/*
 * 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.commons.collections4.comparators;

import java.io.Serializable;
import java.util.Comparator;
import java.util.Objects;

import org.apache.commons.collections4.ComparatorUtils;

/**
 * A Comparator that will compare nulls to be either lower or higher than
 * other objects.
 *
 * @param <E> the type of objects compared by this comparator
 * @since 2.0
 */
public class NullComparator<E> implements Comparator<E>, Serializable {

    /** Serialization version. */
    private static final long serialVersionUID = -5820772575483504339L;

    /**
     *  The comparator to use when comparing two non-{@code null} objects.
     **/
    private final Comparator<? super E> nonNullComparator;

    /**
     *  Specifies whether a {@code null} are compared as higher than
     *  non-{@code null} objects.
     **/
    private final boolean nullsAreHigh;

    /**
     *  Construct an instance that sorts {@code null} higher than any
     *  non-{@code null} object it is compared with. When comparing two
     *  non-{@code null} objects, the {@link ComparableComparator} is
     *  used.
     **/
    public NullComparator() {
        this(ComparatorUtils.NATURAL_COMPARATOR, true);
    }

    /**
     *  Construct an instance that sorts {@code null} higher or lower than
     *  any non-{@code null} object it is compared with.  When comparing
     *  two non-{@code null} objects, the {@link ComparableComparator} is
     *  used.
     *
     *  @param nullsAreHigh a {@code true} value indicates that
     *  {@code null} should be compared as higher than a
     *  non-{@code null} object.  A {@code false} value indicates
     *  that {@code null} should be compared as lower than a
     *  non-{@code null} object.
     **/
    public NullComparator(final boolean nullsAreHigh) {
        this(ComparatorUtils.NATURAL_COMPARATOR, nullsAreHigh);
    }

    /**
     *  Construct an instance that sorts {@code null} higher than any
     *  non-{@code null} object it is compared with.  When comparing two
     *  non-{@code null} objects, the specified {@link Comparator} is
     *  used.
     *
     *  @param nonNullComparator the comparator to use when comparing two
     *  non-{@code null} objects.  This argument cannot be
     *  {@code null}
     *
     *  @throws NullPointerException if {@code nonNullComparator} is
     *  {@code null}
     **/
    public NullComparator(final Comparator<? super E> nonNullComparator) {
        this(nonNullComparator, true);
    }

    /**
     *  Construct an instance that sorts {@code null} higher or lower than
     *  any non-{@code null} object it is compared with.  When comparing
     *  two non-{@code null} objects, the specified {@link Comparator} is
     *  used.
     *
     *  @param nonNullComparator the comparator to use when comparing two
     *  non-{@code null} objects. This argument cannot be
     *  {@code null}
     *
     *  @param nullsAreHigh a {@code true} value indicates that
     *  {@code null} should be compared as higher than a
     *  non-{@code null} object.  A {@code false} value indicates
     *  that {@code null} should be compared as lower than a
     *  non-{@code null} object.
     *
     *  @throws NullPointerException if {@code nonNullComparator} is
     *  {@code null}
     **/
    public NullComparator(final Comparator<? super E> nonNullComparator, final boolean nullsAreHigh) {
        this.nonNullComparator = Objects.requireNonNull(nonNullComparator, "nonNullComparator");
        this.nullsAreHigh = nullsAreHigh;
    }

    /**
     *  Perform a comparison between two objects.  If both objects are
     *  {@code null}, a {@code 0} value is returned.  If one object
     *  is {@code null} and the other is not, the result is determined on
     *  whether the Comparator was constructed to have nulls as higher or lower
     *  than other objects.  If neither object is {@code null}, an
     *  underlying comparator specified in the constructor (or the default) is
     *  used to compare the non-{@code null} objects.
     *
     *  @param o1  the first object to compare
     *  @param o2  the object to compare it to.
     *  @return {@code -1} if {@code o1} is "lower" than (less than,
     *  before, etc.) {@code o2}; {@code 1} if {@code o1} is
     *  "higher" than (greater than, after, etc.) {@code o2}; or
     *  {@code 0} if {@code o1} and {@code o2} are equal.
     **/
    @Override
    public int compare(final E o1, final E o2) {
        if (o1 == o2) {
            return 0;
        }
        if (o1 == null) {
            return nullsAreHigh ? 1 : -1;
        }
        if (o2 == null) {
            return nullsAreHigh ? -1 : 1;
        }
        return nonNullComparator.compare(o1, o2);
    }

    /**
     *  Determines whether the specified object represents a comparator that is
     *  equal to this comparator.
     *
     *  @param obj  the object to compare this comparator with.
     *
     *  @return {@code true} if the specified object is a NullComparator
     *  with equivalent {@code null} comparison behavior
     *  (i.e. {@code null} high or low) and with equivalent underlying
     *  non-{@code null} object comparators.
     **/
    @Override
    public boolean equals(final Object obj) {
        if (obj == null) {
            return false;
        }
        if (obj == this) {
            return true;
        }
        if (!obj.getClass().equals(this.getClass())) {
            return false;
        }

        final NullComparator<?> other = (NullComparator<?>) obj;

        return nullsAreHigh == other.nullsAreHigh &&
                nonNullComparator.equals(other.nonNullComparator);
    }

    /**
     *  Implement a hash code for this comparator that is consistent with
     *  {@link #equals(Object)}.
     *
     *  @return a hash code for this comparator.
     **/
    @Override
    public int hashCode() {
        return (nullsAreHigh ? -1 : 1) * nonNullComparator.hashCode();
    }
}
