Add udf interface

core/src/main/java/org/opensearch/sql/calcite/udf/UserDefinedAggFunction.java

@@ -5,11 +5,34 @@
 
 package org.opensearch.sql.calcite.udf;
 
-public interface UserDefinedAggFunction<S extends Accumulator> {
+public interface UserDefinedAggFunction<S extends UserDefinedAggFunction.Accumulator> {
+  /**
+   * @return {@link Accumulator}
+   */
   S init();
 
+  /**
+   *
+   * @param {@link Accumulator}
+   * @return final result
+   */
   Object result(S accumulator);
 
-  // Add values to the accumulator
+  /**
+   * Add values to the accumulator. Notice some init argument will also be here like the 50 in Percentile(field, 50).
+   * @param acc {@link Accumulator}
+   * @param values the value to add to accumulator
+   * @return {@link Accumulator}
+   */
   S add(S acc, Object... values);
+
+  /**
+   * Maintain the state when {@link UserDefinedAggFunction} add values
+   */
+  interface Accumulator {
+    /**
+      * @return the final aggregation value
+     */
+    Object value();
+  }
 }

core/src/main/java/org/opensearch/sql/calcite/udf/udaf/TakeAggFunction.java

@@ -1,8 +1,13 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
 package org.opensearch.sql.calcite.udf.udaf;
 
 import java.util.ArrayList;
 import java.util.List;
-import org.opensearch.sql.calcite.udf.Accumulator;
+
 import org.opensearch.sql.calcite.udf.UserDefinedAggFunction;
 
 public class TakeAggFunction implements UserDefinedAggFunction<TakeAggFunction.TakeAccumulator> {
@@ -14,7 +19,7 @@ public TakeAccumulator init() {
 
   @Override
   public Object result(TakeAccumulator accumulator) {
-    return accumulator.result();
+    return accumulator.value();
   }
 
   @Override
@@ -40,7 +45,7 @@ public TakeAccumulator() {
     }
 
     @Override
-    public Object result() {
+    public Object value() {
       return hits;
     }
 

core/src/main/java/org/opensearch/sql/calcite/utils/BuiltinFunctionUtils.java

@@ -83,6 +83,16 @@ static SqlOperator translate(String op) {
     }
   }
 
+  /**
+   * Translates function arguments to align with Calcite's expectations, ensuring compatibility
+   * with PPL (Piped Processing Language). This is necessary because Calcite's input argument
+   * order or default values may differ from PPL's function definitions.
+   *
+   * @param op       The function name as a string.
+   * @param argList  A list of {@link RexNode} representing the parsed arguments from the PPL statement.
+   * @param context  The {@link CalcitePlanContext} providing necessary utilities such as {@code rexBuilder}.
+   * @return A modified list of {@link RexNode} that correctly maps to Calcite’s function expectations.
+   */
   static List<RexNode> translateArgument(
       String op, List<RexNode> argList, CalcitePlanContext context) {
     switch (op.toUpperCase(Locale.ROOT)) {