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Some stellar general purpose utils.
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/*
 * Copyright (C) Posten Norge AS
 *
 * 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
 *
 *         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 no.digipost;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Optional;
import java.util.Spliterator;
import java.util.function.Supplier;
import java.util.stream.Collector;
import java.util.stream.Stream;

import static java.util.Collections.unmodifiableList;
import static java.util.Spliterator.IMMUTABLE;
import static java.util.Spliterator.ORDERED;
import static java.util.Spliterators.spliterator;
import static java.util.stream.Collectors.toCollection;
import static java.util.stream.StreamSupport.stream;

/**
 * Utilities for working with {@link Optional}s.
 */
public final class DiggOptionals {

    /**
     * Convert {@link Supplier suppliers} of {@link Optional} to a {@link Stream} consisting of only the
     * {@link Optional#isPresent() present} optionals. The returned stream is {@link Stream#sequential()}, yielding
     * the present values in the order they are given. Depending on how the stream is consumed, only the necessary
     * optional values will be attempted resolved. E.g. calling {@link Stream#findFirst() .findFirst()} will only resolve
     * as many {@code Optional}s needed until the first present one is encountered.
     *
     * Note on parallel streams:
     * The returned stream will be sequential and appropriate for use to resolve several {@link Optional}s in a deterministic
     * order, or for instance to find the first present value of a prioritized set of possible ways to resolve it. One should
     * be very (as always with streams) careful with {@link Stream#parallel() parallelizing} the stream, as you generally
     * have no control with how the resolvers will be run. For instance, {@link Stream#findAny() .parallel().findAny()} will indeed return
     * the first resolved value, but still block until the workers already started by the stream has terminated.
     *
     * @param  The type of the given {@code Optional}s and returned {@code Stream}.
     * @param optionalResolvers The operations resolving {@code Optional}s to convert to a {@code Stream}.
     *
     * @return a {@code Stream} containing the present {@code Optional}s
     */
    @SafeVarargs
    @SuppressWarnings("varargs")
    public static  Stream toStream(Supplier> ... optionalResolvers) {
        Supplier>[] copiedResolvers = Arrays.copyOf(optionalResolvers, optionalResolvers.length);
        Spliterator>> spliterator = spliterator(copiedResolvers, IMMUTABLE | ORDERED);
        return stream(spliterator, false).map(Supplier::get).filter(Optional::isPresent).map(Optional::get);
    }


    /**
     * Convert {@link Optional}s to a {@link Stream} consisting of only the
     * {@link Optional#isPresent() present} optionals.
     *
     * @param  The type of the given {@code Optional}s and returned {@code Stream}.
     * @param optionals The {@code Optional}s to convert to a {@code Stream}.
     *
     * @return a {@code Stream} containing the present {@code Optional}s
     */
    @SafeVarargs
    @SuppressWarnings("varargs")
    public static  Stream toStream(Optional ... optionals) {
        return Stream.of(optionals).filter(Optional::isPresent).map(Optional::get);
    }

    /**
     * Unwrap an {@link Optional} into a zero or one element {@link Collections#unmodifiableList(List) unmodifiable} list.
     *
     * @param  The type of the optional value.
     * @param optional The optional value to be wrapped in a list.
     *
     * @return list containing either exactly one or zero elements depending of the
     *         {@code Optional} being present or not.
     */
    public static  List toList(Optional optional) {
        return unmodifiableList(collect(optional, toCollection(() -> new ArrayList<>(1))));
    }

    /**
     * Unwrap an {@link Optional} into another container using a {@link Collector}.
     *
     * @param  The type of the optional value.
     * @param  The resulting container type.
     *
     * @param optional The optional value to be wrapped in another container.
     * @param collector The collector to use for wrapping into another container.
     *
     * @return The new container with either exactly one or zero elements depending of the
     *         {@code Optional} being present or not.
     */
    public static  R collect(Optional optional, Collector collector) {
        A container = collector.supplier().get();
        optional.ifPresent(o -> collector.accumulator().accept(container, o));
        return collector.finisher().apply(container);
    }


    public DiggOptionals() {}
}