant to JLS9 section 3.8 AND DOES NOT CONTAINS so-called "ignorable" characters.
+   * Ignorable characters are removed by javac silently during compilation and thus may appear only in specially crafted obfuscated classes.
+   * For more information about "ignorable" characters see <a href="https://bugs.openjdk.java.net/browse/JDK-7144981">JDK-7144981</a>.
+   *
+   * @param identifier Identifier to be checked
+   * @return {@code true} in case {@code identifier} passed can be used as an identifier; {@code false} otherwise.
+   */
+  private static boolean isValidIdentifier(boolean isMethod, String identifier) {
+
+    assert identifier != null : "Null identifier passed to the isValidIdentifier() method.";
+    assert !identifier.isEmpty() : "Empty identifier passed to the isValidIdentifier() method.";
+
+    if (isMethod && (identifier.equals(CodeConstants.INIT_NAME) || identifier.equals(CodeConstants.CLINIT_NAME))) {
+      return true;
+    }
+
+    if (!Character.isJavaIdentifierStart(identifier.charAt(0))) {
+      return false;
+    }
+
+    char[] chars = identifier.toCharArray();
+
+    for(int i = 1; i < chars.length; i++) {
+      char ch = chars[i];
+
+      if ((!Character.isJavaIdentifierPart(ch)) || Character.isIdentifierIgnorable(ch)) {
+        return false;
+      }
+    }
+
+    return true;
+
+  }
+
   // TODO: consider possible conflicts with not renamed classes, fields and methods!
   // We should get all relevant information here.
   @Override