/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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
 *
 *     http://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.apache.flink.table.api.functions;

import org.apache.flink.table.api.types.DataType;
import org.apache.flink.util.Collector;

/**
 * Base class for a user-defined table function (UDTF). A user-defined table functions works on
 * zero, one, or multiple scalar values as input and returns multiple rows as output.
 *
 * <p>The behavior of a {@link TableFunction} can be defined by implementing a custom evaluation
 * method. An evaluation method must be declared publicly, not static and named "eval".
 * Evaluation methods can also be overloaded by implementing multiple methods named "eval".
 *
 * <p>User-defined functions must have a default constructor and must be instantiable during
 * runtime.
 *
 * <p>By default the result type of an evaluation method is determined by Flink's type extraction
 * facilities. This is sufficient for basic types or simple POJOs but might be wrong for more
 * complex, custom, or composite types. In these cases {@link DataType} of the result type
 * can be manually defined by overriding {@link #getResultType}.
 *
 * <p>Internally, the Table/SQL API code generation works with primitive values as much as possible.
 * If a user-defined table function should not introduce much overhead during runtime, it is
 * recommended to declare parameters and result types as primitive types instead of their boxed
 * classes. DATE/TIME is equal to int, TIMESTAMP is equal to long.
 *
 * <p>Example:
 *
 * {@code
 *
 *   public class Split extends TableFunction<String> {
 *
 *     // implement an "eval" method with as many parameters as you want
 *     public void eval(String str) {
 *       for (String s : str.split(" ")) {
 *         collect(s);   // use collect(...) to emit an output row
 *       }
 *     }
 *
 *     // you can overload the eval method here ...
 *   }
 *
 *   val tEnv: TableEnvironment = ...
 *   val table: Table = ...    // schema: [a: String]
 *
 *   // for Scala users
 *   val split = new Split()
 *   table.join(split('c) as ('s)).select('a, 's)
 *
 *   // for Java users
 *   tEnv.registerFunction("split", new Split())   // register table function first
 *   table.join(new Table(tEnv, "split(a) as (s)")).select("a, s")
 *
 *   // for SQL users
 *   tEnv.registerFunction("split", new Split())   // register table function first
 *   tEnv.sql("SELECT a, s FROM MyTable, LATERAL TABLE(split(a)) as T(s)")
 *
 * }
 *
 * @param <T> The type of the output row
 */
public abstract class TableFunction<T> extends CustomTypeDefinedFunction {

	/**
	 * Emit an output row.
	 *
	 * @param row the output row
	 */
	public void collect(T row) {
		collector.collect(row);
	}

	// ----------------------------------------------------------------------------------------------

	/**
	 * The code generated collector used to emit row.
	 */
	private Collector<T> collector = null;

	/**
	 * Internal use. Sets the current collector.
	 */
	public final void setCollector(Collector<T> collector) {
		this.collector = collector;
	}
}