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Search code developed from 'Artificial Intelligence a Modern Approach', Prentice Hall.
There is a newer version: 0.9.117
/*
 * Copyright The Sett Ltd, 2005 to 2014.
 *
 * 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.thesett.aima.search;

import java.util.Iterator;

import com.thesett.aima.state.State;
import com.thesett.common.util.logic.UnaryPredicate;

/**
 * The Traversable interface defines methods for a state space that can be traversed by applying operators. Applying a
 * valid operation to a state results in a new state. An operation may have an associated cost.
 *
 * 
 * CRC Card Responsibilities
 * 
 Translate operations into new states
 * 
 Report the cost of an operation
 * 
 Enumerate the valid operations on a state
 * 
 Provide links to successor states
 * 
 *
 * @param   The type of the operator that this traversable uses to traverse states.
 *
 * @author Rupert Smith
 * @todo   State models the state and this class models the operators on a state that lead to new states. Consider
 *         making this not extend state but rather operate on states so that multiple traversers can be applied to the
 *         same state space under different sets of operators.
 */
public interface Traversable extends State
{
    /**
     * Returns the state obtained by applying the specified operation. If the operation is not valid then this may
     * return null. The effect of the operator on the child state does not have to be evaluated immediately. It
     * can be done when this method is called, or when the goal predicate is evaluated.
     *
     * @param  op The operator to apply to the traversable state.
     *
     * @return The new traversable state generated by applying the specified operator.
     */
    public Traversable getChildStateForOperator(Operator op);

    /**
     * Calculates the cost of applying the specified operations.
     *
     * @param  op The operator to apply.
     *
     * @return The cost of applying the specified operation.
     */
    public float costOf(Operator op);

    /**
     * Gets all operators valid from this state.
     *
     * @param  reverse When set, indicates that the successors should be presented in reverse order. This is only
     *                 necessary if the traversal cares about the ordering of the successor states, and is used to
     *                 generate intuitive, left-to-right goal checking in depth first search based searches.
     *
     * @return An iterator over all the operators that may be applied to this state.
     */
    public Iterator> validOperators(boolean reverse);

    /**
     * Gets an enumeration of successors of the state.
     *
     * @param  reverse When set, indicates that the successors should be presented in reverse order. This is only
     *                 necessary if the traversal cares about the ordering of the successor states, and is used to
     *                 generate intuitive, left-to-right goal checking in depth first search based searches.
     *
     * @return An iterator over all the {@link Successor}s that can be reached by applying valid operators to this
     *         state.
     */
    public Iterator> successors(boolean reverse);

    /**
     * Implementations may override this to provide different default goal predicates over the search space. If no
     * default is to be provided, null may be returned.
     *
     * @return The default goal predicate, or null if no default is provided. This implementation returns 
     *         null.
     */
    public UnaryPredicate getDefaultGoalPredicate();
}