Add snake case lambda

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

@@ -394,6 +394,7 @@ protected ImmutableMap.Builder<String, Lambda> addMustacheLambdas() {
         return new ImmutableMap.Builder<String, Mustache.Lambda>()
                 .put("lowercase", new LowercaseLambda().generator(this))
                 .put("uppercase", new UppercaseLambda())
+                .put("snakecase", new SnakecaseLambda())
                 .put("titlecase", new TitlecaseLambda())
                 .put("camelcase", new CamelCaseLambda(true).generator(this))
                 .put("pascalcase", new CamelCaseLambda(false).generator(this))

modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/mustache/SnakecaseLambda.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2018 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.openapitools.codegen.templating.mustache;
+
+import com.samskivert.mustache.Mustache;
+import com.samskivert.mustache.Template;
+
+import java.io.IOException;
+import java.io.Writer;
+import java.util.Locale;
+
+import static org.openapitools.codegen.utils.StringUtils.underscore;
+
+/**
+ * Converts text in a fragment to snake case.
+ *
+ * Register:
+ * <pre>
+ * additionalProperties.put("snakecase", new SnakecaseLambda());
+ * </pre>
+ *
+ * Use:
+ * <pre>
+ * {{#snakecase}}{{summary}}{{/snakecase}}
+ * </pre>
+ */
+public class SnakecaseLambda implements Mustache.Lambda {
+    @Override
+    public void execute(Template.Fragment fragment, Writer writer) throws IOException {
+        writer.write(underscore(fragment.execute()));
+    }
+}

modules/openapi-generator/src/test/java/org/openapitools/codegen/templating/mustache/SnakecaseLambdaTest.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2018 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.openapitools.codegen.templating.mustache;
+
+import java.util.Map;
+
+import org.testng.annotations.Test;
+
+public class SnakecaseLambdaTest extends LambdaTest {
+
+    @Test
+    public void snakecaseTest() {
+        // Given
+        Map<String, Object> ctx = context("snakecase", new SnakecaseLambda());
+
+        // When & Then
+        test("access_code", "{{#snakecase}}accessCode{{/snakecase}}", ctx);
+    }
+
+}