/*
 * Java Genetic Algorithm Library (jenetics-7.2.0).
 * Copyright (c) 2007-2023 Franz Wilhelmstötter
 *
 * 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.
 *
 * Author:
 *    Franz Wilhelmstötter (franz.wilhelmstoetter@gmail.com)
 */
package io.jenetics.internal.math;

import static java.lang.Double.isInfinite;
import static java.lang.Double.isNaN;
import static java.util.Objects.requireNonNull;
import static io.jenetics.internal.util.Hashes.hash;

import java.io.Serial;

/**
 * This class implements the the
 * <a href="http://en.wikipedia.org/wiki/Kahan_summation_algorithm">Kahan
 * summation algorithm</a>, which significantly reduces the numerical error when
 * adding double values.
 *
 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
 * @since 3.0
 * @version 3.0
 */
public final class DoubleAdder
        extends Number
        implements Comparable<DoubleAdder>
{
        @Serial
        private static final long serialVersionUID = 1L;

        private double _sum = 0.0;
        private double _simpleSum = 0.0;
        private double _compensation = 0.0;

        /**
         * Create a new adder with the given default {@code value}.
         *
         * @param value the initial {@code value} of this adder.
         */
        public DoubleAdder(final double value) {
                add(value);
        }

        /**
         * Create a new adder with the initial value of {@code 0.0}.
         */
        public DoubleAdder() {
        }

        /**
         * Reset the adder to the initial value of {@code 0.0}.
         *
         * @return {@code this} adder, for command chaining
         */
        private DoubleAdder reset() {
                _sum = 0.0;
                _simpleSum = 0.0;
                _compensation = 0.0;
                return this;
        }

        /**
         * Set the adder to the given {@code value}.
         *
         * @param value the new adder value
         * @return {@code this} adder, for command chaining
         */
        public DoubleAdder set(final double value) {
                return reset().add(value);
        }

        /**
         * Set the adder to the given {@code value}.
         *
         * @param value the new adder value
         * @return {@code this} adder, for command chaining
         * @throws java.lang.NullPointerException if the given {@code value} is
         *         {@code null}
         */
        public DoubleAdder set(final DoubleAdder value) {
                return reset().add(requireNonNull(value));
        }

        /**
         * Add the given {@code value} to this adder, using the
         * <a href="http://en.wikipedia.org/wiki/Kahan_summation_algorithm">Kahan
         * summation algorithm</a>
         *
         * @param value the {@code value} to add
         * @return {@code this} adder, for command chaining
         */
        public DoubleAdder add(final double value) {
                addWithCompensation(value);
                _simpleSum += value;
                return this;
        }

        /**
         * Add the given values to this adder.
         *
         * @param values the values to add.
         * @return {@code this} adder, for command chaining
         */
        public DoubleAdder add(final double[] values) {
                for (int i = values.length; --i >= 0;) {
                        add(values[i]);
                }

                return this;
        }

        private void addWithCompensation(final double value) {
                final double y = value - _compensation;
                final double t = _sum + y;
                _compensation = t - _sum - y;
                _sum = t;
        }

        /**
         * Add the given {@code value} to this adder, using the
         * <a href="http://en.wikipedia.org/wiki/Kahan_summation_algorithm">Kahan
         * summation algorithm</a>
         *
         * @param value the {@code value} to add
         * @return {@code this} adder, for command chaining
         * @throws java.lang.NullPointerException if the given {@code value} is
         *         {@code null}
         */
        public DoubleAdder add(final DoubleAdder value) {
                addWithCompensation(value._sum);
                addWithCompensation(value._compensation);
                _simpleSum += value._simpleSum;
                return this;
        }

        /**
         * Add the given {@code value} to this adder, using the
         * <a href="http://en.wikipedia.org/wiki/Kahan_summation_algorithm">Kahan
         * summation algorithm</a>
         *
         * @param other the {@code value} to add
         * @return {@code this} adder, for command chaining
         * @throws java.lang.NullPointerException if the given {@code value} is
         *         {@code null}
         */
        public DoubleAdder combine(final DoubleAdder other) {
                return add(other);
        }

        public double value() {
                final double result =  _sum + _compensation;
                return isNaN(result) && isInfinite(_simpleSum) ? _simpleSum : result;
        }

        @Override
        public int intValue() {
                return (int)value();
        }

        @Override
        public long longValue() {
                return (long)value();
        }

        @Override
        public float floatValue() {
                return (float)value();
        }

        @Override
        public double doubleValue() {
                return value();
        }

        @Override
        public int compareTo(final DoubleAdder other) {
                return Double.compare(doubleValue(), other.doubleValue());
        }

        /**
         * Check if {@code this} and the {@code other} {@code DoubleAdder} maintain
         * the same internal state.
         *
         * @param other the other object
         * @return {@code true} if {@code this} and the {@code other} adder have
         *         the same state, {@code false otherwise}
         */
        public boolean sameState(final DoubleAdder other) {
                return Double.compare(_sum, other._sum) == 0 &&
                        Double.compare(_simpleSum, other._simpleSum) == 0 &&
                        Double.compare(_compensation, other._compensation) == 0;
        }

        @Override
        public int hashCode() {
                return hash(doubleValue());
        }

        @Override
        public boolean equals(final Object obj) {
                return obj == this ||
                        obj instanceof DoubleAdder other &&
                        Double.compare(doubleValue(), other.doubleValue()) == 0;
        }

        @Override
        public String toString() {
                return Double.toString(doubleValue());
        }


        /* *************************************************************************
         * Some static helper methods.
         **************************************************************************/

        /**
         * Return the sum of the given double array.
         *
         * @param values the values to sum up.
         * @return the sum of the given {@code values}.
         * @throws NullPointerException if the given array is {@code null}.
         */
        public static double sum(final double[] values) {
                return new DoubleAdder().add(values).doubleValue();
        }
}