HHH-18949 fix underscores within uppercase names

tooling/metamodel-generator/src/main/java/org/hibernate/processor/util/StringUtil.java

@@ -98,18 +98,21 @@ public final class StringUtil {
 	}
 
 	public static String getUpperUnderscoreCaseFromLowerCamelCase(String lowerCamelCaseString) {
-		if ( lowerCamelCaseString.length() == 1 && isUpperCase( lowerCamelCaseString.charAt( 0 ) ) ) {
-			return "_" + lowerCamelCaseString;
-		}
 		final StringBuilder result = new StringBuilder();
 		int position = 0;
+		boolean wasLowerCase = false;
 		while ( position < lowerCamelCaseString.length() ) {
 			final int codePoint = lowerCamelCaseString.codePointAt( position );
-			if ( position>0 && isUpperCase( codePoint ) ) {
+			final boolean isUpperCase = isUpperCase( codePoint );
+			if ( wasLowerCase && isUpperCase ) {
 				result.append('_');
 			}
 			result.appendCodePoint( toUpperCase( codePoint ) );
 			position += charCount( codePoint );
+			wasLowerCase = !isUpperCase;
+		}
+		if ( result.toString().equals( lowerCamelCaseString ) ) {
+			result.insert(0, '_');
 		}
 		return result.toString();
 	}

tooling/metamodel-generator/src/test/java/org/hibernate/processor/test/uppercase/Person.java

@@ -0,0 +1,14 @@
+/*
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ * Copyright Red Hat Inc. and Hibernate Authors
+ */
+package org.hibernate.processor.test.uppercase;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.Id;
+
+@Entity
+public class Person {
+	@Id String SSN;
+	String UserID;
+}

tooling/metamodel-generator/src/test/java/org/hibernate/processor/test/uppercase/UppercaseTest.java

@@ -0,0 +1,29 @@
+/*
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ * Copyright Red Hat Inc. and Hibernate Authors
+ */
+package org.hibernate.processor.test.uppercase;
+
+import org.hibernate.processor.test.util.CompilationTest;
+import org.hibernate.processor.test.util.WithClasses;
+import org.junit.Test;
+
+import static org.hibernate.processor.test.util.TestUtil.assertMetamodelClassGeneratedFor;
+import static org.hibernate.processor.test.util.TestUtil.assertPresenceOfFieldInMetamodelFor;
+import static org.hibernate.processor.test.util.TestUtil.getMetaModelSourceAsString;
+
+public class UppercaseTest extends CompilationTest {
+
+	@Test
+	@WithClasses(value = Person.class)
+	public void test() {
+		System.out.println( getMetaModelSourceAsString( Person.class ) );
+
+		assertMetamodelClassGeneratedFor( Person.class );
+
+		assertPresenceOfFieldInMetamodelFor( Person.class, "SSN" );
+		assertPresenceOfFieldInMetamodelFor( Person.class, "_SSN" );
+		assertPresenceOfFieldInMetamodelFor( Person.class, "UserID" );
+		assertPresenceOfFieldInMetamodelFor( Person.class, "USER_ID" );
+	}
+}