package prefuse.util.force;

/**
 * Represents a constant gravitational force, like the pull of gravity
 * for an object on the Earth (F = mg). The force experienced by a
 * given item is calculated as the product of a GravitationalConstant 
 * parameter and the mass of the item.
 *
 * @author <a href="http://jheer.org">jeffrey heer</a>
 */
public class GravitationalForce extends AbstractForce {

    private static final String[] pnames
        = { "GravitationalConstant", "Direction" };
    
    public static final int GRAVITATIONAL_CONST = 0;
    public static final int DIRECTION = 1;
    
    public static final float DEFAULT_FORCE_CONSTANT = 1E-4f;
    public static final float DEFAULT_MIN_FORCE_CONSTANT = 1E-5f;
    public static final float DEFAULT_MAX_FORCE_CONSTANT = 1E-3f;
    
    public static final float DEFAULT_DIRECTION = (float)-Math.PI/2;
    public static final float DEFAULT_MIN_DIRECTION = (float)-Math.PI;
    public static final float DEFAULT_MAX_DIRECTION = (float)Math.PI;
    
    /**
     * Create a new GravitationForce.
     * @param forceConstant the gravitational constant to use
     * @param direction the direction in which gravity should act,
     * in radians.
     */
    public GravitationalForce(float forceConstant, float direction) {
        params = new float[] { forceConstant, direction };
        minValues = new float[] 
            { DEFAULT_MIN_FORCE_CONSTANT, DEFAULT_MIN_DIRECTION };
        maxValues = new float[] 
            { DEFAULT_MAX_FORCE_CONSTANT, DEFAULT_MAX_DIRECTION };
    }
    
    /**
     * Create a new GravitationalForce with default gravitational
     * constant and direction.
     */
    public GravitationalForce() {
        this(DEFAULT_FORCE_CONSTANT, DEFAULT_DIRECTION);
    }
    
    /**
     * Returns true.
     * @see prefuse.util.force.Force#isItemForce()
     */
    public boolean isItemForce() {
        return true;
    }

    /**
     * @see prefuse.util.force.AbstractForce#getParameterNames()
     */
    protected String[] getParameterNames() {
        return pnames;
    }
    
    /**
     * @see prefuse.util.force.Force#getForce(prefuse.util.force.ForceItem)
     */
    public void getForce(ForceItem item) {
        float theta = params[DIRECTION];
        float coeff = params[GRAVITATIONAL_CONST]*item.mass;
        
        item.force[0] += Math.cos(theta)*coeff;
        item.force[1] += Math.sin(theta)*coeff;
    }

} // end of class GravitationalForce