#include "DateLUTImpl.h"

#include <cctz/civil_time.h>
#include <cctz/time_zone.h>
#include <cctz/zone_info_source.h>
#include <common/getResource.h>
#include <Poco/Exception.h>

#include <algorithm>
#include <cassert>
#include <chrono>
#include <cstring>
#include <iostream>
#include <memory>


namespace
{

UInt8 getDayOfWeek(const cctz::civil_day & date)
{
    cctz::weekday day_of_week = cctz::get_weekday(date);
    switch (day_of_week)
    {
        case cctz::weekday::monday:     return 1;
        case cctz::weekday::tuesday:    return 2;
        case cctz::weekday::wednesday:  return 3;
        case cctz::weekday::thursday:   return 4;
        case cctz::weekday::friday:     return 5;
        case cctz::weekday::saturday:   return 6;
        case cctz::weekday::sunday:     return 7;
    }
    __builtin_unreachable();
}

}


__attribute__((__weak__)) extern bool inside_main;

DateLUTImpl::DateLUTImpl(const std::string & time_zone_)
    : time_zone(time_zone_)
{
    /// DateLUT should not be initialized in global constructors for the following reasons:
    /// 1. It is too heavy.
    if (&inside_main)
        assert(inside_main);

    cctz::time_zone cctz_time_zone;
    if (!cctz::load_time_zone(time_zone, &cctz_time_zone))
        throw Poco::Exception("Cannot load time zone " + time_zone_);

    constexpr cctz::civil_day epoch{1970, 1, 1};
    constexpr cctz::civil_day lut_start{DATE_LUT_MIN_YEAR, 1, 1};
    time_t start_of_day;

    /// Note: it's validated against all timezones in the system.
    static_assert((epoch - lut_start) == daynum_offset_epoch);

    offset_at_start_of_epoch = cctz_time_zone.lookup(cctz_time_zone.lookup(epoch).pre).offset;
    offset_at_start_of_lut = cctz_time_zone.lookup(cctz_time_zone.lookup(lut_start).pre).offset;
    offset_is_whole_number_of_hours_during_epoch = true;

    cctz::civil_day date = lut_start;

    UInt32 i = 0;
    do
    {
        cctz::time_zone::civil_lookup lookup = cctz_time_zone.lookup(date);

        /// Ambiguity is possible if time was changed backwards at the midnight
        /// or after midnight time has been changed back to midnight, for example one hour backwards at 01:00
        /// or after midnight time has been changed to the previous day, for example two hours backwards at 01:00
        /// Then midnight appears twice. Usually time change happens exactly at 00:00 or 01:00.

        /// If transition did not involve previous day, we should use the first midnight as the start of the day,
        /// otherwise it's better to use the second midnight.

        std::chrono::time_point start_of_day_time_point = lookup.trans < lookup.post
            ? lookup.post /* Second midnight appears after transition, so there was a piece of previous day after transition */
            : lookup.pre;

        start_of_day = std::chrono::system_clock::to_time_t(start_of_day_time_point);

        Values & values = lut[i];
        values.year = date.year();
        values.month = date.month();
        values.day_of_month = date.day();
        values.day_of_week = getDayOfWeek(date);
        values.date = start_of_day;

        assert(values.year >= DATE_LUT_MIN_YEAR && values.year <= DATE_LUT_MAX_YEAR + 1);
        assert(values.month >= 1 && values.month <= 12);
        assert(values.day_of_month >= 1 && values.day_of_month <= 31);
        assert(values.day_of_week >= 1 && values.day_of_week <= 7);

        if (values.day_of_month == 1)
        {
            cctz::civil_month month(date);
            values.days_in_month = cctz::civil_day(month + 1) - cctz::civil_day(month);
        }
        else
            values.days_in_month = i != 0 ? lut[i - 1].days_in_month : 31;

        values.time_at_offset_change_value = 0;
        values.amount_of_offset_change_value = 0;

        if (offset_is_whole_number_of_hours_during_epoch && start_of_day > 0 && start_of_day % 3600)
            offset_is_whole_number_of_hours_during_epoch = false;

        /// If UTC offset was changed this day.
        /// Change in time zone without transition is possible, e.g. Moscow 1991 Sun, 31 Mar, 02:00 MSK to EEST
        cctz::time_zone::civil_transition transition{};
        if (cctz_time_zone.next_transition(start_of_day_time_point - std::chrono::seconds(1), &transition)
            && (cctz::civil_day(transition.from) == date || cctz::civil_day(transition.to) == date)
            && transition.from != transition.to)
        {
            values.time_at_offset_change_value = (transition.from - cctz::civil_second(date)) / Values::OffsetChangeFactor;
            values.amount_of_offset_change_value = (transition.to - transition.from) / Values::OffsetChangeFactor;

//            std::cerr << time_zone << ", " << date << ": change from " << transition.from << " to " << transition.to << "\n";
//            std::cerr << time_zone << ", " << date << ": change at " << values.time_at_offset_change() << " with " << values.amount_of_offset_change() << "\n";

            /// We don't support too large changes.
            if (values.amount_of_offset_change_value > 24 * 4)
                values.amount_of_offset_change_value = 24 * 4;
            else if (values.amount_of_offset_change_value < -24 * 4)
                values.amount_of_offset_change_value = -24 * 4;

            /// We don't support cases when time change results in switching to previous day.
            /// Shift the point of time change later.
            if (values.time_at_offset_change_value + values.amount_of_offset_change_value < 0)
                values.time_at_offset_change_value = -values.amount_of_offset_change_value;
        }

        /// Going to next day.
        ++date;
        ++i;
    }
    while (i < DATE_LUT_SIZE && lut[i - 1].year <= DATE_LUT_MAX_YEAR);

    /// Fill excessive part of lookup table. This is needed only to simplify handling of overflow cases.
    while (i < DATE_LUT_SIZE)
    {
        lut[i] = lut[i - 1];
        ++i;
    }

    /// Fill lookup table for years and months.
    size_t year_months_lut_index = 0;
    size_t first_day_of_last_month = 0;

    for (size_t day = 0; day < DATE_LUT_SIZE; ++day)
    {
        const Values & values = lut[day];

        if (values.day_of_month == 1)
        {
            if (values.month == 1)
                years_lut[values.year - DATE_LUT_MIN_YEAR] = day;

            year_months_lut_index = (values.year - DATE_LUT_MIN_YEAR) * 12 + values.month - 1;
            years_months_lut[year_months_lut_index] = day;
            first_day_of_last_month = day;
        }
    }

    /// Fill the rest of lookup table with the same last month (2106-02-01).
    for (; year_months_lut_index < DATE_LUT_YEARS * 12; ++year_months_lut_index)
    {
        years_months_lut[year_months_lut_index] = first_day_of_last_month;
    }
}


#if !defined(ARCADIA_BUILD) /// Arcadia's variant of CCTZ already has the same implementation.

/// Prefer to load timezones from blobs linked to the binary.
/// The blobs are provided by "tzdata" library.
/// This allows to avoid dependency on system tzdata.
namespace cctz_extension
{
    namespace
    {
        class Source : public cctz::ZoneInfoSource
        {
        public:
            Source(const char * data_, size_t size_) : data(data_), size(size_) {}

            size_t Read(void * buf, size_t bytes) override
            {
                if (bytes > size)
                    bytes = size;
                memcpy(buf, data, bytes);
                data += bytes;
                size -= bytes;
                return bytes;
            }

            int Skip(size_t offset) override
            {
                if (offset <= size)
                {
                    data += offset;
                    size -= offset;
                    return 0;
                }
                else
                {
                    errno = EINVAL;
                    return -1;
                }
            }
        private:
            const char * data;
            size_t size;
        };

        std::unique_ptr<cctz::ZoneInfoSource> custom_factory(
            const std::string & name,
            const std::function<std::unique_ptr<cctz::ZoneInfoSource>(const std::string & name)> & fallback)
        {
            std::string_view resource = getResource(name);
            if (!resource.empty())
                return std::make_unique<Source>(resource.data(), resource.size());

            return fallback(name);
        }
    }

    ZoneInfoSourceFactory zone_info_source_factory = custom_factory;
}

#endif