content: /*
   * 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.beam.sdk.transforms;
  
  import static org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.Preconditions.checkNotNull;
  
  import org.apache.beam.sdk.transforms.Contextful.Fn;
  import org.apache.beam.sdk.transforms.WithFailures.ExceptionElement;
  import org.apache.beam.sdk.values.KV;
  import org.apache.beam.sdk.values.PCollection;
  import org.apache.beam.sdk.values.TypeDescriptor;
  import org.apache.beam.sdk.values.TypeParameter;
  import org.checkerframework.checker.nullness.qual.Nullable;
  import org.checkerframework.checker.nullness.qual.RequiresNonNull;
  
  /**
   * {@code MapKeys} maps a {@code SerializableFunction<K1,K2>} over keys of a {@code
   * PCollection<KV<K1,V>>} and returns a {@code PCollection<KV<K2, V>>}.
   *
   * <p>Example of use:
   *
   * <pre>{@code
   * PCollection<KV<Integer, String>> input = ...;
   * PCollection<KV<Double, String> output =
   *      input.apply(MapKeys.into(TypeDescriptors.doubles()).via(Integer::doubleValue));
   * }</pre>
   *
   * <p>See also {@link MapValues}.
   *
   * @param <K1> the type of the keys in the input {@code PCollection}
   * @param <K2> the type of the keys in the output {@code PCollection}
   */
  public class MapKeys<K1, K2, V> extends PTransform<PCollection<KV<K1, V>>, PCollection<KV<K2, V>>> {
  
    private final transient TypeDescriptor<K2> outputType;
    private final @Nullable Contextful<Fn<KV<K1, V>, KV<K2, V>>> fn;
  
    /**
     * Returns a {@code MapKeys<K1, K2, V>} {@code PTransform} for a {@code ProcessFunction<NewK1,
     * K2>} with predefined {@link #outputType}.
     *
     * @param <NewKeyT> the type of the keys in the input {@code PCollection}
     * @param <NewValueT> the type of the values in the input and output {@code PCollection}s
     */
    public <NewValueT, NewKeyT> MapKeys<NewKeyT, K2, NewValueT> via(
        SerializableFunction<NewKeyT, K2> fn) {
      return new MapKeys<>(
          Contextful.fn(
              (element, c) -> KV.of(fn.apply(element.getKey()), element.getValue()),
              Requirements.empty()),
          outputType);
    }
  
    /**
     * Returns a new {@link MapKeys} transform with the given type descriptor for the output type, but
     * the mapping function yet to be specified using {@link #via(SerializableFunction)}.
     */
    public static <K2> MapKeys<?, K2, ?> into(final TypeDescriptor<K2> outputType) {
      return new MapKeys<>(null, outputType);
    }
  
    private MapKeys(
        @Nullable Contextful<Fn<KV<K1, V>, KV<K2, V>>> fn, TypeDescriptor<K2> outputType) {
      this.fn = fn;
      this.outputType = outputType;
    }
  
    /**
     * Returns a new {@link SimpleMapWithFailures} transform that catches exceptions raised while
     * mapping elements, with the given type descriptor used for the failure collection but the
     * exception handler yet to be specified using {@link
     * SimpleMapWithFailures#exceptionsVia(ProcessFunction)}.
     *
     * <p>See {@link WithFailures} documentation for usage patterns of the returned {@link
     * WithFailures.Result}.
     *
     * <p>Example usage:
     *
     * <pre>{@code
     * Result<PCollection<KV<Integer, String>>, String> result =
     *         input.apply(
     *             MapKeys.into(TypeDescriptors.integers())
     *                 .<String, String>via(word -> 1 / word.length)  // Could throw ArithmeticException
     *                 .exceptionsInto(TypeDescriptors.strings())
     *                 .exceptionsVia(ee -> ee.exception().getMessage()));
     * PCollection<KV<Integer, String>> output = result.output();
     * PCollection<String> failures = result.failures();
     * }</pre>
     */
    @RequiresNonNull("fn")
    public <FailureT> SimpleMapWithFailures<KV<K1, V>, KV<K2, V>, FailureT> exceptionsInto(
        TypeDescriptor<FailureT> failureTypeDescriptor) {
      return new SimpleMapWithFailures<>(
          "MapKeysWithFailures", fn, getKvTypeDescriptor(), null, failureTypeDescriptor);
    }
  
    /**
     * Returns a new {@link SimpleMapWithFailures} transform that catches exceptions raised while
     * mapping elements, passing the raised exception instance and the input element being processed
     * through the given {@code exceptionHandler} and emitting the result to a failure collection.
     *
     * <p>This method takes advantage of the type information provided by {@link InferableFunction},
     * meaning that a call to {@link #exceptionsInto(TypeDescriptor)} may not be necessary.
     *
     * <p>See {@link WithFailures} documentation for usage patterns of the returned {@link
     * WithFailures.Result}.
     *
     * <p>Example usage:
     *
     * <pre>{@code
     * Result<PCollection<KV<Integer, String>>, String> result =
     *         input.apply(
     *             MapKeys.into(TypeDescriptors.integers())
     *                 .<String, String>via(word -> 1 / word.length)  // Could throw ArithmeticException
     *                 .exceptionsVia(
     *                     new InferableFunction<ExceptionElement<KV<String, String>>, String>() {
     *                       @Override
     *                       public String apply(ExceptionElement<KV<String, String>> input) {
     *                         return input.exception().getMessage();
     *                       }
     *                     }));
     * PCollection<KV<Integer, String>> output = result.output();
     * PCollection<String> failures = result.failures();
     * }</pre>
     */
    @RequiresNonNull("fn")
    public <FailureT> SimpleMapWithFailures<KV<K1, V>, KV<K2, V>, FailureT> exceptionsVia(
        InferableFunction<ExceptionElement<KV<K1, V>>, FailureT> exceptionHandler) {
      return new SimpleMapWithFailures<>(
          "MapKeysWithFailures",
          fn,
          getKvTypeDescriptor(),
          exceptionHandler,
          exceptionHandler.getOutputTypeDescriptor());
    }
  
    @Override
    public PCollection<KV<K2, V>> expand(PCollection<KV<K1, V>> input) {
      return input.apply(
          "MapKeys",
          MapElements.into(getKvTypeDescriptor())
              .via(checkNotNull(fn, "Must specify a function on MapKeys using .via()")));
    }
  
    private TypeDescriptor<KV<K2, V>> getKvTypeDescriptor() {
      return new TypeDescriptor<KV<K2, V>>() {}.where(new TypeParameter<K2>() {}, outputType);
    }
  }
  