/*
 * Copyright (C) 2009 The Guava Authors
 *
 * 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 com.google.common.collect;

import java.util.Comparator;

import javax.annotation.Nullable;

import com.google.common.annotations.GwtCompatible;
import com.google.common.primitives.Booleans;
import com.google.common.primitives.Ints;
import com.google.common.primitives.Longs;

/**
 * A utility for performing a chained comparison statement. For example:
 * 
 * <pre>
 *    {@code
 *
 *   public int compareTo(Foo that) {
 *     return ComparisonChain.start()
 *         .compare(this.aString, that.aString)
 *         .compare(this.anInt, that.anInt)
 *         .compare(this.anEnum, that.anEnum, Ordering.natural().nullsLast())
 *         .result();
 *   }}
 * </pre>
 *
 * <p>
 * The value of this expression will have the same sign as the <i>first
 * nonzero</i> comparison result in the chain, or will be zero if every
 * comparison result was zero.
 *
 * <p>
 * Performance note: Even though the {@code ComparisonChain} caller always
 * invokes its {@code compare} methods unconditionally, the {@code
 * ComparisonChain} implementation stops calling its inputs'
 * {@link Comparable#compareTo compareTo} and {@link Comparator#compare compare}
 * methods as soon as one of them returns a nonzero result. This optimization is
 * typically important only in the presence of expensive {@code compareTo} and
 * {@code compare} implementations.
 *
 * <p>
 * See the Guava User Guide article on <a href=
 * "http://code.google.com/p/guava-libraries/wiki/CommonObjectUtilitiesExplained#compare/compareTo">
 * {@code ComparisonChain}</a>.
 *
 * @author Mark Davis
 * @author Kevin Bourrillion
 * @since 2.0
 */
@GwtCompatible
public abstract class ComparisonChain {
	private ComparisonChain() {
	}

	/**
	 * Begins a new chained comparison statement. See example in the class
	 * documentation.
	 */
	public static ComparisonChain start() {
		return ACTIVE;
	}

	private static final ComparisonChain ACTIVE = new ComparisonChain() {
		@SuppressWarnings("unchecked")
		@Override
		public ComparisonChain compare(Comparable left, Comparable right) {
			return classify(left.compareTo(right));
		}

		@Override
		public <T> ComparisonChain compare(@Nullable T left, @Nullable T right, Comparator<T> comparator) {
			return classify(comparator.compare(left, right));
		}

		@Override
		public ComparisonChain compare(int left, int right) {
			return classify(Ints.compare(left, right));
		}

		@Override
		public ComparisonChain compare(long left, long right) {
			return classify(Longs.compare(left, right));
		}

		@Override
		public ComparisonChain compare(float left, float right) {
			return classify(Float.compare(left, right));
		}

		@Override
		public ComparisonChain compare(double left, double right) {
			return classify(Double.compare(left, right));
		}

		@Override
		public ComparisonChain compareTrueFirst(boolean left, boolean right) {
			return classify(Booleans.compare(right, left)); // reversed
		}

		@Override
		public ComparisonChain compareFalseFirst(boolean left, boolean right) {
			return classify(Booleans.compare(left, right));
		}

		ComparisonChain classify(int result) {
			return (result < 0) ? LESS : (result > 0) ? GREATER : ACTIVE;
		}

		@Override
		public int result() {
			return 0;
		}
	};

	private static final ComparisonChain LESS = new InactiveComparisonChain(-1);

	private static final ComparisonChain GREATER = new InactiveComparisonChain(1);

	private static final class InactiveComparisonChain extends ComparisonChain {
		final int result;

		InactiveComparisonChain(int result) {
			this.result = result;
		}

		@Override
		public ComparisonChain compare(@Nullable Comparable left, @Nullable Comparable right) {
			return this;
		}

		@Override
		public <T> ComparisonChain compare(@Nullable T left, @Nullable T right, @Nullable Comparator<T> comparator) {
			return this;
		}

		@Override
		public ComparisonChain compare(int left, int right) {
			return this;
		}

		@Override
		public ComparisonChain compare(long left, long right) {
			return this;
		}

		@Override
		public ComparisonChain compare(float left, float right) {
			return this;
		}

		@Override
		public ComparisonChain compare(double left, double right) {
			return this;
		}

		@Override
		public ComparisonChain compareTrueFirst(boolean left, boolean right) {
			return this;
		}

		@Override
		public ComparisonChain compareFalseFirst(boolean left, boolean right) {
			return this;
		}

		@Override
		public int result() {
			return result;
		}
	}

	/**
	 * Compares two comparable objects as specified by {@link Comparable#compareTo},
	 * <i>if</i> the result of this comparison chain has not already been
	 * determined.
	 */
	public abstract ComparisonChain compare(Comparable<?> left, Comparable<?> right);

	/**
	 * Compares two objects using a comparator, <i>if</i> the result of this
	 * comparison chain has not already been determined.
	 */
	public abstract <T> ComparisonChain compare(@Nullable T left, @Nullable T right, Comparator<T> comparator);

	/**
	 * Compares two {@code int} values as specified by {@link Ints#compare},
	 * <i>if</i> the result of this comparison chain has not already been
	 * determined.
	 */
	public abstract ComparisonChain compare(int left, int right);

	/**
	 * Compares two {@code long} values as specified by {@link Longs#compare},
	 * <i>if</i> the result of this comparison chain has not already been
	 * determined.
	 */
	public abstract ComparisonChain compare(long left, long right);

	/**
	 * Compares two {@code float} values as specified by {@link Float#compare},
	 * <i>if</i> the result of this comparison chain has not already been
	 * determined.
	 */
	public abstract ComparisonChain compare(float left, float right);

	/**
	 * Compares two {@code double} values as specified by {@link Double#compare},
	 * <i>if</i> the result of this comparison chain has not already been
	 * determined.
	 */
	public abstract ComparisonChain compare(double left, double right);

	/**
	 * Compares two {@code boolean} values, considering {@code true} to be less than
	 * {@code false}, <i>if</i> the result of this comparison chain has not already
	 * been determined.
	 *
	 * @since 12.0
	 */
	public abstract ComparisonChain compareTrueFirst(boolean left, boolean right);

	/**
	 * Compares two {@code boolean} values, considering {@code false} to be less
	 * than {@code true}, <i>if</i> the result of this comparison chain has not
	 * already been determined.
	 *
	 * @since 12.0 (present as {@code compare} since 2.0)
	 */
	public abstract ComparisonChain compareFalseFirst(boolean left, boolean right);

	/**
	 * Ends this comparison chain and returns its result: a value having the same
	 * sign as the first nonzero comparison result in the chain, or zero if every
	 * result was zero.
	 */
	public abstract int result();
}