/*
 * Copyright 2019 The Android Open Source Project
 *
 * 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 androidx.camera.core.impl.utils;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.core.util.Preconditions;
import androidx.core.util.Supplier;

import java.io.Serializable;

/**
 * An immutable object that may contain a non-null reference to another object. Each instance of
 * this type either contains a non-null reference, or contains nothing (in which case we say that
 * the reference is "absent"); it is never said to "contain {@code null}".
 *
 * <p>A non-null {@code Optional<T>} reference can be used as a replacement for a nullable {@code T}
 * reference. It allows you to represent "a {@code T} that must be present" and a "a {@code T} that
 * might be absent" as two distinct types in your program, which can aid clarity.
 *
 * <p>Some uses of this class include
 *
 * <ul>
 * <li>As a method return type, as an alternative to returning {@code null} to indicate that no
 * value was available
 * <li>To distinguish between "unknown" (for example, not present in a map) and "known to have no
 * value" (present in the map, with value {@code Optional.absent()})
 * <li>To wrap nullable references for storage in a collection that does not support {@code null}
 * (though there are <a
 * href="https://github.com/google/guava/wiki/LivingWithNullHostileCollections">several other
 * approaches to this</a> that should be considered first)
 * </ul>
 *
 * <p>A common alternative to using this class is to find or create a suitable <a
 * href="http://en.wikipedia.org/wiki/Null_Object_pattern">null object</a> for the type in question.
 *
 * <p>This class is not intended as a direct analogue of any existing "option" or "maybe" construct
 * from other programming environments, though it may bear some similarities.
 *
 * <p><b>Comparison to {@code java.util.Optional} (JDK 8 and higher):</b> A new {@code Optional}
 * class was added for Java 8. The two classes are extremely similar, but incompatible (they cannot
 * share a common supertype). <i>All</i> known differences are listed either here or with the
 * relevant methods below.
 *
 * <ul>
 * <li>This class is serializable; {@code java.util.Optional} is not.
 * <li>{@code java.util.Optional} has the additional methods {@code ifPresent}, {@code filter},
 * {@code flatMap}, and {@code orElseThrow}.
 * <li>{@code java.util} offers the primitive-specialized versions {@code OptionalInt}, {@code
 * OptionalLong} and {@code OptionalDouble}, the use of which is recommended; Guava does not
 * have these.
 * </ul>
 *
 * @param <T> the type of instance that can be contained. {@code Optional} is naturally covariant on
 *            this type, so it is safe to cast an {@code Optional<T>} to {@code Optional<S>} for any
 *            supertype {@code S} of {@code T}.
 */
@SuppressWarnings("unused")
public abstract class Optional<T> implements Serializable {
    /**
     * Returns an {@code Optional} instance with no contained reference.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
     * {@code Optional.empty}.
     */
    @NonNull
    public static <T> Optional<T> absent() {
        return Absent.withType();
    }

    /**
     * Returns an {@code Optional} instance containing the given non-null reference. To have {@code
     * null} treated as {@link #absent}, use {@link #fromNullable} instead.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
     *
     * @throws NullPointerException if {@code reference} is null
     */
    @NonNull
    public static <T> Optional<T> of(@NonNull T reference) {
        return new Present<>(Preconditions.checkNotNull(reference));
    }

    /**
     * If {@code nullableReference} is non-null, returns an {@code Optional} instance containing
     * that
     * reference; otherwise returns {@link Optional#absent}.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
     * {@code Optional.ofNullable}.
     */
    @NonNull
    public static <T> Optional<T> fromNullable(@Nullable T nullableReference) {
        return (nullableReference == null) ? Optional.absent() : new Present<>(
                nullableReference);
    }

    Optional() {
    }

    /**
     * Returns {@code true} if this holder contains a (non-null) instance.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
     */
    public abstract boolean isPresent();

    /**
     * Returns the contained instance, which must be present. If the instance might be absent, use
     * {@link #or(Object)} or {@link #orNull} instead.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> when the value is absent, this method
     * throws {@link IllegalStateException}, whereas the Java 8 counterpart throws {@link
     * java.util.NoSuchElementException NoSuchElementException}.
     *
     * @throws IllegalStateException if the instance is absent ({@link #isPresent} returns {@code
     *                               false}); depending on this <i>specific</i> exception type
     *                               (over the more general {@link
     *                               RuntimeException}) is discouraged
     */
    @NonNull
    public abstract T get();

    /**
     * Returns the contained instance if it is present; {@code defaultValue} otherwise. If no
     * default value should be required because the instance is known to be present, use
     * {@link #get()} instead. For a default value of {@code null}, use {@link #orNull}.
     *
     * <p>Note about generics: The signature {@code public T or(T defaultValue)} is overly
     * restrictive. However, the ideal signature, {@code public <S super T> S or(S)}, is not legal
     * Java. As a result, some sensible operations involving subtypes are compile errors:
     *
     * <pre>{@code
     * Optional<Integer> optionalInt = getSomeOptionalInt();
     * Number value = optionalInt.or(0.5); // error
     *
     * FluentIterable<? extends Number> numbers = getSomeNumbers();
     * Optional<? extends Number> first = numbers.first();
     * Number value = first.or(0.5); // error
     * }</pre>
     *
     * <p>As a workaround, it is always safe to cast an {@code Optional<? extends T>} to {@code
     * Optional<T>}. Casting either of the above example {@code Optional} instances to {@code
     * Optional<Number>} (where {@code Number} is the desired output type) solves the problem:
     *
     * <pre>{@code
     * Optional<Number> optionalInt = (Optional) getSomeOptionalInt();
     * Number value = optionalInt.or(0.5); // fine
     *
     * FluentIterable<? extends Number> numbers = getSomeNumbers();
     * Optional<Number> first = (Optional) numbers.first();
     * Number value = first.or(0.5); // fine
     * }</pre>
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's {@code
     * Optional.orElse}, but will not accept {@code null} as a {@code defaultValue} ({@link #orNull}
     * must be used instead). As a result, the value returned by this method is guaranteed non-null,
     * which is not the case for the {@code java.util} equivalent.
     */
    @NonNull
    public abstract T or(@NonNull T defaultValue);

    /**
     * Returns this {@code Optional} if it has a value present; {@code secondChoice} otherwise.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java 8's
     * {@code Optional} class; write {@code thisOptional.isPresent() ? thisOptional : secondChoice}
     * instead.
     */
    @NonNull
    public abstract Optional<T> or(@NonNull Optional<? extends T> secondChoice);

    /**
     * Returns the contained instance if it is present; {@code supplier.get()} otherwise.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's {@code
     * Optional.orElseGet}, except when {@code supplier} returns {@code null}. In this case this
     * method throws an exception, whereas the Java 8 method returns the {@code null} to the caller.
     *
     * @throws NullPointerException if this optional's value is absent and the supplier returns
     *                              {@code null}
     */
    @NonNull
    public abstract T or(@NonNull Supplier<? extends T> supplier);

    /**
     * Returns the contained instance if it is present; {@code null} otherwise. If the instance is
     * known to be present, use {@link #get()} instead.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
     * {@code Optional.orElse(null)}.
     */
    @Nullable
    public abstract T orNull();

    /**
     * Returns {@code true} if {@code object} is an {@code Optional} instance, and either the
     * contained references are {@linkplain Object#equals equal} to each other or both are absent.
     * Note that {@code Optional} instances of differing parameterized types can be equal.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
     */
    @Override
    public abstract boolean equals(@Nullable Object object);

    /**
     * Returns a hash code for this instance.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific choice of
     * hash code unspecified, unlike the Java 8 equivalent.
     */
    @Override
    public abstract int hashCode();

    /**
     * Returns a string representation for this instance.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific string
     * representation unspecified, unlike the Java 8 equivalent.
     */
    @NonNull
    @Override
    public abstract String toString();

    private static final long serialVersionUID = 0;
}