[gnome-calendar] range: Handle zero-length ranges when calculating overlap

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 44e242064533946abe246bc415667983e5d02dee
Author: Sebastian Keller <skeller gnome org>
Date:   Wed Mar 24 14:01:10 2021 +0100

    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 | 20 +++++++++++++++++++-
 tests/test-range.c    | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+), 1 deletion(-)
---
diff --git a/src/core/gcal-range.c b/src/core/gcal-range.c
index 9aeb5c7f..a85b16b8 100644
--- a/src/core/gcal-range.c
+++ b/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;
diff --git a/tests/test-range.c b/tests/test-range.c
index 68d454d5..43a695cb 100644
--- a/tests/test-range.c
+++ b/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