time_zone: Handle offset time zones

time_zone: Remove maybe_unused

time_zone: Use s64 storages

time_zone: Catch by reference

src/common/time_zone.cpp

@@ -2,8 +2,10 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 #include <chrono>
+#include <exception>
 #include <iomanip>
 #include <sstream>
+#include <stdexcept>
 #include <fmt/chrono.h>
 #include <fmt/core.h>
 
@@ -59,56 +61,42 @@ std::chrono::seconds GetCurrentOffsetSeconds() {
     return std::chrono::seconds{seconds};
 }
 
+// Key is [Hours * 100 + Minutes], multiplied by 100 if DST
+const static std::map<s64, const char*> off_timezones = {
+    {530, "Asia/Calcutta"},          {930, "Australia/Darwin"},     {845, "Australia/Eucla"},
+    {103000, "Australia/Adelaide"},  {1030, "Australia/Lord_Howe"}, {630, "Indian/Cocos"},
+    {1245, "Pacific/Chatham"},       {134500, "Pacific/Chatham"},   {-330, "Canada/Newfoundland"},
+    {-23000, "Canada/Newfoundland"}, {430, "Asia/Kabul"},           {330, "Asia/Tehran"},
+    {43000, "Asia/Tehran"},          {545, "Asia/Kathmandu"},       {-930, "Asia/Marquesas"},
+};
+
 std::string FindSystemTimeZone() {
 #if defined(MINGW)
     // MinGW has broken strftime -- https://sourceforge.net/p/mingw-w64/bugs/793/
     // e.g. fmt::format("{:%z}") -- returns "Eastern Daylight Time" when it should be "-0400"
     return timezones[0];
 #else
-    // Time zone offset in seconds from GMT
-    constexpr std::array offsets{
-        0,     0,     3600,  -21600, -19768, 7200,   7509,   -1521, -18000, -18000, -75,    -75,
-        0,     0,     0,     0,      0,      27402,  -36000, -968,  12344,  8454,   -18430, 33539,
-        40160, 3164,  3600,  -25200, -25200, -25196, 41944,  44028, 5040,   -2205,  29143,  -28800,
-        29160, 30472, 24925, 6952,   0,      0,      0,      9017,  0,      0,
-    };
-
-    // If the time zone recognizes Daylight Savings Time
-    constexpr std::array dst{
-        false, false, true,  true,  true,  true,  true,  true,  false, true,  true, true,
-        false, false, false, false, false, true,  false, false, true,  true,  true, true,
-        false, true,  true,  false, true,  true,  true,  true,  true,  true,  true, true,
-        true,  true,  true,  true,  false, false, false, true,  true,  false,
-    };
-
-    static std::string system_time_zone_cached{};
-    if (!system_time_zone_cached.empty()) {
-        return system_time_zone_cached;
-    }
+    const s64 seconds = static_cast<s64>(GetCurrentOffsetSeconds().count());
 
-    const auto now = std::time(nullptr);
-    const struct std::tm& local = *std::localtime(&now);
+    const s64 minutes = seconds / 60;
+    const s64 hours = minutes / 60;
 
-    const s64 system_offset = GetCurrentOffsetSeconds().count() - (local.tm_isdst ? 3600 : 0);
+    const s64 minutes_off = minutes - hours * 60;
 
-    int min = std::numeric_limits<int>::max();
-    int min_index = -1;
-    for (u32 i = 2; i < offsets.size(); i++) {
-        // Skip if system is celebrating DST but considered time zone does not
-        if (local.tm_isdst && !dst[i]) {
-            continue;
-        }
+    if (minutes_off != 0) {
+        const auto the_time = std::time(nullptr);
+        const struct std::tm& local = *std::localtime(&the_time);
+        const bool is_dst = local.tm_isdst != 0;
+
+        const s64 tz_index = (hours * 100 + minutes_off) * (is_dst ? 100 : 1);
 
-        const auto offset = offsets[i];
-        const int difference = static_cast<int>(std::abs(offset - system_offset));
-        if (difference < min) {
-            min = difference;
-            min_index = i;
+        try {
+            return off_timezones.at(tz_index);
+        } catch (std::out_of_range&) {
+            LOG_ERROR(Common, "Time zone {} not handled, defaulting to hour offset.", tz_index);
         }
     }
-
-    system_time_zone_cached = GetTimeZoneStrings()[min_index];
-    return system_time_zone_cached;
+    return fmt::format("Etc/GMT{:s}{:d}", hours > 0 ? "-" : "+", std::abs(hours));
 #endif
 }