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/* Alloy Analyzer 4 -- Copyright (c) 2006-2009, Felix Chang
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files
 * (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify,
 * merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
 * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF
 * OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

package edu.mit.csail.sdg.alloy4;

import java.util.ArrayList;
import java.util.IdentityHashMap;
import java.util.List;
import java.util.Map;

/**
 * Mutable; implements a directed graph; null node is allowed.
 * 

 * Note: it uses n1==n2 for comparing nodes rather than using n1.equals(n2)
 *
 * @param  - the type of node
 */

public final class DirectedGraph {

    /**
     * This substitutes for null nodes. This allows hasPath() method to use put() to
     * both insert and test membership in one step.
     */
    private static final Object            NULL          = new Object();

    /**
     * This field maps each node X to a list of "neighbor nodes" that X can reach by
     * following directed edges zero or more times.
     */
    private final Map> nodeToTargets = new IdentityHashMap>();

    /** Constructs an empty graph. */
    public DirectedGraph() {}

    /**
     * Add a directed edge from start node to end node (if there wasn't such an edge
     * already).
     */
    public void addEdge(N start, N end) {
        if (start == end)
            return;
        Object a = (start == null ? NULL : start);
        Object b = (end == null ? NULL : end);
        List targets = nodeToTargets.get(a);
        if (targets == null) {
            targets = new ArrayList();
            targets.add(b);
            nodeToTargets.put(a, targets);
        } else {
            for (int i = targets.size() - 1; i >= 0; i--)
                if (targets.get(i) == b)
                    return;
            targets.add(b);
        }
    }

    /**
     * Returns whether there is a directed path from start node to end node by
     * following directed edges 0 or more times (breath-first).
     */
    public boolean hasPath(N start, N end) {
        if (start == end)
            return true;
        Object a = (start == null ? NULL : start);
        Object b = (end == null ? NULL : end);
        List todo = new ArrayList();
        Map visited = new IdentityHashMap();
        // The correctness and guaranteed termination relies on following three
        // invariants:
        // (1) Every time we add X to "visited", we also simultaneously add X to
        // "todo".
        // (2) Every time we add X to "todo", we also simultaneously add X to
        // "visited".
        // (3) Nothing is ever removed.
        visited.put(a, a);
        todo.add(a);
        for (int k = 0; k < todo.size(); k++) { // use an integer loop since we
                                               // will be adding to the "todo"
                                               // list as we iterate
            List targets = nodeToTargets.get(todo.get(k));
            if (targets != null)
                for (int i = targets.size() - 1; i >= 0; i--) {
                    Object next = targets.get(i);
                    if (next == b) {
                        addEdge(start, end);
                        return true;
                    } // Cache so that later hasPath(start,end) returns true
                     // immediately
                    if (visited.put(next, next) == null)
                        todo.add(next);
                }
        }
        return false;
    }
}
    

    

    
            
    
            







    
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