MNT: alternate method of identifying 0 length segments

tacaswell · tacaswell · commit d5ae2015ebbd · 2020-01-22T22:11:52.000-08:00
Use length square being close to 0 rather than strict.  I have a worry
that because we are using "close" for identifying the determinant is
0, if we use strict equality for the length, then we may pass through
the length check, but erroneously go through the den == 0 path.
diff --git a/src/_path.h b/src/_path.h
@@ -836,8 +836,12 @@ inline bool segments_intersect(const double &x1,
     const double atol = 1e-13;
 
     // if either segment is 0 length, they do not intersect
+    // length-squared of each segment
+    const double lensq_A = (x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2);
+    const double lenqs_B = (x3 - x4) * (x3 - x4) + (y3 - y4) * (y3 - y4);
 
-    if ((x1 == x2 && y1 == y2) || (x3 == x3 && y3 == y4)) {
+    // one of the segments is 0 length
+    if (isclose(lensq_A, 0, rtol, atol) || isclose(lenqs_B, 0, rtol, atol)) {
         return false;
     }
 
