path: root/src/de/lmu/ifi/dbs/elki/data/spatial/Polygon.java

package de.lmu.ifi.dbs.elki.data.spatial;

/*
 This file is part of ELKI:
 Environment for Developing KDD-Applications Supported by Index-Structures

 Copyright (C) 2014
 Ludwig-Maximilians-Universität München
 Lehr- und Forschungseinheit für Datenbanksysteme
 ELKI Development Team

 This program is free software: you can redistribute it and/or modify
 it under the terms of the GNU Affero General Public License as published by
 the Free Software Foundation, either version 3 of the License, or
 (at your option) any later version.

 This program is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 GNU Affero General Public License for more details.

 You should have received a copy of the GNU Affero General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

import java.util.Iterator;
import java.util.List;

import de.lmu.ifi.dbs.elki.math.linearalgebra.Vector;
import de.lmu.ifi.dbs.elki.utilities.datastructures.iterator.ArrayListIter;

/**
 * Class representing a simple polygon. While you can obviously store non-simple
 * polygons in this, note that many of the supplied methods will assume that the
 * polygons are simple.
 * 
 * @author Erich Schubert
 * 
 * @apiviz.composedOf Vector
 */
public class Polygon implements SpatialComparable {
  /**
   * The actual points
   */
  private List<Vector> points;

  /**
   * Minimum values
   */
  private double[] min = null;

  /**
   * Maximum values
   */
  private double[] max = null;

  /**
   * Constructor.
   * 
   * @param points Polygon points
   */
  public Polygon(List<Vector> points) {
    super();
    this.points = points;
    // Compute the bounds.
    if (points.size() > 0) {
      final Iterator<Vector> iter = points.iterator();
      final Vector first = iter.next();
      final int dim = first.getDimensionality();
      min = first.getArrayCopy();
      max = first.getArrayCopy();
      while (iter.hasNext()) {
        Vector next = iter.next();
        for (int i = 0; i < dim; i++) {
          final double cur = next.get(i);
          min[i] = Math.min(min[i], cur);
          max[i] = Math.max(max[i], cur);
        }
      }
    }
  }

  public Polygon(List<Vector> points, double minx, double maxx, double miny, double maxy) {
    super();
    this.points = points;
    this.min = new double[] { minx, miny };
    this.max = new double[] { maxx, maxy };
  }

  /**
   * Get an iterator to the vector contents.
   * 
   * @return Iterator
   */
  public ArrayListIter<Vector> iter() {
    return new ArrayListIter<>(points);
  }

  /**
   * Append the polygon to the buffer.
   * 
   * @param buf Buffer to append to
   */
  public void appendToBuffer(StringBuilder buf) {
    Iterator<Vector> iter = points.iterator();
    while (iter.hasNext()) {
      double[] data = iter.next().getArrayRef();
      for (int i = 0; i < data.length; i++) {
        if (i > 0) {
          buf.append(",");
        }
        buf.append(data[i]);
      }
      if (iter.hasNext()) {
        buf.append(" ");
      }
    }
  }

  @Override
  public String toString() {
    StringBuilder buf = new StringBuilder();
    appendToBuffer(buf);
    return buf.toString();
  }

  /**
   * Get the polygon length.
   * 
   * @return Polygon length
   */
  public int size() {
    return points.size();
  }

  /**
   * Get a vector by index.
   * 
   * @param idx Index to get
   * @return Vector
   */
  public Vector get(int idx) {
    return points.get(idx);
  }

  @Override
  public int getDimensionality() {
    return min.length;
  }

  @Override
  public double getMin(int dimension) {
    return min[dimension];
  }

  @Override
  public double getMax(int dimension) {
    return max[dimension];
  }

  /**
   * Test polygon orientation.
   * 
   * @return -1, 0, 1 for counterclockwise, undefined and clockwise.
   */
  public int testClockwise() {
    if (points.size() < 3) {
      return 0;
    }
    final int size = points.size();
    // Count the number of positive and negative oriented angles
    int c = 0;

    // TODO: faster when using an iterator?
    for (int i = 0; i < size; i++) {
      // Three consecutive points
      final int j = (i + 1) % size;
      final int k = (i + 2) % size;
      final double dxji = points.get(j).get(0) - points.get(i).get(0);
      final double dykj = points.get(k).get(1) - points.get(j).get(1);
      final double dyji = points.get(j).get(1) - points.get(i).get(1);
      final double dxkj = points.get(k).get(0) - points.get(j).get(0);
      final double z = (dxji * dykj) - (dyji * dxkj);
      if (z < 0) {
        c--;
      } else if (z > 0) {
        c++;
      }
    }
    if (c > 0) {
      return -1;
    } else if (c < 0) {
      return +1;
    } else {
      return 0;
    }
  }

  /**
   * Simple polygon intersection test.
   * 
   * <p>
   * FIXME: while this is found on some web pages as "solution" and satisfies or
   * needs, it clearly is not correct; not even for convex polygons: Consider a
   * cross where the two bars are made out of four vertices each. No vertex is
   * inside the other polygon, yet they intersect.
   * 
   * I knew this before writing this code, but this O(n) code was the simplest
   * thing to come up with, and it would work for our current data sets. A way
   * to fix this is to augment it with the obvious O(n*n) segment intersection
   * test. (Note that you will still need to test for point containment, since
   * the whole polygon could be contained in the other!)
   * </p>
   * 
   * @param other Other polygon
   * @return True when the polygons intersect
   */
  public boolean intersects2DIncomplete(Polygon other) {
    assert (this.getDimensionality() == 2);
    assert (other.getDimensionality() == 2);
    for (Vector v : this.points) {
      if (other.containsPoint2D(v)) {
        return true;
      }
    }
    for (Vector v : other.points) {
      if (this.containsPoint2D(v)) {
        return true;
      }
    }
    return false;
  }

  /**
   * Point in polygon test, based on
   * 
   * http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html
   * 
   * by W. Randolph Franklin
   * 
   * @param v Point to test
   * @return True when contained.
   */
  public boolean containsPoint2D(Vector v) {
    assert (v.getDimensionality() == 2);
    final double testx = v.get(0);
    final double testy = v.get(1);
    boolean c = false;

    Iterator<Vector> it = points.iterator();
    Vector pre = points.get(points.size() - 1);
    while (it.hasNext()) {
      final Vector cur = it.next();
      final double curx = cur.get(0);
      final double cury = cur.get(1);
      final double prex = pre.get(0);
      final double prey = pre.get(1);
      if (((cury > testy) != (prey > testy))) {
        if ((testx < (prex - curx) * (testy - cury) / (prey - cury) + curx)) {
          c = !c;
        }
      }
      pre = cur;
    }
    return c;
  }
}