/***********************************************************************************
 * @author  John Miller
 * @version 1.0
 * @date    Tue Feb  9 18:13:11 EST 2010
 */

import scala.collection.mutable.Set

/***********************************************************************************
 * This class provides a means for building a precedence/directed graph and checking
 * it for cycles.  For cycle detection, nodes are marked with traffic-light colors:
 * Green means go/unexplored, Yellow means caution/been there before, Red mean stop/
 * already fully explored.
 */
class PrecedenceGraph (nNodes: Int)
{
    /** Adjacency set representation of a directed graph
     */
    private val graph = new Array [Set [Int]] (nNodes)

    /** Nodes are marked with traffic-light colors ('G'reen, 'Y'ellow, 'R'ed)
     */
    private val color = new Array [Char] (nNodes)

    {
        for (i <- 0 until nNodes) { graph(i) = Set [Int] (); color(i) = 'G' }
    } // primary constructor

    /*******************************************************************************
     * Add an edge from node i to node j to the graph.
     * @param i  the source node
     * @param j  the target node
     */
    def addEdge (i: Int, j: Int)
    {
        graph(i) += j
    } // addEdge

    /*******************************************************************************
     * Determine whether the graph contains a cycle.
     */
    def hasCycle: Boolean = 
    {
       for (i <- 0 until nNodes if color(i) == 'G' && loopback (i)) return true 
       false
    } // hasCycle

    /*******************************************************************************
     * Search the decendents of node i to see if there is a loopback.
     * @param i  the node where the search starts
     */
    private def loopback (i: Int): Boolean =
    {
        if (color(i) == 'Y') return true
        color(i) = 'Y'
        for (j <- graph(i) if color(j) != 'R' && loopback (j)) return true
        color(i) = 'R'
        false
    } // loopback

    /*******************************************************************************
     * Convert the graph to a string.
     */
    override def toString: String = 
    {
        var s = "PrecedenceGraph ( "
        for (i <- 0 until nNodes) s += "< " + i + " --> " + graph(i) + " > "
        s + ")"
    } // toString
    
} // PrecedenceGraph class

/***********************************************************************************
 * Test the PrecedenceGraph class.
 */
object PrecedenceGraphTest extends Application
{
    val pg = new PrecedenceGraph (3)
    pg.addEdge (0, 1)
    pg.addEdge (0, 2)
    pg.addEdge (1, 2)
    pg.addEdge (2, 0)

    println ("pg = " + pg + "\nhas cycle = " + pg.hasCycle)
    
} // PrecedenceGraphTest