/*
 * Copyright 2007-2011 Tom Castle & Lawrence Beadle
 * Licensed under GNU Lesser General Public License
 * 
 * This file is part of EpochX: genetic programming software for research
 * 
 * EpochX is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * EpochX is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU Lesser General Public License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public License
 * along with EpochX. If not, see <http://www.gnu.org/licenses/>.
 * 
 * The latest version is available from: http://www.epochx.org
 */
package org.epochx.epox.math;

import org.epochx.epox.Node;
import org.epochx.tools.util.*;

/**
 * A function node which performs the mathematical operation of exponentiation.
 */
public class PowerFunction extends Node {

	/**
	 * Constructs a PowerFunction with two <code>null</code> children.
	 */
	public PowerFunction() {
		this(null, null);
	}

	/**
	 * Constructs a PowerFunction with two numerical child nodes. When
	 * evaluated, both children will be evaluated and the first will be raised
	 * to the power of the second.
	 * 
	 * @param base The first child node - the base.
	 * @param exponent The second child node - the exponent.
	 */
	public PowerFunction(final Node base, final Node exponent) {
		super(base, exponent);
	}

	/**
	 * Evaluating a <code>PowerFunction</code> involves raising the first child
	 * to the power of the second, after both children are evaluated. For
	 * performance, this function is evaluated lazily. The second child is
	 * evaluated first, if it evaluates to <code>0.0</code> then the result will
	 * always be <code>1.0</code> and the first child will not be evaluated at
	 * all.
	 */
	/**
	 * Evaluates this function lazily. The second child is evaluated. If its
	 * value is 0.0 then 1.0 will be returned without evaluating the first child
	 * at all. Otherwise the value of the first child will be raised to the
	 * power of the second child. The result is returned as a double value.
	 */
	@Override
	public Double evaluate() {
		final double c2 = NumericUtils.asDouble(getChild(1).evaluate());

		if (c2 == 0.0) {
			return 1.0;
		} else {
			final double c1 = NumericUtils.asDouble(getChild(0).evaluate());

			return Math.pow(c1, c2);
		}
	}

	/**
	 * Returns the identifier of this function which is POW.
	 */
	@Override
	public String getIdentifier() {
		return "POW";
	}

	/**
	 * Returns this function node's return type for the given child input types.
	 * If there are two input types of a numeric type then the return type will
	 * be Double. In all other cases this method will return <code>null</code>
	 * to indicate that the inputs are invalid.
	 * 
	 * @return the Double class or null if the input type is invalid.
	 */
	@Override
	public Class<?> getReturnType(final Class<?> ... inputTypes) {
		if ((inputTypes.length == 2) && TypeUtils.isAllNumericType(inputTypes)) {
			return Double.class;
		} else {
			return null;
		}
	}
}
