fix line intersect divide by zero

Test filinmangust14 exposes two problems:
- finding top of contour can expose divide by zero
- joining partial contour results can add diagonal

The latter makes the test return the wrong result,
and has not been seen in other tests. The fix
is to join disconnected contours by only following
the contours provided as input. Working on that.

The former bug is more straight-forward; just
don't try to compute axis-aligned intersection
if the denominator is zero.

All existing tests prior to the new one work
with this change.

R=caryclark@google.com
Bug: skia:8125
Change-Id: Ic878d090066708d9baca8475f27d4d5aba2294cc
Reviewed-on: https://skia-review.googlesource.com/140121
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
Auto-Submit: Cary Clark <caryclark@skia.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/src/pathops/SkPathWriter.cpp b/src/pathops/SkPathWriter.cpp
index 7291002593..950f8fbce5 100644
--- a/src/pathops/SkPathWriter.cpp
+++ b/src/pathops/SkPathWriter.cpp
@@ -301,6 +301,22 @@ void SkPathWriter::assemble() {
 #endif
         do {
             const SkPath& contour = fPartials[rIndex];
+            if (!first) {
+                SkPoint prior, next;
+                SkAssertResult(fPathPtr->getLastPt(&prior));
+                if (forward) {
+                    next = contour.getPoint(0);
+                } else {
+                    SkAssertResult(contour.getLastPt(&next));
+                }
+                if (prior != next) {
+                    /* TODO: if there is a gap between open path written so far and path to come,
+                       connect by following segments from one to the other, rather than introducing
+                       a diagonal to connect the two.
+                     */
+                    SkDebugf("");
+                }
+            }
             if (forward) {
                 fPathPtr->addPath(contour,
                         first ? SkPath::kAppend_AddPathMode : SkPath::kExtend_AddPathMode);