[gnome-control-center] datetime: Fix handling of Irish timezone on map

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

commit 14a2672e28f983cc72a4c999119ac9a81490ff8c
Author: Colin Watson <cjwatson debian org>
Date:   Mon Sep 27 23:36:29 2021 +0100

    datetime: Fix handling of Irish timezone on map
    
    The timezone map tries to highlight regions of the world that keep the
    same time, approximating this by their non-daylight-savings UTC offset.
    There's no reasonable API for this, and it goes wrong in various cases,
    such as Irish Standard Time which is legally defined as the country's
    summer time with a negative DST offset in winter.
    
    Hardcoding this is unpleasant, but there doesn't seem to be a better
    solution, and in any case there's already similar hardcoding implied by
    the segmented map images in panels/datetime/data/timezone_*.png.  I've
    tried to make it practical to fix other similar disagreements between
    the detected offset and the groupings implied by map images, though for
    now I've conservatively fixed only the case I'm familiar with.
    
    Fixes: #1341

 panels/datetime/cc-timezone-map.c |  4 ++--
 panels/datetime/tz.c              | 36 ++++++++++++++++++++++++++++++++++--
 panels/datetime/tz.h              |  2 +-
 3 files changed, 37 insertions(+), 5 deletions(-)
---
diff --git a/panels/datetime/cc-timezone-map.c b/panels/datetime/cc-timezone-map.c
index 4d5075c88..3c7c8fcd8 100644
--- a/panels/datetime/cc-timezone-map.c
+++ b/panels/datetime/cc-timezone-map.c
@@ -426,8 +426,8 @@ set_location (CcTimezoneMap *map,
 
   info = tz_info_from_location (map->location);
 
-  map->selected_offset = tz_location_get_utc_offset (map->location)
-    / (60.0*60.0) + ((info->daylight) ? -1.0 : 0.0);
+  map->selected_offset = tz_location_get_base_utc_offset (map->location)
+    / (60.0*60.0);
   gtk_widget_queue_draw (GTK_WIDGET (map));
 
   g_signal_emit (map, signals[LOCATION_CHANGED], 0, map->location);
diff --git a/panels/datetime/tz.c b/panels/datetime/tz.c
index 5ef667690..4fe2305b0 100644
--- a/panels/datetime/tz.c
+++ b/panels/datetime/tz.c
@@ -181,14 +181,46 @@ tz_location_get_position (TzLocation *loc, double *longitude, double *latitude)
        *latitude = loc->latitude;
 }
 
+/* For timezone map display purposes, we try to highlight regions of the
+ * world that keep the same time.  There is no reasonable API to discover
+ * this; at the moment we just group timezones by their non-daylight-savings
+ * UTC offset and hope that's good enough.  However, in some cases that
+ * produces confusing results.  For example, Irish Standard Time is legally
+ * defined as the country's summer time, with a negative DST offset in
+ * winter; but this results in the same observed clock times as countries
+ * that observe Western European (Summer) Time, not those that observe
+ * Central European (Summer) Time, so we should group Ireland with the
+ * former, matching the grouping implied by data/timezone_*.png.
+ *
+ * This is something of a hack, and there remain other problems with
+ * timezone grouping: for example, grouping timezones north and south of the
+ * equator together where DST is observed at different times of the year is
+ * dubious.
+ */
+struct {
+       const char *zone;
+       gint offset;
+} base_offset_overrides[] = {
+       { "Europe/Dublin",  0 },
+};
+
 glong
-tz_location_get_utc_offset (TzLocation *loc)
+tz_location_get_base_utc_offset (TzLocation *loc)
 {
        g_autoptr(TzInfo) tz_info = NULL;
        glong offset;
+       guint i;
 
        tz_info = tz_info_from_location (loc);
-       offset = tz_info->utc_offset;
+       offset = tz_info->utc_offset + (tz_info->daylight ? -3600 : 0);
+
+       for (i = 0; i < G_N_ELEMENTS (base_offset_overrides); i++) {
+               if (g_str_equal (loc->zone, base_offset_overrides[i].zone)) {
+                       offset = base_offset_overrides[i].offset;
+                       break;
+               }
+       }
+
        return offset;
 }
 
diff --git a/panels/datetime/tz.h b/panels/datetime/tz.h
index b6b7ab9d0..feef16580 100644
--- a/panels/datetime/tz.h
+++ b/panels/datetime/tz.h
@@ -78,7 +78,7 @@ void       tz_location_get_position   (TzLocation *loc,
 char      *tz_location_get_country    (TzLocation *loc);
 gchar     *tz_location_get_zone       (TzLocation *loc);
 gchar     *tz_location_get_comment    (TzLocation *loc);
-glong      tz_location_get_utc_offset (TzLocation *loc);
+glong      tz_location_get_base_utc_offset (TzLocation *loc);
 gint       tz_location_set_locally    (TzLocation *loc);
 TzInfo    *tz_info_from_location      (TzLocation *loc);
 void       tz_info_free               (TzInfo *tz_info);