split: fix line splitting to avoid creating zero length line segments

When an endpoint of a line intersects another entity, line splitting
creates two segments with one of them having zero length, unnecessarily
discarding all constraints on it in the process as well.

Change the behaviour to not split the line in that case, and instead
just use the intersecting endpoint for the new coincident constraint.

Author: iscgar

src/modify.cpp

@@ -448,6 +448,14 @@ hEntity GraphicsWindow::SplitLine(hEntity he, Vector pinter) {
     Vector p0 = SK.GetEntity(hep0)->PointGetNum(),
            p1 = SK.GetEntity(hep1)->PointGetNum();
 
+    if(p0.Equals(pinter)) {
+        return hep0;
+    }
+
+    if(p1.Equals(pinter)) {
+        return hep1;
+    }
+
     // Add the two line segments this one gets split into.
     hRequest r0i = AddRequest(Request::Type::LINE_SEGMENT, /*rememberForUndo=*/false),
              ri1 = AddRequest(Request::Type::LINE_SEGMENT, /*rememberForUndo=*/false);
@@ -582,7 +590,6 @@ hEntity GraphicsWindow::SplitCubic(hEntity he, Vector pinter) {
 
 hEntity GraphicsWindow::SplitEntity(hEntity he, Vector pinter) {
     Entity *e = SK.GetEntity(he);
-    Entity::Type entityType = e->type;
 
     hEntity ret;
     if(e->IsCircle()) {
@@ -597,22 +604,20 @@ hEntity GraphicsWindow::SplitEntity(hEntity he, Vector pinter) {
     }
 
     // Finally, delete the request that generated the original entity.
-    Request::Type reqType = EntReqTable::GetRequestForEntity(entityType);
-    SK.request.ClearTags();
-    for(auto &r : SK.request) {
-        if(r.group != activeGroup)
-            continue;
-        if(r.type != reqType)
-            continue;
-
-        // If the user wants to keep the old entities around, they can just
-        // mark them construction first.
-        if(he == r.h.entity(0) && !r.construction) {
-            r.tag = 1;
-            break;
+    if(he.isFromRequest() && he == he.request().entity(0)) {
+        hRequest hr = he.request();
+        // Only delete the original request if we actually made a split
+        // (i.e. the split point is not from the original request)
+        if(hr != ret.request()) {
+            Request *r = SK.GetRequest(hr);
+            // If the user wants to keep the old entities around, they can just
+            // mark them construction first.
+            if(r->group == activeGroup && !r->construction) {
+                r->tag = 1;
+                DeleteTaggedRequests();
+            }
         }
     }
-    DeleteTaggedRequests();
 
     return ret;
 }