Date.h

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/*
 * Copyright(c) Sophist Solutions, Inc. 1990-2025.  All rights reserved
 */
#ifndef _Stroika_Foundation_Time_Date_h_
#define _Stroika_Foundation_Time_Date_h_ 1
 
#include "Stroika/Foundation/StroikaPreComp.h"
 
#include <chrono>
#include <climits>
#include <compare>
#include <string>
 
#include "Stroika/Foundation/Characters/String.h"
#include "Stroika/Foundation/Common/Common.h"
#include "Stroika/Foundation/Common/Enumeration.h"
#include "Stroika/Foundation/DataExchange/ValidationStrategy.h"
#include "Stroika/Foundation/Execution/Exceptions.h"
#include "Stroika/Foundation/Time/Common.h"
#include "Stroika/Foundation/Traversal/Range.h"
 
/**
 *  \file
 *
 *  \note Code-Status:  <a href="Code-Status.md#Release">Release</a>
 *
 * TODO:
 *      @todo   Could optimize the Format/Parse calls for case without locale to just hardwire implementation
 *              using sprintf/scanf (as we had before 2.1b10); only performance optimization and unclear it would help
 *
 *      @todo   I'm not sure eCurrentLocale_WithZerosStripped is a good idea. Not sure if better
 *              to use separate format print arg or???
 *
 *      @todo   It would be highly desirable to allow this date code to represent larger/smaller dates
 *              (without the Julian calendar restriction).
 *              That maybe a longer term issue.
 *
 *              Consider representing as big struct
 *              o   Like with STRUCT DATETIME or struct tm
 *                  o   Int year
 *                  o   Int month
 *              o   And maybe store cached string reps for common cases as optimization and
 *                  store cached second-offset (mutable) for quick compares
 *              o   Note in docs for future versions the min/max date COULD be expanded
 */
 
namespace Stroika::Foundation::Time {
 
    class Duration;
 
    using Characters::String;
 
    using chrono::month;
    using chrono::months;
 
    // clang-format off
    using chrono::January;
    using chrono::February;
    using chrono::March;
    using chrono::April;
    using chrono::May;
    using chrono::June;
    using chrono::July;
    using chrono::August;
    using chrono::September;
    using chrono::October;
    using chrono::November;
    using chrono::December;
    // clang-format on
 
    using chrono::weekday;
 
    // clang-format off
    using chrono::Sunday;
    using chrono::Monday;
    using chrono::Tuesday;
    using chrono::Wednesday;
    using chrono::Thursday;
    using chrono::Friday;
    using chrono::Saturday;
    // clang-format on
 
    using chrono::day;
    using chrono::days;
 
    using chrono::year;
 
    using chrono::year_month_day;
 
    DISABLE_COMPILER_MSC_WARNING_START (4455)
    using std::literals::chrono_literals::operator"" d; // day
    using std::literals::chrono_literals::operator"" y; // year
    DISABLE_COMPILER_MSC_WARNING_END (4455)
    using std::chrono::operator/; // year/month/day -> year_month_day
 
    /**
     *  \brief Simple wrapper on std::chrono::weekday, with some helpful validation properties (assures constructed 'ok'). But not necessary to use - use just 'weekday' in most places
     * 
     *  \note - DayOfWeek was an enum in Stroika v2.1, so this is a significant change.
     * 
     *  \note DayOfWeek can be converted from/to unsigned int.
     *
     *  \note not [[nodiscard]] cuz can use CTOR just for quiet validation
     */
    struct DayOfWeek : weekday {
        /**
         *  For the purpose of integer constructors, 0==Sunday, 1==Monday, and so on
         */
        constexpr DayOfWeek (weekday w, DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
        constexpr DayOfWeek (unsigned int w, DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
 
    public:
        [[deprecated ("Since Stroika v3.0d1, use chrono::Sunday")]] static constexpr weekday    eSunday{Sunday};
        [[deprecated ("Since Stroika v3.0d1, use chrono::Monday")]] static constexpr weekday    eMonday{Monday};
        [[deprecated ("Since Stroika v3.0d1, use chrono::Tuesday")]] static constexpr weekday   eTuesday{Tuesday};
        [[deprecated ("Since Stroika v3.0d1, use chrono::Wednesday")]] static constexpr weekday eWednesday{Wednesday};
        [[deprecated ("Since Stroika v3.0d1, use chrono::Thursday")]] static constexpr weekday  eThursday{Thursday};
        [[deprecated ("Since Stroika v3.0d1, use chrono::Friday")]] static constexpr weekday    eFriday{Friday};
        [[deprecated ("Since Stroika v3.0d1, use chrono::Saturday")]] static constexpr weekday  eSaturday{Saturday};
    };
 
    /**
     *  \brief Simple wrapper on std::chrono::month, with some helpful validation properties (assures constructed 'ok'). But not necessary to use - use just 'month' in most places
     * 
     *  \note - MonthOfYear was an enum in Stroika v2.1, so this is a significant change.
     * 
     *  \note MonthOfYear can be converted from/to unsigned int.
     *
     *  \note not [[nodiscard]] cuz can use CTOR just for quiet validation
     */
    struct MonthOfYear : month {
        /**
         *  For the purpose of integer constructors, 1==January, 2==February and so on (no zero).
         */
        constexpr MonthOfYear (month m, DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
        constexpr MonthOfYear (unsigned int m, DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
 
    public:
        [[deprecated ("Since Stroika v3.0d1, use chrono::January")]] static constexpr chrono::month   eJanuary{January};
        [[deprecated ("Since Stroika v3.0d1, use chrono::February")]] static constexpr chrono::month  eFebruary{February};
        [[deprecated ("Since Stroika v3.0d1, use chrono::March")]] static constexpr chrono::month     eMarch{March};
        [[deprecated ("Since Stroika v3.0d1, use chrono::April")]] static constexpr chrono::month     eApril{April};
        [[deprecated ("Since Stroika v3.0d1, use chrono::May")]] static constexpr chrono::month       eMay{May};
        [[deprecated ("Since Stroika v3.0d1, use chrono::June")]] static constexpr chrono::month      eJune{June};
        [[deprecated ("Since Stroika v3.0d1, use chrono::July")]] static constexpr chrono::month      eJuly{July};
        [[deprecated ("Since Stroika v3.0d1, use chrono::August")]] static constexpr chrono::month    eAugust{August};
        [[deprecated ("Since Stroika v3.0d1, use chrono::September")]] static constexpr chrono::month eSeptember{September};
        [[deprecated ("Since Stroika v3.0d1, use chrono::October")]] static constexpr chrono::month   eOctober{October};
        [[deprecated ("Since Stroika v3.0d1, use chrono::November")]] static constexpr chrono::month  eNovember{November};
        [[deprecated ("Since Stroika v3.0d1, use chrono::December")]] static constexpr chrono::month  eDecember{December};
    };
 
    /**
     *  \brief Simple wrapper on std::chrono::day, with some helpful validation properties (assures constructed 'ok'). But not necessary to use - use just 'day' in most places
     * 
     *  \note - DayOfMonth was an enum in Stroika v2.1, so this is a significant change.
     * 
     *  \note DayOfMonth can be converted from/to unsigned int.
     *  \note You can use the suffix 'd' instead of DayOfMonth{n}
     *
     *  \note not [[nodiscard]] cuz can use CTOR just for quiet validation
     */
    struct DayOfMonth : day {
        /**
         *  For the purpose of integer constructors, 1==1st, 2==2nd, and so on (no zero)
         */
        constexpr DayOfMonth (day d, DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
        constexpr DayOfMonth (unsigned int d, DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
 
    public:
        [[deprecated ("Since Stroika v3.0d1, use day{1}")]] static constexpr day  e1{1};
        [[deprecated ("Since Stroika v3.0d1, use day{2}")]] static constexpr day  e2{2};
        [[deprecated ("Since Stroika v3.0d1, use day{3}")]] static constexpr day  e3{3};
        [[deprecated ("Since Stroika v3.0d1, use day{4}")]] static constexpr day  e4{4};
        [[deprecated ("Since Stroika v3.0d1, use day{5}")]] static constexpr day  e5{5};
        [[deprecated ("Since Stroika v3.0d1, use day{6}")]] static constexpr day  e6{6};
        [[deprecated ("Since Stroika v3.0d1, use day{7}")]] static constexpr day  e7{7};
        [[deprecated ("Since Stroika v3.0d1, use day{8}")]] static constexpr day  e8{8};
        [[deprecated ("Since Stroika v3.0d1, use day{9}")]] static constexpr day  e9{9};
        [[deprecated ("Since Stroika v3.0d1, use day{10}")]] static constexpr day e10{10};
        [[deprecated ("Since Stroika v3.0d1, use day{11}")]] static constexpr day e11{11};
        [[deprecated ("Since Stroika v3.0d1, use day{12}")]] static constexpr day e12{12};
        [[deprecated ("Since Stroika v3.0d1, use day{13}")]] static constexpr day e13{13};
        [[deprecated ("Since Stroika v3.0d1, use day{14}")]] static constexpr day e14{14};
        [[deprecated ("Since Stroika v3.0d1, use day{15}")]] static constexpr day e15{15};
        [[deprecated ("Since Stroika v3.0d1, use day{16}")]] static constexpr day e16{16};
        [[deprecated ("Since Stroika v3.0d1, use day{17}")]] static constexpr day e17{17};
        [[deprecated ("Since Stroika v3.0d1, use day{18}")]] static constexpr day e18{18};
        [[deprecated ("Since Stroika v3.0d1, use day{19}")]] static constexpr day e19{19};
        [[deprecated ("Since Stroika v3.0d1, use day{20}")]] static constexpr day e20{20};
        [[deprecated ("Since Stroika v3.0d1, use day{21}")]] static constexpr day e21{21};
        [[deprecated ("Since Stroika v3.0d1, use day{22}")]] static constexpr day e22{22};
        [[deprecated ("Since Stroika v3.0d1, use day{23}")]] static constexpr day e23{23};
        [[deprecated ("Since Stroika v3.0d1, use day{24}")]] static constexpr day e24{24};
        [[deprecated ("Since Stroika v3.0d1, use day{25}")]] static constexpr day e25{25};
        [[deprecated ("Since Stroika v3.0d1, use day{26}")]] static constexpr day e26{26};
        [[deprecated ("Since Stroika v3.0d1, use day{27}")]] static constexpr day e27{27};
        [[deprecated ("Since Stroika v3.0d1, use day{28}")]] static constexpr day e28{28};
        [[deprecated ("Since Stroika v3.0d1, use day{29}")]] static constexpr day e29{29};
        [[deprecated ("Since Stroika v3.0d1, use day{30}")]] static constexpr day e30{30};
        [[deprecated ("Since Stroika v3.0d1, use day{31}")]] static constexpr day e31{31};
    };
 
    /**
     *  \brief Simple wrapper on std::chrono::year, with some helpful validation properties (assures constructed 'ok'). But not necessary to use - use just 'year' in most places
     * 
     *  \note - Year was an enum in Stroika v2.1, so this is a significant change.
     * 
     *  \note Year can be converted from/to signed int.
     * 
     *  \note you can use the suffix y instead of Year{N} (assuming using namespace Foundation::Time).
     *
     *  \note not [[nodiscard]] cuz can use CTOR just for quiet validation
     */
    struct Year : year {
        /**
         */
        constexpr Year (year y, DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
        constexpr Year (int y, DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
 
    public:
        /**
         *  \brief 4713 BC (no year zero)
         * 
         *      Was 1752 in Stroika v2.1
         */
        static constexpr year eFirstYear{-4712};
 
    public:
        /**
         *  Reason for current max-date:
         *      C:\Program Files (x86)\Windows Kits\10\Source\10.0.22000.0\ucrt\time\wcsftime.cpp
         *      _VALIDATE_RETURN(timeptr->tm_year >= -1900 && timeptr->tm_year <= 8099, EINVAL, false);
         *      -- LGP 2022-11-09
         *  Was SHRT_MAX - 1 in Stroika v2.1
         */
        static constexpr year eLastYear{8099};
    };
 
    /**
     *  \brief this defines undefined but important properties of the %x (read/write date) format string in stdc++ time_get/time_put
     * 
     *  This (rather important detail) appears to be left out of the std c++ specification for date/time formatting, and as a result
     *  each implementation varies. Attempt to capture the actual choices - so at least I can write regression tests that properly
     *  reflect expected Stroika behavior. And - perhaps - avoid the %x format string!
     * 
     *  @todo see if other format strings similarly unreliable. MAYBE the biggest lesson (obvious) is to not count on reading/writing dates
     *  in any locale-defined format (but for UIs this is a tricky constraint)?
     * 
     *  \note - These values all derived empirically (and checked in Stroika regression tests)
     */
    struct StdCPctxTraits {
        static constexpr bool kLocaleClassic_Write4DigitYear = false;
        static constexpr bool kLocaleENUS_Write4DigitYear =
#if defined(_MSC_VER)
            true
#elif defined(_GLIBCXX_RELEASE)
            false
#elif defined(_LIBCPP_VERSION)
            true
#else
            true
#endif
            ;
    };
 
    /**
     * Description:
     *      The Date class is (originally) based on SmallTalk-80, The Language & Its Implementation,
     *      page 108 (apx) - but changed to use Gregorian instead of Julian calendar.
     *
     *  \note   This class integrates neatly with the C++20 chrono date support. You can easily
     *          go back and forth (e.g. Date{std::chrono::year_month_day...}} or d.As<year_month_day> ())
     * 
     *          The main features of the Stroika Data class (compared to the std c++ date support):
     * 
     *              o   Simpler to use/understand (Stroika 'Date' does about the same thing as a bevy of
     *                  different chrono classes)
     *              o   Stroika Date immutable (OK - difference not clearly advantage)
     *              o   Validation (constructor DataExchange::ValidationStrategy)
     *                  (no non-ok () Date objects in Stroika). Nice clear semantics about exceptions
     *                  and assertions.
     *              o   Easier to use formatting
     *              o   ISO8601 formatting
     *              o   Wraps needlessly complicated locale/facet API for formatting dates as Strings, or
     *                  parsing them from strings.
     *              o   Builtin support for Julian calendar (again - maybe this is a difference not advantage?)
     *
     *  \note
     *      o   Date stores date's internally as Julian days, and so is valid for any date > January 1, −4713;
     *          Also note sizeof (year_month_day) == sizeof (Date) == 4
     *
     *  \par Miscellaneous references
     *      o   According to https://en.wikipedia.org/wiki/Gregorian_calendar
     *          Britain and the British Empire (including the eastern part of what is
     *          now the United States) adopted the Gregorian calendar in 1752
     *      o   https://aa.usno.navy.mil/data/JulianDate
     *          Best Julian date calculator I found (bad but best)
     *      o   Proleptic Gregorian Calendar
     *          https://en.wikipedia.org/wiki/Gregorian_calendar#Proleptic_Gregorian_calendar
     *      o   Julian Day Numer
     *          https://en.wikipedia.org/wiki/Julian_day
     * 
     *  Class Date knows about some obvious information:
     *      ->  there are seven days in a week, each day having a symbolic name and
     *          an index 1..7
     *      ->  there are twelve months in a year, each having a symbolic name and
     *          an index 1..12.
     *      ->  months have 28..31 days and
     *      ->  a particular year might be a leap year."
     *
     *          NB: Date implies NO NOTION of timezone.
     *
     *  \note   The entire Date API is immutable - meaning that all methods are const (except constructor and assignment operator)
     *
     *  \note   Date constructors REQUIRE valid inputs, and any operations which might overflow throw range_error
     *          instead of creating invalid values.
     *
     *  \note   Would like to make Date inherit from Debug::AssertExternallySynchronizedMutex to assure its not accidentially modified, but
     *          that's difficult because its sometimes uses as a constexpr
     *
     *  \note   \em Thread-Safety   <a href="Thread-Safety.md#C++-Standard-Thread-Safety">C++-Standard-Thread-Safety</a>
     *
     *  \note <a href="Design-Overview.md#Comparisons">Comparisons</a>:
     *        static_assert (totally_ordered<Date>);
     */
    class [[nodiscard]] Date {
    public:
        /*
         *  This refers to Julian Day Number - JDN - https://en.wikipedia.org/wiki/Julian_day
         * 
         *  \note This was called JulianRepType in Stroika v2.1
         */
        using JulianDayNumber = uint32_t;
 
    public:
        /**
         *  Sometimes want a signed type to compute differences.
         */
        using SignedJulianDayNumber = make_signed_t<JulianDayNumber>;
 
    public:
        /**
         *  Define a few 'reference' points, which define the correspondence between year/month/day in the Gregorian(ish)
         *  calendar with.
         * 
         *  Generally, users of the Date class can ignore this detail. Its used to 'calibrate' the mapping between Julian dates and
         *  year month day dates.
         */
        struct ReferencePoint {
            year_month_day  fYMD;
            JulianDayNumber fJulianRep;
        };
 
    public:
        /**
         *  Start of Julian Calendar (see https://docs.kde.org/trunk5/en/kstars/kstars/ai-julianday.html)
         *  JD=0, is January 1, 4713 BC (or -4712 January 1, since there was no year '0').
         */
        static constexpr ReferencePoint kStartOfJulianCalendar{-4712y / January / 1, 0};
 
    public:
        /**
         *  Start of UNIX time (see https://docs.kde.org/trunk5/en/kstars/kstars/ai-julianday.html, https://aa.usno.navy.mil/data/JulianDate)
         */
        static constexpr ReferencePoint kUNIXEpoch{1970y / January / 1, 2440588};
 
    public:
        /**
         *  See https://en.wikipedia.org/wiki/Gregorian_calendar
         *      September, 14d, 1752 (even this not sure of, but used this in Stroika v2.1)
         * 
         *      "Algorithm 199 from Communications of the ACM, Volume 6, No. 8,
                 * (Aug. 1963), p. 444.  Gregorian calendar started on Sep. 14, 1752"
         */
        static constexpr ReferencePoint kGregorianCalendarEpoch{1752y / September / 14d, 2361222};
 
    public:
        /**
         *  The Stroika Date class works with any date after this date (apx 4000 BC), but mostly
         *  just very accurate post Gregorian Calendar era (1753 apx).
         */
        static constexpr ReferencePoint kMinDateReference = kStartOfJulianCalendar;
        static_assert (kMinDateReference.fYMD.year () == Year::eFirstYear);
 
    public:
        /**
         *  Very hard to figure out how todo this. But this algorithm appears correct at least for dates > Gregorian Calendar era.
         * 
         *  Also, web is littered with Julian date converters that are wrong, somewhat wrong, or very wrong (at least all disagreeing).
         *  I used https://aa.usno.navy.mil/data/JulianDate as my reference/final arbiter/check (at least for dates past 1800).
         */
        constexpr static JulianDayNumber ToJulianRep (month m, day d, year y,
                                                      DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
        constexpr static JulianDayNumber
        ToJulianRep (year_month_day ymd, DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
 
    public:
        /**
         *  Compute the month/day/year associated with a given Julian day number. NOTE, this is only
         *  really accurate (as of Stroika v3.0d1) for dates in the Gregorian Calendar epoch (roughly since 1753).
         */
        constexpr static year_month_day
        FromJulianRep (JulianDayNumber j, DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
 
    public:
        /**
         *  kMinJulianRep is defined (later) constexpr.
         * 
         *  \note In Stroika v2.1, 2361222, aka Date::ToJulianRep (September, 14d, year{1752})
         *        but now its around 4000BC (see kMinDateReference)
         */
        static const JulianDayNumber kMinJulianRep = kMinDateReference.fJulianRep;
 
    public:
        /**
         *  kMaxJulianRep is defined (later) constexpr.
        */
        static const JulianDayNumber kMaxJulianRep; // = Date::ToJulianRep (December, 31d, Year::eLastYear)
 
    public:
        class FormatException;
 
    public:
        /**
         *  if DataExchange::ValidationStrategy is NOT specified, or == DataExchange::ValidationStrategy::eAssertion, then
         *      \pre kMinJulianRep <= julianRep <= kMaxJulianRep AND Date::kMin <= d <= Date::kMax
         *  else if eThrow, then throw when arguments out of range.
         * 
         *  \par Example Usage
         *      \code
         *          Assert (1906y/May/12d == Date{1906y, May, 12d});
         *      \endcode
         */
        constexpr Date (Date&& src) noexcept      = default;
        constexpr Date (const Date& src) noexcept = default;
        explicit constexpr Date (JulianDayNumber                  julianRep,
                                 DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
        constexpr Date (year y, month m, day d, DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
        constexpr Date (year_month_day ymd, DataExchange::ValidationStrategy validationStrategy = DataExchange::ValidationStrategy::eAssertion);
 
    public:
        /**
         */
        nonvirtual Date& operator= (Date&& rhs) noexcept = default;
        nonvirtual Date& operator= (const Date& rhs)     = default;
 
    public:
        /**
         *  Return the current Date - shorthand for DateTime::Now ().GetDate () (so uses localtime)
         */
        static Date Now () noexcept;
 
    public:
        /**
         *  \brief Y-M-D format - locale independent, and ISO-8601 date format standard
         *
         *  \note sometimes represented as %F (see https://en.cppreference.com/w/cpp/chrono/c/wcsftime), but that's not supported in https://en.cppreference.com/w/cpp/locale/time_get/get.
         *        so equivalent to %Y-%m-%d
         *  \note this is LOCALE-INDEPENDENT
         *  \see kMonthDayYearFormat
         * 
         *  \note also used for XML
         */
        static constexpr string_view kISO8601Format = "%Y-%m-%d"sv;
 
    public:
        /**
         *  \note https://en.cppreference.com/w/cpp/locale/time_get/get
         */
        static constexpr string_view kLocaleStandardFormat = "%x"sv;
 
    public:
        /**
         *  \note https://en.cppreference.com/w/cpp/locale/time_get/get 
         */
        static constexpr string_view kLocaleStandardAlternateFormat = "%Ex"sv;
 
    public:
        /**
         *  \brief classic (american) month-day-year format, but unlike %D, this uses %Y, so the 4-digit form of year
         * 
         *  \note https://en.cppreference.com/w/cpp/locale/time_get/get 
         *  \note This format is LOCALE INDEPENDENT (according to https://en.cppreference.com/w/cpp/locale/time_get/get)
         *  \see kISO8601Format
         */
        static constexpr string_view kMonthDayYearFormat = "%m/%d/%Y"sv;
 
    public:
        /**
         *  Default formats used by Date::Parse () to parse time strings. The first of these - kLocaleStandardFormat, is
         *  the locale-specific date format.
         */
        static const Traversal::Iterable<String> kDefaultParseFormats;
 
    public:
        /**
         * Note that for the consumedCharsInStringUpTo overload, the consumedCharsInStringUpTo is filled in with the position after the last
         * character read (so before the next character to be read).
         *
         *  \note Parse (... locale) with no formats specified, defaults to parsing with kDefaultParseFormats formats.
         *
         *  \note if the locale is not specified, its assumed to be the current locale (locale{}))
         *
         *  \note an empty string produces BadFormat exception.
         *
         *  \see https://en.cppreference.com/w/cpp/locale/time_get/get for allowed formatPatterns
         * 
         *  \note when calling Parse with a format string and no locale, the default locale is assumed
         */
        static Date Parse (const String& rep, const locale& l = locale{});
        static Date Parse (const String& rep, const locale& l, const Traversal::Iterable<String>& formatPatterns);
        static Date Parse (const String& rep, const locale& l, const Traversal::Iterable<String>& formatPatterns, size_t* consumedCharsInStringUpTo);
        static Date Parse (const String& rep, const locale& l, size_t* consumedCharsInStringUpTo);
        static Date Parse (const String& rep, const locale& l, const String& formatPattern);
        static Date Parse (const String& rep, const locale& l, const String& formatPattern, size_t* consumedCharsInStringUpTo);
        static Date Parse (const String& rep, const Traversal::Iterable<String>& formatPatterns);
        static Date Parse (const String& rep, const Traversal::Iterable<String>& formatPatterns, size_t* consumedCharsInStringUpTo);
        static Date Parse (const String& rep, const String& formatPattern);
        static Date Parse (const String& rep, const String& formatPattern, size_t* consumedCharsInStringUpTo);
 
    public:
        /**
         *  \brief like Parse(), but returns nullopt on parse error, not throwing exception.
         * if locale is missing, and formatPattern is not locale independent, the current locale (locale{}) is used.
         *  if rep is empty, this will return nullopt
         */
        static optional<Date> ParseQuietly (const String& rep, const String& formatPattern);
        static optional<Date> ParseQuietly (const String& rep, const locale& l, const String& formatPattern);
 
    private:
        static optional<Date> ParseQuietly_ (const wstring& rep, const time_get<wchar_t>& tmget, const String& formatPattern,
                                             size_t* consumedCharsInStringUpTo);
 
    private:
        static Date Parse_ (const String& rep, const locale& l, const Traversal::Iterable<String>& formatPatterns, size_t* consumedCharsInStringUpTo);
 
    public:
        /**
         *  Date::kMin is the first date this Date class supports representing.
         */
        static const Date kMin; // defined constexpr
 
    public:
        /**
         * Date::kMax is the last date this Date class supports representing.
         */
        static const Date kMax; // defined constexpr
 
    public:
        /**
         */
        nonvirtual constexpr year GetYear () const;
 
    public:
        /**
         */
        nonvirtual constexpr month GetMonth () const;
 
    public:
        /**
         */
        nonvirtual constexpr day GetDayOfMonth () const;
 
    public:
        /**
         */
        nonvirtual weekday GetDayOfWeek () const;
 
    public:
        /**
         *  \brief return the Julian Day Number (JDN) - corresponding to this date object (https://en.wikipedia.org/wiki/Julian_day) - days since Monday, January 1, 4713 BC
         */
        nonvirtual constexpr JulianDayNumber GetJulianRep () const;
 
    public:
        /**
         *  \brief  DisplayFormat is a representation which a date can be transformed in and out of
         *
         *  eCurrentLocale_WithZerosStripped
         *      eCurrentLocale_WithZerosStripped is locale{}, but with many cases of leading zero's,
         *      stripped, so for example, 03/05/2013 becomes 3/5/2013. This only affects the day/month, and not the
         *      year.
         *
         *  \note   Common::DefaultNames<> supported
         */
        enum class NonStandardPrintFormat : uint8_t {
            eCurrentLocale_WithZerosStripped,
 
            eDEFAULT = eCurrentLocale_WithZerosStripped,
 
            Stroika_Define_Enum_Bounds (eCurrentLocale_WithZerosStripped, eCurrentLocale_WithZerosStripped)
        };
 
    public:
        static constexpr NonStandardPrintFormat eCurrentLocale_WithZerosStripped = NonStandardPrintFormat::eCurrentLocale_WithZerosStripped;
 
    public:
        /**
         *  For formatPattern, see http://en.cppreference.com/w/cpp/locale/time_put/put
         *  If only formatPattern specified, and no locale, use default (global) locale.
         */
        nonvirtual String Format (NonStandardPrintFormat pf = NonStandardPrintFormat::eDEFAULT) const;
        nonvirtual String Format (const locale& l) const;
        nonvirtual String Format (const locale& l, const String& formatPattern) const;
        nonvirtual String Format (const String& formatPattern) const;
 
    public:
        /**
         *  @see Characters::ToString ()
         */
        nonvirtual String ToString () const;
 
    public:
        /**
         * Returns a new Date object based on this Date, with 'dayCount' days added.
         *
         *  \par Example Usage
         *      \code
         *          Date d = 1906y/May/12d;
         *          d = d.Add (1);      // OR d = d + 1;
         *          Assert (d == 1906y/May/13d);
         *      \endcode
         * 
         *  \note - a duration is much more precise than a day, so that overload rounds.
         */
        nonvirtual [[nodiscard]] Date Add (int d) const;
        nonvirtual [[nodiscard]] Date Add (days d) const;
        nonvirtual [[nodiscard]] Date Add (const Duration& d) const;
 
    public:
        /**
         *  returns number of days between a start day and end day: Never less than zero.
         *  No argument version computes days between *this and 'now'.
         */
        static days     Since (Date dStart, Date dEnd);
        nonvirtual days Since () const;
 
    public:
        /**
         *  \brief Returns the difference (*this - rhs) between the two Date records;
         *
         *  \note Before Stroika v3.0d1, this returned SignedJulianDayNumber.
         */
        nonvirtual days Difference (const Date& rhs) const;
 
    public:
        /**
         *  \brief  Syntactic sure for Add (n);
         *
         *  \par Example Usage
         *      \code
         *          Date d = 1906y/May/12d;
         *          d = d + 1;
         *          Assert (d == 1906y/May/13d);
         *      \endcode
         */
        nonvirtual Date operator+ (int daysOffset) const;
        nonvirtual Date operator+ (days daysOffset) const;
        nonvirtual Date operator+ (const Duration& d) const;
 
    public:
        /**
         *  days operator- (const Date& rhs): Syntactic sugar on Difference()
         *  Date                operator- (days or int daysOffset)  Syntactic sugar on Add(-arg)
         * 
         *  \note subtracting by duration 'd' rounds to days...
         */
        nonvirtual days operator- (const Date& rhs) const;
        nonvirtual Date operator- (int daysOffset) const;
        nonvirtual Date operator- (days daysOffset) const;
        nonvirtual Date operator- (const Duration& d) const;
 
    public:
        /**
         */
        constexpr strong_ordering operator<=> (const Date& rhs) const = default;
 
    public:
        /**
         * Defined for
         *      struct tm
         *      year_month_day
         * 
         *  Generally constexpr where possible.
         */
        template <typename T>
        nonvirtual T As () const;
 
    public:
        using JulianRepType [[deprecated ("Since Stroika v3.0d1 - use JulianDayNumber")]]             = JulianDayNumber;
        using SignedJulianRepType [[deprecated ("Since Stroika v3.0d1 - use SignedJulianDayNumber")]] = SignedJulianDayNumber;
        [[deprecated ("Since Stroika v3.0d1 - use.Add () - now Date immutable")]] Date& operator++ ()
        {
            *this = Add (1);
            return *this;
        }
        [[deprecated ("Since Stroika v3.0d1 - use.Add () - now Date immutable")]] Date operator++ (int)
        {
            return *this + 1;
        }
        [[deprecated ("Since Stroika v3.0d1, use As<year_month_day> ()")]] void mdy (month* m, day* d, year* y) const
        {
            RequireNotNull (m);
            RequireNotNull (d);
            RequireNotNull (y);
            *m = fRep_.month ();
            *d = fRep_.day ();
            *y = fRep_.year ();
        }
        [[deprecated ("Since Stroika v3.0d1 - use Add(days)")]] Date AddDays (SignedJulianDayNumber dayCount) const
        {
            return Add (chrono::days{dayCount});
        }
        [[deprecated ("Since Stroika 3.0d1 - use Since")]] JulianDayNumber DaysSince () const
        {
            return static_cast<JulianDayNumber> (Since ().count ());
        }
 
    private:
        static optional<Date> LocaleFreeParseQuietly_kMonthDayYearFormat_ (const wstring& rep, size_t* consumedCharsInStringUpTo);
 
    private:
        static constexpr int kTM_Year_RelativeToYear_{1900}; // see https://man7.org/linux/man-pages/man3/ctime.3.html
 
    private:
        static Date AsDate_ (const ::tm& when);
 
    private:
        year_month_day fRep_;
    };
    static_assert (sizeof (Date) == sizeof (year_month_day)); // generally 4 bytes
    static_assert (totally_ordered<Date>);
 
    template <>
    constexpr ::tm Date::As () const;
    template <>
    constexpr year_month_day Date::As () const;
 
    class Date::FormatException : public Execution::RuntimeErrorException<> {
    public:
        FormatException ();
 
    public:
        /**
         */
        static const FormatException kThe;
    };
    inline const Date::FormatException       Date::FormatException::kThe;
    inline const Traversal::Iterable<String> Date::kDefaultParseFormats{
        kLocaleStandardFormat,          // x (kLocaleStandardFormat) parses the locale's standard date representation
        kLocaleStandardAlternateFormat, // Ex (kLocaleStandardAlternateFormat) parses the locale's alternative date representation, e.g. expecting 平成23年 (year Heisei 23) instead of 2011年 (year 2011) in ja_JP locale
        kMonthDayYearFormat,            // Before Stroika 2.1b10, this was L"%D" (=="%m/%d/%y) which is the 2-digit year
        kISO8601Format,
    };
 
    Date::SignedJulianDayNumber DayDifference (const Date& lhs, const Date& rhs);
    int                         YearDifference (const Date& lhs, const Date& rhs);
    float                       YearDifferenceF (const Date& lhs, const Date& rhs);
 
    String GetFormattedAge (const optional<Date>& birthDate, const optional<Date>& deathDate = {}); // returns ? if not a good src date
    String GetFormattedAgeWithUnit (const optional<Date>& birthDate, const optional<Date>& deathDate = {}, bool abbrevUnit = true); // returns ? if not a good src date
 
}
 
namespace Stroika::Foundation::Traversal::RangeTraits {
 
    template <>
    struct DefaultOpenness<Time::Date> : ExplicitOpenness<Openness::eClosed, Openness::eClosed> {};
    template <>
    struct DefaultDifferenceTypes<Time::Date> : ExplicitDifferenceTypes<chrono::days> {};
    template <>
    struct Default<Time::Date> : ExplicitOpennessAndDifferenceType<Time::Date> {
        static const Time::Date kLowerBound;
        static const Time::Date kUpperBound;
 
        static Time::Date GetNext (Time::Date n);
        static Time::Date GetPrevious (Time::Date n);
 
        static size_t DifferenceToSizeT (chrono::days s)
        {
            return size_t (s.count ());
        }
    };
 
}
 
/*
 ********************************************************************************
 ***************************** Implementation Details ***************************
 ********************************************************************************
 */
#include "Date.inl"
 
#endif /*_Stroika_Foundation_Time_Date_h_*/