Fix LineSegment.project to handle segments projecting onto a single endpoint (#1179)

Author: dr-jts

modules/app/src/main/java/org/locationtech/jtstest/function/LineSegmentFunctions.java

@@ -116,4 +116,25 @@ public static Geometry reflectPoint(Geometry g1, Geometry g2)
     return g1.getFactory().createPoint(reflectPt);
   }
 
+  public static Geometry project(Geometry g1, Geometry g2)
+  {
+    LineSegment seg1 = toLineSegment(g1);
+    Coordinate[] line2 = g2.getCoordinates();
+    if (line2.length == 1) {
+      Coordinate pt = line2[0];
+      Coordinate result = seg1.project(pt); 
+      return g1.getFactory().createPoint(result);
+    }
+    LineSegment seg2 = new LineSegment(line2[0], line2[1]);
+    LineSegment result = seg1.project(seg2);
+    if (result == null)
+      return g1.getFactory().createLineString();
+    return result.toGeometry(g1.getFactory());
+  }
+
+  private static LineSegment toLineSegment(Geometry g) {
+    Coordinate[] line = g.getCoordinates();
+    return new LineSegment(line[0], line[1]);
+  }
+
 }

modules/core/src/main/java/org/locationtech/jts/geom/LineSegment.java

@@ -468,16 +468,30 @@ public LineSegment project(LineSegment seg)
     double pf0 = projectionFactor(seg.p0);
     double pf1 = projectionFactor(seg.p1);
     // check if segment projects at all
-    if (pf0 >= 1.0 && pf1 >= 1.0) return null;
-    if (pf0 <= 0.0 && pf1 <= 0.0) return null;
+    if (pf0 > 1.0 && pf1 > 1.0) return null;
+    if (pf0 < 0.0 && pf1 < 0.0) return null;
 
-    Coordinate newp0 = project(seg.p0, pf0);
-    if (pf0 < 0.0) newp0 = p0;
-    if (pf0 > 1.0) newp0 = p1;
+    Coordinate newp0;
+    if (pf0 < 0.0) {
+      newp0 = p0;
+    }
+    else if (pf0 > 1.0) {
+      newp0 = p1;
+    }
+    else {
+      newp0 = project(seg.p0, pf0);
+    }
 
-    Coordinate newp1 = project(seg.p1, pf1);
-    if (pf1 < 0.0) newp1 = p0;
-    if (pf1 > 1.0) newp1 = p1;
+    Coordinate newp1;
+    if (pf1 < 0.0) {
+      newp1 = p0;
+    }
+    else if (pf1 > 1.0) {
+      newp1 = p1;
+    }
+    else {
+      newp1 = project(seg.p1, pf1);
+    }
 
     return new LineSegment(newp0, newp1);
   }

modules/core/src/test/java/org/locationtech/jts/geom/LineSegmentTest.java

@@ -11,14 +11,17 @@
  */
 package org.locationtech.jts.geom;
 
-import junit.framework.TestCase;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
 import junit.textui.TestRunner;
+import test.jts.GeometryTestCase;
 
 
 /**
  * Test LineSegment methods
  */
-public class LineSegmentTest extends TestCase {
+public class LineSegmentTest extends GeometryTestCase {
 
   public static void main(String args[]) {
     TestRunner.run(LineSegmentTest.class);
@@ -53,6 +56,60 @@ public void testProjectionFactor()
     assertTrue(seg2.projectionFactor(new Coordinate(11, 0)) == 0.1);
   }
 
+  public void testProjectPoint() {
+    //-- interior point
+    checkProjectPoint("LINESTRING (4 0, 8 0)", "POINT (5 2)", 5, 0);
+    //-- endpoint
+    checkProjectPoint("LINESTRING (4 0, 8 0)", "POINT (8 2)", 8, 0);
+    //-- beyond end
+    checkProjectPoint("LINESTRING (4 0, 8 0)", "POINT (9 2)", 9, 0);
+    //-- before end
+    checkProjectPoint("LINESTRING (4 0, 8 0)", "POINT (3 2)", 3, 0);
+    //-- collinear
+    checkProjectPoint("LINESTRING (4 0, 8 0)", "POINT (2 0)", 2, 0);
+  }
+  
+  private void checkProjectPoint(String wkt1, String wkt2, double x, double y) {
+    LineSegment seg1 = readLineSegment(wkt1);
+    Point pt = (Point) read(wkt2);
+    Coordinate p = pt.getCoordinate();
+    Coordinate actual = seg1.project(p);
+    
+    checkEqualXY(new Coordinate(x, y), actual, 0.0001);
+  }
+
+  public void testProjectSegment() {
+    //-- project onto interior segment
+    checkProjectSegment("LINESTRING (0 0, 8 0)", "LINESTRING (1 2, 2 3)", "LINESTRING(1 0, 2 0)");
+    //-- project onto interior point
+    checkProjectSegment("LINESTRING (0 0, 8 0)", "LINESTRING (1 2, 1 4)", "LINESTRING(1 0, 1 0)");
+    //-- projection includes endpoint
+    checkProjectSegment("LINESTRING (0 0, 8 0)", "LINESTRING (0 2, 1 4)", "LINESTRING(0 0, 1 0)");
+    //- projection onto endpoint
+    checkProjectSegment("LINESTRING (0 0, 8 0)", "LINESTRING (8 2, 8 4)",  "LINESTRING(8 0, 8 0)");
+    checkProjectSegment("LINESTRING (0 0, 8 0)", "LINESTRING (0 2, 0 4)",  "LINESTRING(0 0, 0 0)");
+    checkProjectSegment("LINESTRING (0 0, 8 0)", "LINESTRING (0 2, -1 4)", "LINESTRING(0 0, 0 0)");
+    checkProjectSegment("LINESTRING (0 0, 8 0)", "LINESTRING (9 1, 8 0)",  "LINESTRING(8 0, 8 0)");
+    checkProjectSegment("LINESTRING (0 0, 8 0)", "LINESTRING (9 1, 8 1)",  "LINESTRING(8 0, 8 0)");
+    //-- no projection
+    checkProjectSegment("LINESTRING (0 0, 8 0)", "LINESTRING (9 1, 9 2)", null);
+  }
+
+  private void checkProjectSegment(String wkt1, String wkt2, String wktExpected) {
+    LineSegment seg1 = readLineSegment(wkt1);
+    LineSegment seg2 = readLineSegment(wkt2);
+    LineSegment actual = seg1.project(seg2);
+    
+    LineSegment expected = wktExpected == null ? null : readLineSegment(wktExpected);
+    checkEqual(expected, actual, 0.0001);
+  }
+  
+  private LineSegment readLineSegment(String wkt) {
+    Geometry g = read(wkt);
+    LineString line = (LineString) g;
+    return new LineSegment(line.getCoordinateN(0), line.getCoordinateN(1));
+  }
+
   public void testLineIntersection() {
     // simple case
     checkLineIntersection(

modules/core/src/test/java/test/jts/GeometryTestCase.java

@@ -233,7 +233,27 @@ protected void checkEqualXY(String message, Coordinate expected, Coordinate actu
     assertEquals(message + " X", expected.getX(), actual.getX(), tolerance);
     assertEquals(message + " Y", expected.getY(), actual.getY(), tolerance);
   }
- 
+  
+  protected void checkEqual(LineSegment expected, LineSegment actual, double tolerance) {
+    boolean equal;
+    if (actual == null || expected == null) {
+      equal = actual == null && expected == null;
+    }
+    else {
+      equal = isEqual(actual, expected, tolerance);
+    }
+    if (! equal) {
+      System.out.format(CHECK_EQUAL_FAIL_MSG, expected, actual );
+    }
+    assertTrue(equal);
+  }
+  
+  private boolean isEqual(LineSegment actual, LineSegment expected, double tolerance) {
+    return expected.getCoordinate(0).equals2D(actual.getCoordinate(0), tolerance)
+        && expected.getCoordinate(1).equals2D(actual.getCoordinate(1), tolerance);
+    
+  }
+
   protected void checkNoAlias(Geometry geom, Geometry geom2) {
     Geometry geom2Copy = geom2.copy();
     geom.apply(new CoordinateFilter() {