src/main/java/com/montecarlo/geometry/Triangle.java - master - shvidkuli/kkrjava

src/main/java/com/montecarlo/geometry/Triangle.java
99 строк3 KB
kroc400
first_commit
30 апр 2026, 22:44
30 апр 2026, 22:442347fdb
package com.montecarlo.geometry;

/**
 * A filled triangle defined by three vertices in 2D space.
 *
 * <p>Point containment is tested with the cross-product (half-plane) method:
 * a point lies inside (or on the boundary of) the triangle if all three
 * signed cross products {@code (edge × point)} have the same sign (or are zero).</p>
 */
public class Triangle implements Shape {

    private final Point p1;
    private final Point p2;
    private final Point p3;

    /**
     * Constructs a Triangle with the given three vertices.
     * Vertex order (clockwise or counterclockwise) does not matter.
     *
     * @param p1 first vertex
     * @param p2 second vertex
     * @param p3 third vertex
     */
    public Triangle(Point p1, Point p2, Point p3) {
        this.p1 = p1;
        this.p2 = p2;
        this.p3 = p3;
    }

    /** @return first vertex */
    public Point getP1() { return p1; }

    /** @return second vertex */
    public Point getP2() { return p2; }

    /** @return third vertex */
    public Point getP3() { return p3; }

    /**
     * Signed cross product of edge vector A→B and vector A→P.
     * Positive means P is to the left of A→B; negative means to the right.
     */
    private double cross(Point a, Point b, Point p) {
        return (b.getX() - a.getX()) * (p.getY() - a.getY())
             - (b.getY() - a.getY()) * (p.getX() - a.getX());
    }

    /**
     * Returns {@code true} if point {@code p} is inside or on the boundary of this triangle.
     *
     * <p>Uses the sign-consistency of cross products: a point is inside iff the signs of
     * cross(p1,p2,p), cross(p2,p3,p), cross(p3,p1,p) are all non-negative or all
     * non-positive (allowing zeros for boundary points).</p>
     *
     * @param p the point to test
     * @return {@code true} if inside or on the boundary
     */
    @Override
    public boolean contains(Point p) {
        double d1 = cross(p1, p2, p);
        double d2 = cross(p2, p3, p);
        double d3 = cross(p3, p1, p);
        boolean hasNeg = (d1 < 0) || (d2 < 0) || (d3 < 0);
        boolean hasPos = (d1 > 0) || (d2 > 0) || (d3 > 0);
        return !(hasNeg && hasPos);
    }

    /**
     * Returns the area of this triangle using the shoelace formula.
     *
     * @return non-negative area
     */
    @Override
    public double analyticalArea() {
        return 0.5 * Math.abs(
            (p2.getX() - p1.getX()) * (p3.getY() - p1.getY())
          - (p3.getX() - p1.getX()) * (p2.getY() - p1.getY())
        );
    }

    /**
     * Returns the smallest axis-aligned bounding box enclosing this triangle.
     *
     * @return the bounding box
     */
    @Override
    public BoundingBox getBoundingBox() {
        double minX = Math.min(p1.getX(), Math.min(p2.getX(), p3.getX()));
        double minY = Math.min(p1.getY(), Math.min(p2.getY(), p3.getY()));
        double maxX = Math.max(p1.getX(), Math.max(p2.getX(), p3.getX()));
        double maxY = Math.max(p1.getY(), Math.max(p2.getY(), p3.getY()));
        return new BoundingBox(minX, minY, maxX, maxY);
    }

    @Override
    public String toString() {
        return String.format("Triangle(%s, %s, %s)", p1, p2, p3);
    }
}