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AIMA-Java Core Algorithms from the book Artificial Intelligence a Modern Approach 3rd Ed.
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package aima.core.environment.xyenv;

import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.Map;
import java.util.Set;

import aima.core.agent.Action;
import aima.core.agent.Agent;
import aima.core.agent.EnvironmentObject;
import aima.core.agent.EnvironmentState;
import aima.core.agent.Percept;
import aima.core.agent.impl.AbstractEnvironment;
import aima.core.agent.impl.DynamicPercept;
import aima.core.util.datastructure.XYLocation;

/**
 * @author Ravi Mohan
 * @author Ciaran O'Reilly
 */
public class XYEnvironment extends AbstractEnvironment {
	private XYEnvironmentState envState = null;

	//
	// PUBLIC METHODS
	//
	public XYEnvironment(int width, int height) {
		assert (width > 0);
		assert (height > 0);

		envState = new XYEnvironmentState(width, height);
	}

	/** Does nothing (don't ask me why...). */
	@Override
	public void executeAction(Agent a, Action action) {
	}

	@Override
	public Percept getPerceptSeenBy(Agent anAgent) {
		return new DynamicPercept();
	}

	public void addObjectToLocation(EnvironmentObject eo, XYLocation loc) {
		moveObjectToAbsoluteLocation(eo, loc);
	}

	public void moveObjectToAbsoluteLocation(EnvironmentObject eo, XYLocation loc) {
		// Ensure the object is not already at a location
		envState.moveObjectToAbsoluteLocation(eo, loc);

		// Ensure is added to the environment
		addEnvironmentObject(eo);
	}

	public void moveObject(EnvironmentObject eo, XYLocation.Direction direction) {
		XYLocation presentLocation = envState.getCurrentLocationFor(eo);

		if (null != presentLocation) {
			XYLocation locationToMoveTo = presentLocation.locationAt(direction);
			if (!(isBlocked(locationToMoveTo))) {
				moveObjectToAbsoluteLocation(eo, locationToMoveTo);
			}
		}
	}

	public XYLocation getCurrentLocationFor(EnvironmentObject eo) {
		return envState.getCurrentLocationFor(eo);
	}

	public Set getObjectsAt(XYLocation loc) {
		return envState.getObjectsAt(loc);
	}

	public Set getObjectsNear(Agent agent, int radius) {
		return envState.getObjectsNear(agent, radius);
	}

	public boolean isBlocked(XYLocation loc) {
		for (EnvironmentObject eo : envState.getObjectsAt(loc)) {
			if (eo instanceof Wall) {
				return true;
			}
		}
		return false;
	}

	public void makePerimeter() {
		for (int i = 0; i < envState.width; i++) {
			XYLocation loc = new XYLocation(i, 0);
			XYLocation loc2 = new XYLocation(i, envState.height - 1);
			envState.moveObjectToAbsoluteLocation(new Wall(), loc);
			envState.moveObjectToAbsoluteLocation(new Wall(), loc2);
		}

		for (int i = 0; i < envState.height; i++) {
			XYLocation loc = new XYLocation(0, i);
			XYLocation loc2 = new XYLocation(envState.width - 1, i);
			envState.moveObjectToAbsoluteLocation(new Wall(), loc);
			envState.moveObjectToAbsoluteLocation(new Wall(), loc2);
		}
	}
}

class XYEnvironmentState implements EnvironmentState {
	int width;
	int height;

	private Map> objsAtLocation = new LinkedHashMap>();

	public XYEnvironmentState(int width, int height) {
		this.width = width;
		this.height = height;
		for (int h = 1; h <= height; h++) {
			for (int w = 1; w <= width; w++) {
				objsAtLocation.put(new XYLocation(h, w), new LinkedHashSet());
			}
		}
	}

	public void moveObjectToAbsoluteLocation(EnvironmentObject eo, XYLocation loc) {
		// Ensure is not already at another location
		for (Set eos : objsAtLocation.values()) {
			if (eos.remove(eo)) {
				break; // Should only every be at 1 location
			}
		}
		// Add it to the location specified
		getObjectsAt(loc).add(eo);
	}

	public Set getObjectsAt(XYLocation loc) {
		Set objectsAt = objsAtLocation.get(loc);
		if (null == objectsAt) {
			// Always ensure an empty Set is returned
			objectsAt = new LinkedHashSet();
			objsAtLocation.put(loc, objectsAt);
		}
		return objectsAt;
	}

	public XYLocation getCurrentLocationFor(EnvironmentObject eo) {
		for (XYLocation loc : objsAtLocation.keySet()) {
			if (objsAtLocation.get(loc).contains(eo)) {
				return loc;
			}
		}
		return null;
	}

	public Set getObjectsNear(Agent agent, int radius) {
		Set objsNear = new LinkedHashSet();

		XYLocation agentLocation = getCurrentLocationFor(agent);
		for (XYLocation loc : objsAtLocation.keySet()) {
			if (withinRadius(radius, agentLocation, loc)) {
				objsNear.addAll(objsAtLocation.get(loc));
			}
		}
		// Ensure the 'agent' is not included in the Set of
		// objects near
		objsNear.remove(agent);

		return objsNear;
	}

	@Override
	public String toString() {
		return "XYEnvironmentState:" + objsAtLocation.toString();
	}

	//
	// PRIVATE METHODS
	//
	private boolean withinRadius(int radius, XYLocation agentLocation, XYLocation objectLocation) {
		int xdifference = agentLocation.getXCoOrdinate() - objectLocation.getXCoOrdinate();
		int ydifference = agentLocation.getYCoOrdinate() - objectLocation.getYCoOrdinate();
		return Math.sqrt((xdifference * xdifference) + (ydifference * ydifference)) <= radius;
	}
}