LengthIndexOfPoint.java

/*
* The JTS Topology Suite is a collection of Java classes that
* implement the fundamental operations required to validate a given
* geo-spatial data set to a known topological specification.
*
* Copyright (C) 2001 Vivid Solutions
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library 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
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
*
* For more information, contact:
*
*     Vivid Solutions
*     Suite #1A
*     2328 Government Street
*     Victoria BC  V8T 5G5
*     Canada
*
*     (250)385-6040
*     www.vividsolutions.com
*/

package com.vividsolutions.jts.linearref;

import com.vividsolutions.jts.geom.*;
import com.vividsolutions.jts.util.Assert;

/**
 * Computes the length index of the point
 * on a linear {@link Geometry} nearest a given {@link Coordinate}.
 * The nearest point is not necessarily unique; this class
 * always computes the nearest point closest to
 * the start of the geometry.
 */
class LengthIndexOfPoint
{
  public static double indexOf(Geometry linearGeom, Coordinate inputPt)
  {
    LengthIndexOfPoint locater = new LengthIndexOfPoint(linearGeom);
    return locater.indexOf(inputPt);
  }

  public static double indexOfAfter(Geometry linearGeom, Coordinate inputPt, double minIndex)
  {
    LengthIndexOfPoint locater = new LengthIndexOfPoint(linearGeom);
    return locater.indexOfAfter(inputPt, minIndex);
  }

  private Geometry linearGeom;

  public LengthIndexOfPoint(Geometry linearGeom) {
    this.linearGeom = linearGeom;
  }

  /**
   * Find the nearest location along a linear {@link Geometry} to a given point.
   *
   * @param inputPt the coordinate to locate
   * @return the location of the nearest point
   */
  public double indexOf(Coordinate inputPt)
  {
    return indexOfFromStart(inputPt, -1.0);
  }

  /**
   * Finds the nearest index along the linear {@link Geometry}
   * to a given {@link Coordinate}
   * after the specified minimum index.
   * If possible the location returned will be strictly greater than the
   * <code>minLocation</code>.
   * If this is not possible, the
   * value returned will equal <code>minLocation</code>.
   * (An example where this is not possible is when
   * minLocation = [end of line] ).
   *
   * @param inputPt the coordinate to locate
   * @param minIndex the minimum location for the point location
   * @return the location of the nearest point
   */
  public double indexOfAfter(Coordinate inputPt, double minIndex)
  {
    if (minIndex < 0.0) return indexOf(inputPt);

    // sanity check for minIndex at or past end of line
    double endIndex = linearGeom.getLength();
    if (endIndex < minIndex)
      return endIndex;

    double closestAfter = indexOfFromStart(inputPt, minIndex);
    /**
     * Return the minDistanceLocation found.
     */
    Assert.isTrue(closestAfter >= minIndex,
                  "computed index is before specified minimum index");
    return closestAfter;
  }

  private double indexOfFromStart(Coordinate inputPt, double minIndex)
  {
    double minDistance = Double.MAX_VALUE;

    double ptMeasure = minIndex;
    double segmentStartMeasure = 0.0;
    LineSegment seg = new LineSegment();
    LinearIterator it = new LinearIterator(linearGeom);
    while (it.hasNext()) {
      if (! it.isEndOfLine()) {
        seg.p0 = it.getSegmentStart();
        seg.p1 = it.getSegmentEnd();
        double segDistance = seg.distance(inputPt);
        double segMeasureToPt = segmentNearestMeasure(seg, inputPt, segmentStartMeasure);
        if (segDistance < minDistance
            && segMeasureToPt > minIndex) {
          ptMeasure = segMeasureToPt;
          minDistance = segDistance;
        }
        segmentStartMeasure += seg.getLength();
      }
      it.next();
    }
    return ptMeasure;
  }

  private double segmentNearestMeasure(LineSegment seg, Coordinate inputPt,
                            double segmentStartMeasure)
  {
    // found new minimum, so compute location distance of point
    double projFactor = seg.projectionFactor(inputPt);
    if (projFactor <= 0.0)
      return segmentStartMeasure;
    if (projFactor <= 1.0)
      return segmentStartMeasure + projFactor * seg.getLength();
    // projFactor > 1.0
    return segmentStartMeasure + seg.getLength();
  }
}