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  package java.util.stream;
  
  import java.util.Collections;
  import java.util.EnumSet;
  import java.util.Objects;
  import java.util.Set;
  import java.util.function.BiConsumer;
  import java.util.function.BinaryOperator;
  import java.util.function.Function;
  import java.util.function.Supplier;
  
  /**
   * A <a href="package-summary.html#Reduction">mutable reduction operation</a> that
   * accumulates input elements into a mutable result container, optionally transforming
   * the accumulated result into a final representation after all input elements
   * have been processed.  Reduction operations can be performed either sequentially
   * or in parallel.
   *
   * <p>Examples of mutable reduction operations include:
   * accumulating elements into a {@code Collection}; concatenating
   * strings using a {@code StringBuilder}; computing summary information about
   * elements such as sum, min, max, or average; computing "pivot table" summaries
   * such as "maximum valued transaction by seller", etc.  The class {@link Collectors}
   * provides implementations of many common mutable reductions.
   *
   * <p>A {@code Collector} is specified by four functions that work together to
   * accumulate entries into a mutable result container, and optionally perform
   * a final transform on the result.  They are: <ul>
   *     <li>creation of a new result container ({@link #supplier()})</li>
   *     <li>incorporating a new data element into a result container ({@link #accumulator()})</li>
   *     <li>combining two result containers into one ({@link #combiner()})</li>
   *     <li>performing an optional final transform on the container ({@link #finisher()})</li>
   * </ul>
   *
   * <p>Collectors also have a set of characteristics, such as
   * {@link Characteristics#CONCURRENT}, that provide hints that can be used by a
   * reduction implementation to provide better performance.
   *
   * <p>A sequential implementation of a reduction using a collector would
   * create a single result container using the supplier function, and invoke the
   * accumulator function once for each input element.  A parallel implementation
   * would partition the input, create a result container for each partition,
   * accumulate the contents of each partition into a subresult for that partition,
   * and then use the combiner function to merge the subresults into a combined
   * result.
   *
   * <p>To ensure that sequential and parallel executions produce equivalent
   * results, the collector functions must satisfy an <em>identity</em> and an
   * <a href="package-summary.html#Associativity">associativity</a> constraints.
   *
   * <p>The identity constraint says that for any partially accumulated result,
   * combining it with an empty result container must produce an equivalent
   * result.  That is, for a partially accumulated result {@code a} that is the
   * result of any series of accumulator and combiner invocations, {@code a} must
   * be equivalent to {@code combiner.apply(a, supplier.get())}.
   *
   * <p>The associativity constraint says that splitting the computation must
   * produce an equivalent result.  That is, for any input elements {@code t1}
   * and {@code t2}, the results {@code r1} and {@code r2} in the computation
   * below must be equivalent:
   * <pre>{@code
   *     A a1 = supplier.get();
   *     accumulator.accept(a1, t1);
   *     accumulator.accept(a1, t2);
   *     R r1 = finisher.apply(a1);  // result without splitting
   *
   *     A a2 = supplier.get();
   *     accumulator.accept(a2, t1);
   *     A a3 = supplier.get();
   *     accumulator.accept(a3, t2);
   *     R r2 = finisher.apply(combiner.apply(a2, a3));  // result with splitting
   * } </pre>
   *
   * <p>For collectors that do not have the {@code UNORDERED} characteristic,
   * two accumulated results {@code a1} and {@code a2} are equivalent if
   * {@code finisher.apply(a1).equals(finisher.apply(a2))}.  For unordered
   * collectors, equivalence is relaxed to allow for non-equality related to
   * differences in order.  (For example, an unordered collector that accumulated
   * elements to a {@code List} would consider two lists equivalent if they
   * contained the same elements, ignoring order.)
   *
   * <p>Libraries that implement reduction based on {@code Collector}, such as
   * {@link Stream#collect(Collector)}, must adhere to the following constraints:
   * <ul>
   *     <li>The first argument passed to the accumulator function, both
   *     arguments passed to the combiner function, and the argument passed to the
   *     finisher function must be the result of a previous invocation of the
   *     result supplier, accumulator, or combiner functions.</li>
   *     <li>The implementation should not do anything with the result of any of
   *     the result supplier, accumulator, or combiner functions other than to
   *     pass them again to the accumulator, combiner, or finisher functions,
   *     or return them to the caller of the reduction operation.</li>
   *     <li>If a result is passed to the combiner or finisher
   *     function, and the same object is not returned from that function, it is
   *     never used again.</li>
   *     <li>Once a result is passed to the combiner or finisher function, it
   *     is never passed to the accumulator function again.</li>
   *     <li>For non-concurrent collectors, any result returned from the result
   *     supplier, accumulator, or combiner functions must be serially
   *     thread-confined.  This enables collection to occur in parallel without
   *     the {@code Collector} needing to implement any additional synchronization.
   *     The reduction implementation must manage that the input is properly
   *     partitioned, that partitions are processed in isolation, and combining
   *     happens only after accumulation is complete.</li>
   *     <li>For concurrent collectors, an implementation is free to (but not
   *     required to) implement reduction concurrently.  A concurrent reduction
   *     is one where the accumulator function is called concurrently from
   *     multiple threads, using the same concurrently-modifiable result container,
   *     rather than keeping the result isolated during accumulation.
   *     A concurrent reduction should only be applied if the collector has the
   *     {@link Characteristics#UNORDERED} characteristics or if the
   *     originating data is unordered.</li>
   * </ul>
   *
   * <p>In addition to the predefined implementations in {@link Collectors}, the
   * static factory methods {@link #of(Supplier, BiConsumer, BinaryOperator, Characteristics...)}
   * can be used to construct collectors.  For example, you could create a collector
   * that accumulates widgets into a {@code TreeSet} with:
   *
   * <pre>{@code
   *     Collector<Widget, ?, TreeSet<Widget>> intoSet =
   *         Collector.of(TreeSet::new, TreeSet::add,
   *                      (left, right) -> { left.addAll(right); return left; });
   * }</pre>
   *
   * (This behavior is also implemented by the predefined collector
   * {@link Collectors#toCollection(Supplier)}).
   *
   * @apiNote
   * Performing a reduction operation with a {@code Collector} should produce a
   * result equivalent to:
   * <pre>{@code
   *     R container = collector.supplier().get();
   *     for (T t : data)
   *         collector.accumulator().accept(container, t);
   *     return collector.finisher().apply(container);
   * }</pre>
   *
   * <p>However, the library is free to partition the input, perform the reduction
   * on the partitions, and then use the combiner function to combine the partial
   * results to achieve a parallel reduction.  (Depending on the specific reduction
   * operation, this may perform better or worse, depending on the relative cost
   * of the accumulator and combiner functions.)
   *
   * <p>Collectors are designed to be <em>composed</em>; many of the methods
   * in {@link Collectors} are functions that take a collector and produce
   * a new collector.  For example, given the following collector that computes
   * the sum of the salaries of a stream of employees:
   *
   * <pre>{@code
   *     Collector<Employee, ?, Integer> summingSalaries
   *         = Collectors.summingInt(Employee::getSalary))
   * }</pre>
   *
   * If we wanted to create a collector to tabulate the sum of salaries by
   * department, we could reuse the "sum of salaries" logic using
   * {@link Collectors#groupingBy(Function, Collector)}:
   *
   * <pre>{@code
   *     Collector<Employee, ?, Map<Department, Integer>> summingSalariesByDept
   *         = Collectors.groupingBy(Employee::getDepartment, summingSalaries);
   * }</pre>
   *
   * @see Stream#collect(Collector)
   * @see Collectors
   *
   * @param <T> the type of input elements to the reduction operation
   * @param <A> the mutable accumulation type of the reduction operation (often
   *            hidden as an implementation detail)
   * @param <R> the result type of the reduction operation
   * @since 1.8
   */
  public interface Collector<T, A, R> {
      /**
       * A function that creates and returns a new mutable result container.
       *
       * @return a function which returns a new, mutable result container
       */
      Supplier<A> supplier();
  
      /**
       * A function that folds a value into a mutable result container.
       *
       * @return a function which folds a value into a mutable result container
       */
      BiConsumer<A, T> accumulator();
  
      /**
       * A function that accepts two partial results and merges them.  The
       * combiner function may fold state from one argument into the other and
       * return that, or may return a new result container.
       *
       * @return a function which combines two partial results into a combined
       * result
       */
      BinaryOperator<A> combiner();
  
      /**
       * Perform the final transformation from the intermediate accumulation type
       * {@code A} to the final result type {@code R}.
       *
       * <p>If the characteristic {@code IDENTITY_TRANSFORM} is
       * set, this function may be presumed to be an identity transform with an
       * unchecked cast from {@code A} to {@code R}.
       *
       * @return a function which transforms the intermediate result to the final
       * result
       */
      Function<A, R> finisher();
  
      /**
       * Returns a {@code Set} of {@code Collector.Characteristics} indicating
       * the characteristics of this Collector.  This set should be immutable.
       *
       * @return an immutable set of collector characteristics
       */
      Set<Characteristics> characteristics();
  
      /**
       * Returns a new {@code Collector} described by the given {@code supplier},
       * {@code accumulator}, and {@code combiner} functions.  The resulting
       * {@code Collector} has the {@code Collector.Characteristics.IDENTITY_FINISH}
       * characteristic.
       *
       * @param supplier The supplier function for the new collector
       * @param accumulator The accumulator function for the new collector
       * @param combiner The combiner function for the new collector
       * @param characteristics The collector characteristics for the new
       *                        collector
       * @param <T> The type of input elements for the new collector
       * @param <R> The type of intermediate accumulation result, and final result,
       *           for the new collector
       * @throws NullPointerException if any argument is null
       * @return the new {@code Collector}
       */
      public static<T, R> Collector<T, R, R> of(Supplier<R> supplier,
                                                BiConsumer<R, T> accumulator,
                                                BinaryOperator<R> combiner,
                                                Characteristics... characteristics) {
          Objects.requireNonNull(supplier);
          Objects.requireNonNull(accumulator);
          Objects.requireNonNull(combiner);
          Objects.requireNonNull(characteristics);
          Set<Characteristics> cs = (characteristics.length == 0)
                                    ? Collectors.CH_ID
                                    : Collections.unmodifiableSet(EnumSet.of(Characteristics.IDENTITY_FINISH,
                                                                             characteristics));
          return new Collectors.CollectorImpl<>(supplier, accumulator, combiner, cs);
      }
  
      /**
       * Returns a new {@code Collector} described by the given {@code supplier},
       * {@code accumulator}, {@code combiner}, and {@code finisher} functions.
       *
       * @param supplier The supplier function for the new collector
       * @param accumulator The accumulator function for the new collector
       * @param combiner The combiner function for the new collector
       * @param finisher The finisher function for the new collector
       * @param characteristics The collector characteristics for the new
       *                        collector
       * @param <T> The type of input elements for the new collector
       * @param <A> The intermediate accumulation type of the new collector
       * @param <R> The final result type of the new collector
       * @throws NullPointerException if any argument is null
       * @return the new {@code Collector}
       */
      public static<T, A, R> Collector<T, A, R> of(Supplier<A> supplier,
                                                   BiConsumer<A, T> accumulator,
                                                   BinaryOperator<A> combiner,
                                                   Function<A, R> finisher,
                                                   Characteristics... characteristics) {
          Objects.requireNonNull(supplier);
          Objects.requireNonNull(accumulator);
          Objects.requireNonNull(combiner);
          Objects.requireNonNull(finisher);
          Objects.requireNonNull(characteristics);
          Set<Characteristics> cs = Collectors.CH_NOID;
          if (characteristics.length > 0) {
              cs = EnumSet.noneOf(Characteristics.class);
              Collections.addAll(cs, characteristics);
              cs = Collections.unmodifiableSet(cs);
          }
          return new Collectors.CollectorImpl<>(supplier, accumulator, combiner, finisher, cs);
      }
  
      /**
       * Characteristics indicating properties of a {@code Collector}, which can
       * be used to optimize reduction implementations.
       */
      enum Characteristics {
          /**
           * Indicates that this collector is <em>concurrent</em>, meaning that
           * the result container can support the accumulator function being
           * called concurrently with the same result container from multiple
           * threads.
           *
           * <p>If a {@code CONCURRENT} collector is not also {@code UNORDERED},
           * then it should only be evaluated concurrently if applied to an
           * unordered data source.
           */
          CONCURRENT,
  
          /**
           * Indicates that the collection operation does not commit to preserving
           * the encounter order of input elements.  (This might be true if the
           * result container has no intrinsic order, such as a {@link Set}.)
           */
          UNORDERED,
  
          /**
           * Indicates that the finisher function is the identity function and
           * can be elided.  If set, it must be the case that an unchecked cast
           * from A to R will succeed.
           */
          IDENTITY_FINISH
      }
  }
  