package cz.cuni.amis.pogamut.ut2004.agent.navigation.floydwarshall;
   
   import java.util.ArrayList;
   import java.util.Collection;
   import java.util.HashMap;
   import java.util.List;
   import java.util.Map;
   import java.util.logging.Level;
   import java.util.logging.Logger;
  
  import javax.vecmath.Point3d;
  
  import cz.cuni.amis.pogamut.base.agent.IGhostAgent;
  import cz.cuni.amis.pogamut.base.agent.module.SensorModule;
  import cz.cuni.amis.pogamut.base.agent.navigation.IPathFuture;
  import cz.cuni.amis.pogamut.base.agent.navigation.IPathPlanner;
  import cz.cuni.amis.pogamut.base.agent.navigation.impl.PrecomputedPathFuture;
  import cz.cuni.amis.pogamut.base.communication.worldview.event.IWorldEventListener;
  import cz.cuni.amis.pogamut.base3d.worldview.object.ILocated;
  import cz.cuni.amis.pogamut.base3d.worldview.object.IViewable;
  import cz.cuni.amis.pogamut.base3d.worldview.object.Location;
  import cz.cuni.amis.pogamut.unreal.communication.messages.UnrealId;
  import cz.cuni.amis.pogamut.ut2004.agent.module.sensor.NavigationGraphBuilder;
  import cz.cuni.amis.pogamut.ut2004.communication.messages.INavPoint;
  import cz.cuni.amis.pogamut.ut2004.communication.messages.gbinfomessages.Item;
  import cz.cuni.amis.pogamut.ut2004.communication.messages.gbinfomessages.NavPoint;
  import cz.cuni.amis.pogamut.ut2004.communication.messages.gbinfomessages.NavPointNeighbourLink;
  import cz.cuni.amis.pogamut.ut2004.communication.messages.gbinfomessages.Player;
  import cz.cuni.amis.pogamut.ut2004.communication.translator.shared.events.MapPointListObtained;
  import cz.cuni.amis.pogamut.ut2004.utils.LinkFlag;
  import cz.cuni.amis.utils.IFilter;
  import cz.cuni.amis.utils.collections.MyCollections;
  import cz.cuni.amis.utils.exception.PogamutException;
  
  /**
   * Private map using Floyd-Warshall for path-finding.
   * <p><p>
   * It should be initialized upon receiving {@link MapPointListObtained} event.
   * It precomputes all the paths inside the environment using Floyd-Warshall
   * algorithm (O(n^3)). Use getReachable(), getDistance(), getPath() to obtain
   * the info about the path.
   * <p><p>
   * If needed you may call {@link FloydWarshallMap#refreshPathMatrix()} to recompute Floyd-Warshall. Especially useful
   * if you're using {@link NavigationGraphBuilder} to modify the underlying navpoints/edges.
   * <p><p>
   * Based upon the implementation from Martin Krulis with his kind consent -
   * Thank you!
   * <p><p>
   * NOTE: requires O(navpoints.size^3) of memory ~ which is 10000^3 at max for
   * UT2004 (usually the biggest maps have 3000 navpoints max, but small maps, e.g., DM-1on1-Albatross has 200 navpoints).
   * <p><p>
   * Because the algorithm is so space-hungery-beast, there is option to disable it
   * by calling {@link FloydWarshallMap#setEnabled(boolean)} with 'false' argument. See the method for
   * further documentation about the object behavior. 
   * 
   * @author Martin Krulis
   * @author Jimmy
   */
  public class FloydWarshallMap extends SensorModule<IGhostAgent> implements IPathPlanner<NavPoint> {
  
  	public static class PathMatrixNode {
  
  		private float distance = Float.POSITIVE_INFINITY;
  		private Integer viaNode = null;
  		private List<NavPoint> path = null;
  
  		/**
  		 * Doesn't leading anywhere
  		 */
  		public PathMatrixNode() {
  		}
  
  		public PathMatrixNode(float distance) {
  			this.distance = distance;
  		}
  
  		public float getDistance() {
  			return distance;
  		}
  
  		public void setDistance(float distance) {
  			this.distance = distance;
  		}
  
  		/**
  		 * Returns indice.
  		 * 
  		 * @return
  		 */
  		public Integer getViaNode() {
  			return viaNode;
  		}
  
  		public void setViaNode(Integer indice) {
  			this.viaNode = indice;
  		}
  
  		public List<NavPoint> getPath() {
  			return path;
 		}
 
 		public void setPath(List<NavPoint> path) {
 			this.path = path;
 		}
 
 	}
 
 	private IWorldEventListener<MapPointListObtained> mapListener = new IWorldEventListener<MapPointListObtained>() {
 
 		@Override
 		public void notify(MapPointListObtained event) {
 			if (log.isLoggable(Level.INFO)) log.info("Map point list obtained.");
 			performFloydWarshall(event);
 		}
 	};
 
 	/**
 	 * Flag mask representing unusable edge.
 	 */
 	public static final int BAD_EDGE_FLAG = LinkFlag.FLY.get()
 			| LinkFlag.LADDER.get() | LinkFlag.PROSCRIBED.get()
 			| LinkFlag.SWIM.get() | LinkFlag.PLAYERONLY.get();
 	
 	public static boolean isWalkable(int flag) {
 		return ((flag & BAD_EDGE_FLAG) == 0) && ((flag & LinkFlag.SPECIAL.get()) == 0); 
 	}
 
 	/**
 	 * Prohibited edges.
 	 */
 	protected int badEdgeFlag = 0;
 
 	/**
 	 * Hash table converting navPoint IDs to our own indices.
 	 */
 	protected Map<UnrealId, Integer> navPointIndices;
 
 	/**
 	 * Mapping indices to nav points.
 	 * <p>
 	 * <p>
 	 * WILL BE NULL AFTER CONSTRUCTION! SERVES AS A TEMPORARY "GLOBAL VARIABLE"
 	 * DURING FLOYD-WARSHALL COMPUTATION AND PATH RECONSTRUCTION.
 	 */
 	protected Map<Integer, NavPoint> indicesNavPoints;
 
 	// Warshall's matrix of distances.
 	protected PathMatrixNode[][] pathMatrix;
 	
 	/**
 	 * Whether the this object is enabled and the path is going to be actually computed.
 	 * <p><p>
 	 * Enabled as default. 
 	 */
 	private boolean enabled = true;
 	
 	/**
 	 * Synchronizing access to object with respect to {@link FloydWarshallMap#enabled}.
 	 */
 	protected Object mutex = new Object();
 
 	public FloydWarshallMap(IGhostAgent bot) {
 		this(bot, BAD_EDGE_FLAG, null);
 	}
 
 	public FloydWarshallMap(IGhostAgent bot, Logger log) {
 		this(bot, BAD_EDGE_FLAG, log);
 	}
 
 	public FloydWarshallMap(IGhostAgent bot, int badEdgeFlag, Logger log) {
 		super(bot, log);
 		this.badEdgeFlag = badEdgeFlag;
 		worldView.addEventListener(MapPointListObtained.class, mapListener);
 	}
 	
 	/**
 	 * Whether the object is active, see {@link FloydWarshallMap#setEnabled(boolean)} for more documentation.
 	 * <p><p>
 	 * Default: true
 	 * 
 	 * @return
 	 */
 	public boolean isEnabled() {
 		return enabled;
 	}
 
 	/**
 	 * Enables/disables object. As default, object is initialized as 'enabled'. 
 	 * <p><p>
 	 * If you "disable" the object (passing 'false' as an argument), it will {@link FloydWarshallMap#cleanUp()} itself
 	 * dropping any info it has about paths, i.e., method {@link FloydWarshallMap#computePath(NavPoint, NavPoint)} will start throwing exceptions
 	 * at you.
 	 * <p><p>
 	 * Note that if you "enable" the object (passing 'true' as an argument), it won't AUTOMATICALLY trigger the computation of the algorithm,
 	 * you should manually {@link FloydWarshallMap#refreshPathMatrix()} when it is appropriate (unless you enable it before list of navpoints
 	 * is received, in that case the path will get computed automatically).
 	 * 
 	 * @param enabled
 	 */
 	public void setEnabled(boolean enabled) {
 		synchronized(mutex) {
 			this.enabled = enabled;
 			if (!enabled) {
 				cleanUp();
 			}
 		}
 	}
 	
 	
 	/**
 	 * Force FloydWarshall to run again, useful if you modify navpoints using {@link NavigationGraphBuilder}.
 	 */
 	public void refreshPathMatrix() {
 		synchronized(mutex) {
 			if (!enabled) {
 				if (log.isLoggable(Level.WARNING)) log.fine(this + ": Won't refresh path matrix, object is disabled.");
 				return;
 			}
 			if (log.isLoggable(Level.FINE)) log.fine(this + ": Refreshing path matrix...");
 			List<NavPoint> navPoints = MyCollections.asList(agent.getWorldView().getAll(NavPoint.class).values());
 			performFloydWarshall(navPoints);
 			if (log.isLoggable(Level.INFO)) log.info(this + ": Path matrix refreshed!");
 		}		
 	}
 	
 	protected void performFloydWarshall(MapPointListObtained map) {
 		List<NavPoint> navPoints = MyCollections.asList(map.getNavPoints().values());
 		performFloydWarshall(navPoints);
 	}
 
 	protected void performFloydWarshall(List<NavPoint> navPoints) {
 		if (!enabled) {
 			if (log.isLoggable(Level.WARNING)) log.fine(this + ": Should not be running Floyd-Warshall, object disabled.");
 			return;
 		}
 		if (log.isLoggable(Level.FINE)) log.fine(this + ": Beginning Floyd-Warshall algorithm...");
 		long start = System.currentTimeMillis();
 
 		// prepares data structures
 		int size = navPoints.size();
 		navPointIndices = new HashMap<UnrealId, Integer>(size);
 		indicesNavPoints = new HashMap<Integer, NavPoint>(size);
 		pathMatrix = new PathMatrixNode[size][size];
 
 		// Initialize navPoint indices mapping.
 		for (int i = 0; i < navPoints.size(); ++i) {
 			navPointIndices.put(navPoints.get(i).getId(), i);
 			indicesNavPoints.put(i, navPoints.get(i));
 		}
 
 		// Initialize distance matrix.
 		for (int i = 0; i < size; i++) {
 			for (int j = 0; j < size; j++) {
 				pathMatrix[i][j] = new PathMatrixNode((i == j) ? 0.0f
 						: Float.POSITIVE_INFINITY);
 			}
 		}
 
 		// Set edge lengths into distance matrix.
 		for (int i = 0; i < size; i++) {
 			Point3d location = navPoints.get(i).getLocation().getPoint3d();
 			for (NavPointNeighbourLink link : navPoints.get(i)
 					.getOutgoingEdges().values()) {
 				if (checkLink(link)) {
 					pathMatrix[i][navPointIndices.get(link.getToNavPoint()
 							.getId())].setDistance((float) location
 							.distance(link.getToNavPoint().getLocation()
 									.getPoint3d()));
 				}
 			}
 		}
 
 		// Perform Floyd-Warshall.
 		for (int k = 0; k < size; k++) {
 			for (int i = 0; i < size; i++) {
 				for (int j = 0; j < size; j++) {
 					float newLen = pathMatrix[i][k].getDistance()
 							+ pathMatrix[k][j].getDistance();
 					if (pathMatrix[i][j].getDistance() > newLen) {
 						pathMatrix[i][j].setDistance(newLen);
 						pathMatrix[i][j].setViaNode(k);
 					}
 				}
 			}
 		}
 
 		// Construct paths + log unreachable paths.
 		int count = 0;
 		for (int i = 0; i < size; i++) {
 			for (int j = 0; j < size; j++) {
 				if (pathMatrix[i][j].getDistance() == Float.POSITIVE_INFINITY) {
 					if (log.isLoggable(Level.FINER)) log.warning("Unreachable path from " + navPoints.get(i).getId().getStringId()
 							+ " -> " + navPoints.get(j).getId().getStringId());
 					count++;
 				} else {
 					// path exists ... retrieve it
 					pathMatrix[i][j].setPath(retrievePath(i, j));
 				}
 			}
 		}
 
 		if (count > 0) {
 			if (log.isLoggable(Level.WARNING)) log.warning(this + ": There are " + count + " unreachable nav point pairs (if you wish to see more, set logging to Level.FINER).");
 		}
 
 		if (log.isLoggable(Level.INFO)) log.info(this + ": computation for " + size + " navpoints took " + (System.currentTimeMillis() - start) + " millis");
 
 		// null unneeded field to free some memory
 		indicesNavPoints = null;
 	}
 
 	/**
 	 * Checks whether the edge is usable.
 	 * 
 	 * @param from
 	 *            Starting nav point.
 	 * @param edge
 	 *            NeighNav object representing the edge.
 	 * @return boolean
 	 */
 	public boolean checkLink(NavPointNeighbourLink edge) {
 		// Bad flags (prohibited edges, swimming, flying...).
 		if ((edge.getFlags() & badEdgeFlag) != 0)
 			return false;
 
 		// Lift flags.
 		if ((edge.getFlags() & LinkFlag.SPECIAL.get()) != 0)
 			return true;
 
 		// Check whether the climbing angle is not so steep.
 //		if ((edge.getFromNavPoint().getLocation().getPoint3d().distance(
 //				edge.getToNavPoint().getLocation().getPoint3d()) < (edge
 //				.getToNavPoint().getLocation().z - edge.getFromNavPoint()
 //				.getLocation().z))
 //				&& (edge.getFromNavPoint().getLocation().getPoint3d().distance(
 //						edge.getToNavPoint().getLocation().getPoint3d()) > 100)) {
 //			return false;
 //		}
 
 		// Check whether the jump is not so high.
 //		if (((edge.getFlags() & LinkFlag.JUMP.get()) != 0)
 //				&& (edge.getToNavPoint().getLocation().z
 //						- edge.getFromNavPoint().getLocation().z > 80)) {
 //			return false;
 //		}
                 //Check whether there is NeededJump attribute set - this means the bot has to 
                 //provide the jump himself - if the Z of the jump is too high it means he
                 //needs to rocket jump or impact hammer jump - we will erase those links
                 //as our bots are not capable of this
                 if (edge.getNeededJump() != null && edge.getNeededJump().z > 680) {
                     return false;
                 }
 
 		return true;
 	}
 
 	/**
 	 * Sub-routine of retrievePath - do not use! ... Well you may, it returns
 	 * path without 'from', 'to' or null if such path dosn't exist.
 	 * <p>
 	 * <p>
 	 * DO NOT USE OUTSIDE CONSTRUCTOR (relies on indicesNavPoints).
 	 * 
 	 * @param from
 	 * @param to
 	 * @return
 	 */
 	private List<NavPoint> retrievePathInner(Integer from, Integer to) {
 		PathMatrixNode node = pathMatrix[from][to];
 		if (node.getDistance() == Float.POSITIVE_INFINITY)
 			return null;
 		if (node.getViaNode() == null) {
 			return new ArrayList<NavPoint>(0);
 		}
 		if (node.getViaNode() == null)
 			return new ArrayList<NavPoint>(0);
 
 		List<NavPoint> path = new ArrayList<NavPoint>();
 		path.addAll(retrievePathInner(from, node.getViaNode()));
 		path.add(indicesNavPoints.get(node.getViaNode()));
 		path.addAll(retrievePathInner(node.getViaNode(), to));
 
 		return path;
 	}
 
 	/**
 	 * Returns path between from-to or null if path doesn't exist. Path begins
 	 * with 'from' and ends with 'to'.
 	 * <p>
 	 * <p>
 	 * DO NOT USE OUTSIDE CONSTRUCTOR (relies on indicesNavPoints).
 	 * 
 	 * @param from
 	 * @param to
 	 * @return
 	 */
 	private List<NavPoint> retrievePath(Integer from, Integer to) {
 		List<NavPoint> path = new ArrayList<NavPoint>();
 		path.add(indicesNavPoints.get(from));
 		path.addAll(retrievePathInner(from, to));
 		path.add(indicesNavPoints.get(to));
 		return path;
 	}
 
 	protected PathMatrixNode getPathMatrixNode(NavPoint np1, NavPoint np2) {
 		return pathMatrix[navPointIndices.get(np1.getId())][navPointIndices
 				.get(np2.getId())];
 	}
 
 	/**
 	 * Whether navpoint 'to' is reachable from the navpoint 'from'.
 	 * <p><p>
 	 * Throws exception if object is disabled, see {@link FloydWarshallMap#setEnabled(boolean)}. Note that the object
 	 * is enabled by default.
 	 * 
 	 * @param from
 	 * @param to
 	 * @return
 	 */
 	public boolean reachable(NavPoint from, NavPoint to) {
 		if ((from == null) || (to == null))
 			return false;
 		return getPathMatrixNode(from, to).getDistance() != Float.POSITIVE_INFINITY;
 	}
 
 	/**
 	 * Calculate's distance between two nav points (using pathfinding).
 	 * <p><p>
 	 * Throws exception if object is disabled, see {@link FloydWarshallMap#setEnabled(boolean)}. Note that the object
 	 * is enabled by default.
 	 * 
 	 * @return Distance or POSITIVE_INFINITY if there's no path.
 	 */
 	public float getDistance(NavPoint from, NavPoint to) {
 		if ((from == null) || (to == null))
 			return Float.POSITIVE_INFINITY;
 		return getPathMatrixNode(from, to).getDistance();
 	}
 
 	/**
 	 * Returns path between navpoints 'from' -> 'to'. The path begins with
 	 * 'from' and ends with 'to'. If such path doesn't exist, returns null.
 	 * <p><p>
 	 * Throws exception if object is disabled, see {@link FloydWarshallMap#setEnabled(boolean)}. Note that the object
 	 * is enabled by default.
 	 * 
 	 * @param from
 	 * @param to
 	 * @return
 	 */
 	public List<NavPoint> getPath(NavPoint from, NavPoint to) {
 		synchronized(mutex) {
 			if (!enabled) {
 				throw new PogamutException("Can't return path as the object is disabled, call .setEnabled(true) & .refreshPathMatrix() first!", log, this);
 			}
 			if ((from == null) || (to == null))
 				return null;
 			if (log.isLoggable(Level.FINE)) log.fine("Retrieving path: " + from.getId().getStringId() + "[" + from.getLocation() + "] -> " + to.getId().getStringId() + "[" + to.getLocation() + "]");
 			List<NavPoint> path = getPathMatrixNode(from, to).getPath();
 			if (path == null) {
 				if (log.isLoggable(Level.WARNING)) log.warning("PATH NOT EXIST: " + from.getId().getStringId() + "[" + from.getLocation() + "] -> " + to.getId().getStringId() + "[" + to.getLocation() + "]");
 			} else {
 				if (log.isLoggable(Level.FINE)) log.fine("Path exists - " + path.size() + " navpoints.");
 			}
 			return path;
 		}
 	}
 
 	@Override
 	protected void cleanUp() {
 		super.cleanUp();
 		pathMatrix = null;
 		navPointIndices = null;
 	}
 	
 	@Override
 	public String toString() {
 		return "FloydWarshallMap";
 	}
 
 	/**
 	 * Returns path between navpoints 'from' -> 'to'. The path begins with
 	 * 'from' and ends with 'to'. If such path does not exist, it returns zero-sized path.
 	 * <p><p>
 	 * Throws exception if object is disabled, see {@link FloydWarshallMap#setEnabled(boolean)}. Note that the object
 	 * is enabled by default.
 	 * 
 	 * @param from
 	 * @param to
 	 * @return
 	 */
 	@Override
 	public IPathFuture<NavPoint> computePath(NavPoint from, NavPoint to) {
 		return new PrecomputedPathFuture<NavPoint>(from, to, getPath(from, to));
 	}
 	
 	// ===============================
 	// PATH-DISTANCE FILTERING METHODS
 	// ===============================
 	
 	/**
      * Returns the nearest target (distance == path distance between 'from' and 'target').
      * <p><p>
      * WARNING: O(n) complexity!
      * 
      * @param <T>
      * @param locations
      * @param target
      * @return nearest object from collection of objects
      */
     public <T extends NavPoint> T getNearestNavPoint(Collection<T> locations, T target) {
     	if (locations == null) return null;
     	if (target == null) return null;
         T nearest = null;
         double minDistance = Double.MAX_VALUE;
         double d;
         for(T l : locations) {
         	if (l.getLocation() == null) continue;
             d = getDistance(target, l);
             if(d < minDistance) {
                 minDistance = d;
                 nearest = l;
             }
         }
         return nearest;
     }
     
     /**
      * Returns the nearest target (distance == path distance between 'from' and 'target') that is not further than 'maxDistance'.
      * <p><p>
      * WARNING: O(n) complexity!
      * 
      * @param <T>
      * @param locations
      * @param target
      * @param maxDistance
      * @return nearest object from collection of objects that is not further than 'maxDistance'.
      */
     public <T extends NavPoint> T getNearestNavPoint(Collection<T> locations, NavPoint target, double maxDistance) {
     	if (locations == null) return null;
     	if (target == null) return null;
         T nearest = null;
         double minDistance = Double.MAX_VALUE;
         double d;
         for(T l : locations) {
         	d = getDistance(target, l);
             if (d > maxDistance) continue;
             if(d < minDistance) {
                 minDistance = d;
                 nearest = l;
             }
         }
         return nearest;
     }
     
     /**
      * Returns the nearest target (distance == path distance between 'from' and 'target').
      * <p><p>
      * WARNING: O(n) complexity!
      * 
      * @param <T>
      * @param locations
      * @param target
      * @return nearest object from collection of objects
      */
     public <T extends NavPoint> T getNearestFilteredNavPoint(Collection<T> locations, NavPoint target, IFilter<T> filter) {
     	if (locations == null) return null;
     	if (target == null) return null;
         T nearest = null;        
         double minDistance = Double.MAX_VALUE;
         double d;
         for(T l : locations) {
         	if (!filter.isAccepted(l)) continue;
             d = getDistance(target, l);
             if(d < minDistance) {
                 minDistance = d;
                 nearest = l;
             }
         }
         return nearest;
     }
 
     /**
      * Returns the second nearest target (distance == path distance between 'from' and 'target').
      * <p><p>
      * WARNING: O(n) complexity!
      * 
      * @param <T>
      * @param locations
      * @param target
      * @return nearest object from collection of objects
      */
     public <T extends NavPoint> T getSecondNearestNavPoint(Collection<T> locations, NavPoint target) {
     	if (locations == null) return null;
     	if (target == null) return null;
     	T secondNearest = null;
         T nearest = null;
         double closestDistance = Double.MAX_VALUE;
         double secondClosestDistance = Double.MAX_VALUE;
 
         for (T l : locations) {
         	double distance = getDistance(target, l);
             if (distance < closestDistance) {
                 secondClosestDistance = closestDistance;
                 secondNearest = nearest;
 
                 closestDistance = distance;
                 nearest = l;
             } else {
                 if(distance < secondClosestDistance) {
                     secondClosestDistance = distance;
                     secondNearest = l;
                 }
             }
         }
         return secondNearest;
     }
     
     /**
      * Returns the nearest target (distance == path distance between 'from' and 'target').
      * <p><p>
      * WARNING: O(n) complexity!
      * 
      * @param <T>
      * @param locations
      * @param target
      * @return nearest object from collection of objects
      */
     public <T extends Item> T getNearestItem(Collection<T> locations, NavPoint target) {
     	if (locations == null) return null;
     	if (target == null) return null;
         T nearest = null;
         double minDistance = Double.MAX_VALUE;
         double d;
         for(T l : locations) {
         	if (l.getLocation() == null) continue;
             d = getDistance(target, l.getNavPoint());
             if(d < minDistance) {
                 minDistance = d;
                 nearest = l;
             }
         }
         return nearest;
     }
     
     /**
      * Returns the nearest target (distance == path distance between 'from' and 'target') that is not further than 'maxDistance'.
      * <p><p>
      * WARNING: O(n) complexity!
      * 
      * @param <T>
      * @param locations
      * @param target
      * @param maxDistance
      * @return nearest object from collection of objects that is not further than 'maxDistance'.
      */
     public <T extends Item> T getNearestItem(Collection<T> locations, NavPoint target, double maxDistance) {
     	if (locations == null) return null;
     	if (target == null) return null;
         T nearest = null;
         double minDistance = Double.MAX_VALUE;
         double d;
         for(T l : locations) {
         	d = getDistance(target, l.getNavPoint());
             if (d > maxDistance) continue;
             if(d < minDistance) {
                 minDistance = d;
                 nearest = l;
             }
         }
         return nearest;
     }
     
     /**
      * Returns the nearest target (distance == path distance between 'from' and 'target').
      * <p><p>
      * WARNING: O(n) complexity!
      * 
      * @param <T>
      * @param locations
      * @param target
      * @return nearest object from collection of objects
      */
     public <T extends Item> T getNearestFilteredItem(Collection<T> locations, NavPoint target, IFilter<T> filter) {
     	if (locations == null) return null;
     	if (target == null) return null;
         T nearest = null;        
         double minDistance = Double.MAX_VALUE;
         double d;
         for(T l : locations) {
         	if (!filter.isAccepted(l)) continue;
             d = getDistance(target, l.getNavPoint());
             if(d < minDistance) {
                 minDistance = d;
                 nearest = l;
             }
         }
         return nearest;
     }
 
     /**
      * Returns the second nearest target (distance == path distance between 'from' and 'target').
      * <p><p>
      * WARNING: O(n) complexity!
      * 
      * @param <T>
      * @param targets
      * @param from
      * @return nearest object from collection of objects
      */
     public <T extends Item> T getSecondNearestItem(Collection<T> targets, NavPoint from) {
     	if (targets == null) return null;
     	if (from == null) return null;
     	T secondNearest = null;
         T nearest = null;
         double closestDistance = Double.MAX_VALUE;
         double secondClosestDistance = Double.MAX_VALUE;
 
         for (T l : targets) {
         	double distance = getDistance(from, l.getNavPoint());
             if (distance < closestDistance) {
                 secondClosestDistance = closestDistance;
                 secondNearest = nearest;
 
                 closestDistance = distance;
                 nearest = l;
             } else {
                 if(distance < secondClosestDistance) {
                     secondClosestDistance = distance;
                     secondNearest = l;
                 }
             }
         }
         return secondNearest;
     }
     
 }