Fix ExactMaxInscribedCircle to handle collinear input

Author: dr-jts

modules/core/src/main/java/org/locationtech/jts/algorithm/Angle.java

@@ -185,6 +185,7 @@ public static double angleBetweenOriented(Coordinate tip1, Coordinate tail,
    * Computes the angle of the unoriented bisector 
    * of the smallest angle between two vectors.
    * The computed angle will be in the range (-Pi, Pi].
+   * Collinear inputs are handled.
    * 
    * @param tip1 the tip of v1
    * @param tail the tail of each vector

modules/core/src/main/java/org/locationtech/jts/algorithm/construct/ExactMaxInscribedCircle.java

@@ -163,8 +163,11 @@ private static LineSegment computeConvexBisector(Coordinate[] pts, int index, do
     int iNext = index >= pts.length ? 0 : index + 1;
     Coordinate pPrev = pts[iPrev];
     Coordinate pNext = pts[iNext];
-    if (! isConvex(pPrev, basePt, pNext))
-      throw new IllegalArgumentException("Input is not convex");
+    
+    //-- this should never happen, since only convex quads are handled
+    if (isConcave(pPrev, basePt, pNext)) 
+      throw new IllegalStateException("Input is not convex");
+    
     double bisectAng = Angle.bisector(pPrev, basePt, pNext);
     Coordinate endPt = Angle.project(basePt, bisectAng, len);
     return new LineSegment(basePt.copy(), endPt);
@@ -201,8 +204,8 @@ else if (orient != ringOrient) {
     return true;
   }
 
-  private static boolean isConvex(Coordinate p0, Coordinate p1, Coordinate p2) {
-    return Orientation.CLOCKWISE == Orientation.index(p0, p1, p2);
+  private static boolean isConcave(Coordinate p0, Coordinate p1, Coordinate p2) {
+    return Orientation.COUNTERCLOCKWISE == Orientation.index(p0, p1, p2);
   }
 
   private static boolean isPointInConvexRing(Coordinate[] ringCW, Coordinate p) {

modules/core/src/test/java/org/locationtech/jts/algorithm/construct/MaximumInscribedCircleTest.java

@@ -112,6 +112,11 @@ public void testVeryThin() {
        0.01 );
   }
 
+  public void testQuadWithCollinearVertex() {
+    checkCircle("POLYGON ((1 5, 5 5, 9 5, 5 1, 1 5))", 
+       0.001, 5.0, 3.3431,  1.65685 );
+  }
+  
   /**
    * A coarse distance check, mainly testing 
    * that there is not a huge number of iterations.