Improve overall camelization

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

@@ -60,6 +60,7 @@ import org.openapitools.codegen.model.OperationsMap;
 import org.openapitools.codegen.serializer.SerializerUtils;
 import org.openapitools.codegen.templating.MustacheEngineAdapter;
 import org.openapitools.codegen.templating.mustache.*;
+import org.openapitools.codegen.utils.CamelizeOption;
 import org.openapitools.codegen.utils.ModelUtils;
 import org.openapitools.codegen.utils.OneOfImplementorAdditionalData;
 import org.slf4j.Logger;
@@ -5913,7 +5914,7 @@ public class DefaultCodegen implements CodegenConfig {
         String[] splitString = name.split(nonNameElementPattern);
 
         if (splitString.length > 0) {
-            splitString[0] = lowerStartOfWord(splitString[0]);
+            splitString[0] = camelize(splitString[0], CamelizeOption.LOWERCASE_FIRST_CHAR);
         }
 
         String result = Arrays.stream(splitString)
@@ -5925,6 +5926,9 @@ public class DefaultCodegen implements CodegenConfig {
         return result;
     }
 
+
+    private static Pattern capitalLetterPattern = Pattern.compile("([A-Z]+)([A-Z][a-z][a-z]+)");
+
     /**
      * Puts the first letters to lowercase. If the word starts with multiple capital letters, all of them
      * will be converted to lowercase.

modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/StringUtils.java

@@ -118,6 +118,7 @@ public class StringUtils {
 
     private static Pattern camelizeSlashPattern = Pattern.compile("\\/(.?)");
     private static Pattern camelizeUppercasePattern = Pattern.compile("(\\.?)(\\w)([^\\.]*)$");
+    private static Pattern camelizeUppercaseStartPattern = Pattern.compile("^([A-Z]+)(([A-Z][a-z].*)|([^a-zA-Z].*)|$)$");
     private static Pattern camelizeUnderscorePattern = Pattern.compile("(_)(.)");
     private static Pattern camelizeHyphenPattern = Pattern.compile("(-)(.)");
     private static Pattern camelizeDollarPattern = Pattern.compile("\\$");
@@ -136,8 +137,15 @@ public class StringUtils {
         return camelizedWordsCache.get(key, pair -> {
             String word = pair.getKey();
             CamelizeOption option = pair.getValue();
+
+            // Lowercase acronyms at start of word if not UPPERCASE_FIRST_CHAR
+            Matcher m = camelizeUppercaseStartPattern.matcher(word);
+            if (camelizeOption != UPPERCASE_FIRST_CHAR && m.find()) {
+                word = m.group(1).toLowerCase(Locale.ROOT) + m.group(2);
+            }
+
             // Replace all slashes with dots (package separator)
-            Matcher m = camelizeSlashPattern.matcher(word);
+            m = camelizeSlashPattern.matcher(word);
             while (m.find()) {
                 word = m.replaceFirst("." + m.group(1)/*.toUpperCase()*/);
                 m = camelizeSlashPattern.matcher(word);

modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultCodegenTest.java

@@ -4730,7 +4730,10 @@ public class DefaultCodegenTest {
         final String alreadyCamelCase = "aVATRate";
         Assert.assertEquals(codegen.removeNonNameElementToCamelCase(alreadyCamelCase), alreadyCamelCase);
 
-        final String startWithCapitals = "DELETE_Invoice";
-        Assert.assertEquals(codegen.removeNonNameElementToCamelCase(startWithCapitals), "deleteInvoice");
+        final String startWithCapitals = "VATRate";
+        Assert.assertEquals(codegen.removeNonNameElementToCamelCase(startWithCapitals), "vatRate");
+
+        final String startWithCapitalsThenNonNameElement = "DELETE_Invoice";
+        Assert.assertEquals(codegen.removeNonNameElementToCamelCase(startWithCapitalsThenNonNameElement), "deleteInvoice");
     }
 }

modules/openapi-generator/src/test/java/org/openapitools/codegen/kotlin/AbstractKotlinCodegenTest.java

@@ -278,7 +278,7 @@ public class AbstractKotlinCodegenTest {
         // Assert the enum default value is properly generated
         CodegenProperty cp1 = cm1.vars.get(0);
         Assert.assertEquals(cp1.getEnumName(), "PropertyName");
-        Assert.assertEquals(cp1.getDefaultValue(), "PropertyName.vALUE");
+        Assert.assertEquals(cp1.getDefaultValue(), "PropertyName.`value`");
     }
 
     @Test(description = "Issue #10792")

modules/openapi-generator/src/test/java/org/openapitools/codegen/utils/StringUtilsTest.java

@@ -35,6 +35,14 @@ public class StringUtilsTest {
 
         Assert.assertEquals(camelize("some-value", LOWERCASE_FIRST_CHAR), "someValue");
         Assert.assertEquals(camelize("$type", LOWERCASE_FIRST_CHAR), "$Type");
+
+        Assert.assertEquals(camelize("aVATRate", LOWERCASE_FIRST_CHAR), "aVATRate");
+        Assert.assertEquals(camelize("VATRate", LOWERCASE_FIRST_CHAR), "vatRate");
+        Assert.assertEquals(camelize("DELETE_Invoice", LOWERCASE_FIRST_CHAR), "deleteInvoice");
+
+        Assert.assertEquals(camelize("aVATRate"), "AVATRate");
+        Assert.assertEquals(camelize("VATRate"), "VATRate");
+        Assert.assertEquals(camelize("DELETE_Invoice"), "DELETEInvoice");
     }
 
     @Test