[plasma/kdeplasma-addons] plasmacalendarplugins/alternatecalendar/provider: [Vietnamese Lunar Calendar] Move away from ICU :(

Git commit 4de2c318a02f446262736afd8ff3cacac897e114 by Nate Graham, on behalf of Trần Nam Tuấn.
Committed on 27/07/2026 at 19:07.
Pushed by ngraham into branch 'master'.

[Vietnamese Lunar Calendar] Move away from ICU :(

My assumption that we can just set `UCAL_*` and trick ICU to give us the
formatted string of the date is wrong. ICU definitely checks the input date and
tries it best to fix the date, which we don't want. This behavior can be seen
with 20 May 2031, which should display lunar date 30 March (Leap) 2031, but
shows April first 2031 instead. Since we use Vietnamese regardless of locale,
it's easier to just hardcode the ganzhi names ourself rather than trying to
wrangle ICU.

M  +58   -61   plasmacalendarplugins/alternatecalendar/provider/vietnamesecalendar.cpp

https://invent.kde.org/plasma/kdeplasma-addons/-/commit/4de2c318a02f446262736afd8ff3cacac897e114

diff --git a/plasmacalendarplugins/alternatecalendar/provider/vietnamesecalendar.cpp b/plasmacalendarplugins/alternatecalendar/provider/vietnamesecalendar.cpp
index 2a14d29ce..61caca800 100644
--- a/plasmacalendarplugins/alternatecalendar/provider/vietnamesecalendar.cpp
+++ b/plasmacalendarplugins/alternatecalendar/provider/vietnamesecalendar.cpp
@@ -6,7 +6,6 @@
 
 #include "vietnamesecalendar.h"
 
-#include "icucalendar_p.h"
 #include "solarutils.h"
 
 #include <KLocalizedString>
@@ -17,6 +16,12 @@
 using namespace Qt::StringLiterals;
 
 /*
+    Vietnamese lunar calendar implementation.
+
+    This work is technically duplicated with ICU Chinese calendar.
+    However, ICU public API does not allow changing the timezone used for calcuation.
+    This code should be revised once upstream provides support Vietnamese calendar.
+
     The algorithm used is based on Hồ Ngọc Đức's work.
     It has been adapted to use what available in `solarutils.h`.
     While providing greater accuracy, it is (in theory) more computational expensive.
@@ -26,6 +31,7 @@ using namespace Qt::StringLiterals;
     E.g. Vietnamese is UTC+7, while Chinese is UTC+8.
 
     References:
+    - https://unicode-org.atlassian.net/browse/ICU-11973
     - https://www.xemamlich.uhm.vn/calrules.html
  */
 
@@ -54,6 +60,20 @@ constexpr double s_J2000NewMoon = 2451550.09766;
  */
 constexpr int s_vietnameseUtcOffset = 7;
 
+/*!
+ * \brief The 10 heavenly stems in Vietnamese.
+ *
+ * \internal
+ */
+const std::array s_heavenlyStems{u"Giáp"_s, u"Ất"_s, u"Bính"_s, u"Đinh"_s, u"Mậu"_s, u"Kỷ"_s, u"Canh"_s, u"Tân"_s, u"Nhâm"_s, u"Quý"_s};
+
+/*!
+ * \brief The 12 earthly branches in Vietnamese.
+ *
+ * \internal
+ */
+const std::array s_earthlyBranches{u"Tý"_s, u"Sửu"_s, u"Dần"_s, u"Mão"_s, u"Thìn"_s, u"Tỵ"_s, u"Ngọ"_s, u"Mùi"_s, u"Thân"_s, u"Dậu"_s, u"Tuất"_s, u"Hợi"_s};
+
 struct LunarDate {
     int year;
     int month;
@@ -65,6 +85,7 @@ std::shared_mutex s_cacheMutex;
 constinit QHash<int /* k-index */, double> s_winterSolsticeCache;
 constinit QHash<int /* k-index */, double> s_newMoonCache;
 constinit QHash<QDate, LunarDate> s_lunarDateCache;
+constinit QHash<int /* lunar year */, QString> s_ganzhiYearCache;
 
 /*!
  * \brief Get the local midnight UTC+7 for \a{dayIndex}.
@@ -327,97 +348,73 @@ LunarDate getLunarDate(const QDate &date)
 
     return lunarDate;
 }
+
+/*!
+   \brief Get the ganzhi name for \a{year}.
+   \internal
+ */
+QString getYearGanzhi(int year)
+{
+    {
+        std::shared_lock lock(s_cacheMutex);
+        if (s_ganzhiYearCache.contains(year)) {
+            return s_ganzhiYearCache[year];
+        }
+    }
+    return s_ganzhiYearCache[year] = s_heavenlyStems.at((year + 6) % 10) + ' '_L1 + s_earthlyBranches.at((year + 8) % 12);
+}
 }
 
-class VietnameseCalendarProviderPrivate : public ICUCalendarPrivate
+class VietnameseCalendarProviderPrivate
 {
     Q_DISABLE_COPY(VietnameseCalendarProviderPrivate)
-
-    const icu::Locale m_locale;
-
 public:
-    explicit VietnameseCalendarProviderPrivate();
-
-    bool setDate(const QDate &date);
-
-    bool isLeapMonth() const;
-
-    QString formattedDateString(const icu::UnicodeString &str) const;
+    VietnameseCalendarProviderPrivate() = default;
 
     QCalendar::YearMonthDay fromGregorian(const QDate &date);
     CalendarEvents::CalendarEventsPlugin::SubLabel subLabel(const QDate &date);
-};
 
-VietnameseCalendarProviderPrivate::VietnameseCalendarProviderPrivate()
-    : m_locale{"vi", nullptr, nullptr, "calendar=chinese"} // Force using the Vietnamese translation
-{
-    m_calendar.reset(icu::Calendar::createInstance(m_locale, m_errorCode));
-}
+private:
+    QDate m_date;
+    LunarDate m_lunarDate{};
+
+    bool setDate(const QDate &date);
+};
 
 bool VietnameseCalendarProviderPrivate::setDate(const QDate &date)
 {
     if (!date.isValid()) {
         return false;
     }
-
-    const auto lunarDate = getLunarDate(date);
-
-    m_calendar->clear(); // Clear existing time to avoid carry-over "noise".
-
-    // Calculate the Chinese year from Gregorian year, since the lunar date calculation returns the Gregorian one.
-    // 2637 BCE epoch was the first year of the Chinese calendar in Gregorian calendar used by ICU.
-    m_calendar->set(UCAL_YEAR, lunarDate.year + 2637);
-
-    m_calendar->set(UCAL_MONTH, lunarDate.month - 1); // Zero-index month.
-    m_calendar->set(UCAL_DATE, lunarDate.day);
-    m_calendar->set(UCAL_IS_LEAP_MONTH, lunarDate.isLeapMonth ? 1 : 0);
-
-    return !U_FAILURE(m_errorCode);
-}
-
-bool VietnameseCalendarProviderPrivate::isLeapMonth() const
-{
-    const int32_t leap = m_calendar->get(UCAL_IS_LEAP_MONTH, m_errorCode);
-
-    if (U_FAILURE(m_errorCode)) {
-        return -1;
-    }
-
-    return leap != 0;
-}
-
-QString VietnameseCalendarProviderPrivate::formattedDateString(const icu::UnicodeString &str) const
-{
-    UErrorCode errorCode = U_ZERO_ERROR;
-    icu::UnicodeString dateString;
-    icu::SimpleDateFormat formatter(str, m_locale, errorCode);
-    formatter.setCalendar(*m_calendar);
-    formatter.format(m_calendar->getTime(errorCode), dateString);
-
-    return QStringView(dateString.getBuffer(), dateString.length()).toString();
+    m_date = date;
+    m_lunarDate = getLunarDate(date);
+    return true;
 }
 
 QCalendar::YearMonthDay VietnameseCalendarProviderPrivate::fromGregorian(const QDate &date)
 {
-    if (U_FAILURE(m_errorCode) || !date.isValid() || setDate(date)) {
+    if (!setDate(date)) {
         return {};
     }
-    return this->date();
+    return {m_lunarDate.year, m_lunarDate.month, m_lunarDate.year};
 }
 
 CalendarEvents::CalendarEventsPlugin::SubLabel VietnameseCalendarProviderPrivate::subLabel(const QDate &date)
 {
-    if (U_FAILURE(m_errorCode) || !date.isValid() || !setDate(date)) {
+    if (!setDate(date)) {
         return {};
     }
 
     CalendarEvents::CalendarEventsPlugin::SubLabel sublabel;
     sublabel.priority = CalendarEvents::CalendarEventsPlugin::SubLabelPriority::Low;
 
-    sublabel.dayLabel = QString::number(day());
-    sublabel.monthLabel = formattedDateString("MMMM");
-    sublabel.yearLabel = formattedDateString("U r");
-    sublabel.label = formattedDateString("'Ngày' d 'tháng' MMMM 'năm' U r");
+    sublabel.dayLabel = QString::number(m_lunarDate.day);
+    sublabel.monthLabel = QString::number(m_lunarDate.month);
+    if (m_lunarDate.isLeapMonth) {
+        sublabel.monthLabel.append(u" Nhuận"_s);
+    }
+    sublabel.yearLabel = getYearGanzhi(m_lunarDate.year).append(' '_L1).append(QString::number(m_lunarDate.year));
+    sublabel.label = u"Ngày %1 tháng %2 năm %3"_s.arg(sublabel.dayLabel, sublabel.monthLabel, sublabel.yearLabel);
 
     return sublabel;
 }