package de.intevation.flys.geom;

import java.awt.geom.Point2D;

public final class VectorUtils
{
    public static final double EPSILON = 1e-4;

    private VectorUtils() {
    }

    public static final double X(Point2D p) {
        return p.getX();
    }

    public static final double Y(Point2D p) {
        return p.getY();
    }

    public static final Point2D sub(Point2D a, Point2D b) {
        return new Point2D.Double(X(a)-X(b), Y(a)-Y(b));
    }

    public static final double dot(Point2D a, Point2D b) {
        return X(a)*X(b) + Y(a)*Y(b);
    }

    public static final Point2D add(Point2D a, Point2D b) {
        return new Point2D.Double(X(a)+X(b), Y(a)+Y(b));
    }

    public static final Point2D negate(Point2D a) {
        return new Point2D.Double(-X(a), -Y(a));
    }

    public static final Point2D ortho(Point2D a) {
        return new Point2D.Double(-Y(a), X(a));
    }

    public static final Point2D scale(Point2D a, double s) {
        return new Point2D.Double(s*X(a), s*Y(a));
    }

    public static final double lengthSq(Point2D a) {
        double x = X(a);
        double y = Y(a);
        return x*x + y*y;
    }

    public static final double length(Point2D a) {
        return Math.sqrt(lengthSq(a));
    }

    public static final Point2D normalize(Point2D a) {
        double length = length(a);
        return length != 0d
            ? scale(a, 1d/length)
            : new Point2D.Double(X(a), Y(a));
    }

    public static final double L1(Point2D a, Point2D b) {
        return Math.abs(X(a)-X(b)) + Math.abs(Y(a)-Y(b));
    }

    public static final boolean collinear(Point2D a, Point2D b, Point2D c) {
        double x1 = X(a);
        double y1 = Y(a);
        double x2 = X(b);
        double y2 = Y(b);
        double x3 = X(c);
        double y3 = Y(c);

        return Math.abs((x2-x1)*(y3-y1)-(y2-y1)*(x3-x1)) < EPSILON;
    }

    public static boolean epsilonEquals(Point2D a, Point2D b) {
        return Math.abs(X(a)-X(b)) < EPSILON
            && Math.abs(Y(a)-Y(b)) < EPSILON;
    }

    public static final Point2D intersection(
        Point2D p1, Point2D p2,
        Point2D p3, Point2D p4
    ) {
        double x1 = X(p1);
        double y1 = Y(p1);
        double x2 = X(p2);
        double y2 = Y(p2);
        double x3 = X(p3);
        double y3 = Y(p3);
        double x4 = X(p4);
        double y4 = Y(p4);

        // Compute a1, b1, c1, where line joining points 1 and 2
        // is "a1 x + b1 y + c1 = 0".
        double a1 = y2 - y1;
        double b1 = x1 - x2;
        double c1 = x2*y1 - x1*y2;

        // Compute r3 and r4.
        double r3 = a1*x3 + b1*y3 + c1;
        double r4 = a1*x4 + b1*y4 + c1;

        if (r3 != 0d && r4 != 0d && r3*r4 >= 0) {
            return null;
        }

        // Compute a2, b2, c2
        double a2 = y4 - y3;
        double b2 = x3 - x4;
        double c2 = (x4 * y3) - (x3 * y4);

        // Compute r1 and r2
        double r1 = a2*x1 + b2*y1 + c2;
        double r2 = a2*x2 + b2*y2 + c2;

        if (r1 != 0d && r2 != 0d && r1*r2 >= 0) {
            return null;
        }

        // Line segments intersect: compute intersection point.
        double denom = a1*b2 - a2*b1;

        if (denom == 0d) { // collinear
            return null;
        }

        double offset = Math.abs(denom)/2d;

        // The denom/2 is to get rounding instead of truncating. It
        // is added or subtracted to the numerator, depending upon the
        // sign of the numerator.
        double num = b1*c2 - b2*c1;

        double x = num < 0d
            ? (num - offset)/denom
            : (num + offset)/denom;

        num = a2*c1 - a1*c2;

        double y = num < 0d
            ? (num - offset)/denom
            : (num + offset)/denom;

        return new Point2D.Double(x, y);
    }

}
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