common: Move system time zone string detection

Moves it from Settings to Common::TimeZone, since this algorithm doesn't
depend on the setting. It also lets us use it in other libraries.

common: Various fixes

time_zone: Don't double up the std::abs

Too many absolute values were causing mirrored time zones to resolve
as the same.

3 files changed, 84 insertions, 76 deletions

diff --git a/src/common/settings.cpp b/src/common/settings.cpp
index ed15b8b7d..115fba27d 100644
--- a/src/common/settings.cpp
+++ b/src/common/settings.cpp
@@ -3,18 +3,14 @@
 
 #if __cpp_lib_chrono >= 201907L
 #include <chrono>
-#else
-#include <ctime>
-#include <limits>
 #endif
 #include <string_view>
-#include <fmt/chrono.h>
-#include <fmt/core.h>
 
 #include "common/assert.h"
 #include "common/fs/path_util.h"
 #include "common/logging/log.h"
 #include "common/settings.h"
+#include "common/time_zone.h"
 
 namespace Settings {
 
@@ -22,83 +18,22 @@ Values values;
 static bool configuring_global = true;
 
 std::string GetTimeZoneString() {
-    static constexpr std::array timezones{
-        "GMT",       "GMT",       "CET", "CST6CDT", "Cuba",    "EET",    "Egypt",     "Eire",
-        "EST",       "EST5EDT",   "GB",  "GB-Eire", "GMT",     "GMT+0",  "GMT-0",     "GMT0",
-        "Greenwich", "Hongkong",  "HST", "Iceland", "Iran",    "Israel", "Jamaica",   "Japan",
-        "Kwajalein", "Libya",     "MET", "MST",     "MST7MDT", "Navajo", "NZ",        "NZ-CHAT",
-        "Poland",    "Portugal",  "PRC", "PST8PDT", "ROC",     "ROK",    "Singapore", "Turkey",
-        "UCT",       "Universal", "UTC", "W-SU",    "WET",     "Zulu",
-    };
-
     const auto time_zone_index = static_cast<std::size_t>(values.time_zone_index.GetValue());
-    ASSERT(time_zone_index < timezones.size());
+    ASSERT(time_zone_index < Common::TimeZone::GetTimeZoneStrings().size());
+
     std::string location_name;
-    switch (time_zone_index) {
-    case 0: { // Auto
+    if (time_zone_index == 0) { // Auto
 #if __cpp_lib_chrono >= 201907L
         const struct std::chrono::tzdb& time_zone_data = std::chrono::get_tzdb();
         const std::chrono::time_zone* current_zone = time_zone_data.current_zone();
         std::string_view current_zone_name = current_zone->name();
         location_name = current_zone_name;
-#elif 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"
-        location_name = timezones[0];
-        break;
 #else
-        static 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,
-        };
-
-        static 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,
-        };
-
-        const auto now = std::time(nullptr);
-        const struct std::tm& local = *std::localtime(&now);
-        const std::string clock_offset_s = fmt::format("{:%z}", local);
-        if (clock_offset_s.empty()) {
-            location_name = timezones[0];
-            break;
-        }
-        const int hours_offset = std::stoi(clock_offset_s) / 100;
-        const int minutes_offset = std::stoi(clock_offset_s) - hours_offset * 100;
-        const int system_offset =
-            hours_offset * 3600 + minutes_offset * 60 - (local.tm_isdst ? 3600 : 0);
-
-        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;
-            }
-
-            const auto offset = offsets[i];
-            const int difference = std::abs(std::abs(offset) - std::abs(system_offset));
-            if (difference < min) {
-                min = difference;
-                min_index = i;
-            }
-        }
-
-        location_name = timezones[min_index];
+        location_name = Common::TimeZone::FindSystemTimeZone();
 #endif
-        break;
+    } else {
+        location_name = Common::TimeZone::GetTimeZoneStrings()[time_zone_index];
     }
-    default:
-        location_name = timezones[time_zone_index];
-        break;
-    }
-
     return location_name;
 }
 
diff --git a/src/common/time_zone.cpp b/src/common/time_zone.cpp
index 126836b01..717d751ba 100644
--- a/src/common/time_zone.cpp
+++ b/src/common/time_zone.cpp
@@ -4,12 +4,29 @@
 #include <chrono>
 #include <iomanip>
 #include <sstream>
+#include <fmt/chrono.h>
+#include <fmt/core.h>
 
 #include "common/logging/log.h"
+#include "common/settings.h"
 #include "common/time_zone.h"
 
 namespace Common::TimeZone {
 
+// Time zone strings
+constexpr std::array timezones{
+    "GMT",       "GMT",       "CET", "CST6CDT", "Cuba",    "EET",    "Egypt",     "Eire",
+    "EST",       "EST5EDT",   "GB",  "GB-Eire", "GMT",     "GMT+0",  "GMT-0",     "GMT0",
+    "Greenwich", "Hongkong",  "HST", "Iceland", "Iran",    "Israel", "Jamaica",   "Japan",
+    "Kwajalein", "Libya",     "MET", "MST",     "MST7MDT", "Navajo", "NZ",        "NZ-CHAT",
+    "Poland",    "Portugal",  "PRC", "PST8PDT", "ROC",     "ROK",    "Singapore", "Turkey",
+    "UCT",       "Universal", "UTC", "W-SU",    "WET",     "Zulu",
+};
+
+const std::array<const char*, 46>& GetTimeZoneStrings() {
+    return timezones;
+}
+
 std::string GetDefaultTimeZone() {
     return "GMT";
 }
@@ -18,10 +35,7 @@ static std::string GetOsTimeZoneOffset() {
     const std::time_t t{std::time(nullptr)};
     const std::tm tm{*std::localtime(&t)};
 
-    std::stringstream ss;
-    ss << std::put_time(&tm, "%z"); // Get the current timezone offset, e.g. "-400", as a string
-
-    return ss.str();
+    return fmt::format("{:%z}", tm);
 }
 
 static int ConvertOsTimeZoneOffsetToInt(const std::string& timezone) {
@@ -45,4 +59,57 @@ std::chrono::seconds GetCurrentOffsetSeconds() {
     return std::chrono::seconds{seconds};
 }
 
+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 auto now = std::time(nullptr);
+    const struct std::tm& local = *std::localtime(&now);
+
+    const s64 system_offset = GetCurrentOffsetSeconds().count() - (local.tm_isdst ? 3600 : 0);
+
+    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;
+        }
+
+        const auto offset = offsets[i];
+        const int difference = static_cast<int>(std::abs(offset - system_offset));
+        if (difference < min) {
+            min = difference;
+            min_index = i;
+        }
+    }
+
+    system_time_zone_cached = GetTimeZoneStrings()[min_index];
+    return system_time_zone_cached;
+#endif
+}
+
 } // namespace Common::TimeZone
diff --git a/src/common/time_zone.h b/src/common/time_zone.h
index 99cae6ef2..f574d5c04 100644
--- a/src/common/time_zone.h
+++ b/src/common/time_zone.h
@@ -3,15 +3,21 @@
 
 #pragma once
 
+#include <array>
 #include <chrono>
 #include <string>
 
 namespace Common::TimeZone {
 
+[[nodiscard]] const std::array<const char*, 46>& GetTimeZoneStrings();
+
 /// Gets the default timezone, i.e. "GMT"
 [[nodiscard]] std::string GetDefaultTimeZone();
 
 /// Gets the offset of the current timezone (from the default), in seconds
 [[nodiscard]] std::chrono::seconds GetCurrentOffsetSeconds();
 
+/// Searches time zone offsets for the closest offset to the system time zone
+[[nodiscard]] std::string FindSystemTimeZone();
+
 } // namespace Common::TimeZone