range: Handle zero-length ranges when calculating overlap

Zero length ranges, such as for events without a DTEND could be
considered to overlap a range if they were at exactly the end of the
range. The end of a range however is the first date that is supposed to
not overlap, so a zero-length range on exactly this point should not be
considered overlapping.

This fixes cases where all-day events from the next month could show up
on the same day of the current month if they did not have a DTEND.

This also adds tests for all possible cases of such zero-length events
overlapping.

Fixes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/701

src/core/gcal-range.c

@@ -320,7 +320,25 @@ gcal_range_calculate_overlap (GcalRange         *a,
         }
       else if (a_end_b_end_diff == 0)
         {
-          if (a_start_b_start_diff < 0)
+          gint a_start_b_end_diff;
+          gint a_end_b_start_diff;
+
+          a_start_b_end_diff = compare_func (a->range_start, b->range_end);
+          a_end_b_start_diff = compare_func (a->range_end, b->range_start);
+
+          if (a_start_b_end_diff >= 0)
+            {
+              /* Case 5.i for zero length A */
+              overlap = GCAL_RANGE_NO_OVERLAP;
+              position = GCAL_RANGE_AFTER;
+            }
+          else if (a_end_b_start_diff <= 0)
+            {
+              /* Case 5.ii for zero length B */
+              overlap = GCAL_RANGE_NO_OVERLAP;
+              position = GCAL_RANGE_BEFORE;
+            }
+          else if (a_start_b_start_diff < 0)
             {
               /* Case 2.iii */
               overlap = GCAL_RANGE_SUPERSET;

tests/test-range.c

@@ -159,6 +159,38 @@ static struct {
     GCAL_RANGE_NO_OVERLAP,
     GCAL_RANGE_AFTER,
   },
+
+  /* Zero length */
+  {
+    { "2020-03-10T00:00:00", "2020-03-15T00:00:00" },
+    { "2020-03-15T00:00:00", "2020-03-15T00:00:00" },
+    GCAL_RANGE_NO_OVERLAP,
+    GCAL_RANGE_BEFORE,
+  },
+  {
+    { "2020-03-10T00:00:00", "2020-03-15T00:00:00" },
+    { "2020-03-10T00:00:00", "2020-03-10T00:00:00" },
+    GCAL_RANGE_SUPERSET,
+    GCAL_RANGE_AFTER,
+  },
+  {
+    { "2020-03-15T00:00:00", "2020-03-15T00:00:00" },
+    { "2020-03-10T00:00:00", "2020-03-15T00:00:00" },
+    GCAL_RANGE_NO_OVERLAP,
+    GCAL_RANGE_AFTER,
+  },
+  {
+    { "2020-03-10T00:00:00", "2020-03-10T00:00:00" },
+    { "2020-03-10T00:00:00", "2020-03-15T00:00:00" },
+    GCAL_RANGE_SUBSET,
+    GCAL_RANGE_BEFORE,
+  },
+  {
+    { "2020-03-10T00:00:00", "2020-03-10T00:00:00" },
+    { "2020-03-10T00:00:00", "2020-03-10T00:00:00" },
+    GCAL_RANGE_EQUAL,
+    GCAL_RANGE_MATCH,
+  },
 };
 
 static void