using System;

namespace AcDream.Core.World;

/// <summary>
/// Asheron's Call "Portal Year" calendar + time-of-day math — faithful
/// port of retail's <c>DerethDateTime</c> (r12 §1.2 + ACE
/// <c>DerethDateTime.cs</c>).
///
/// <para>
/// The server transports an absolute tick count as a double (seconds
/// since a seed chosen at boot). The client doesn't need to know which
/// seed was chosen — it just needs the tick count to derive:
/// <list type="bullet">
///   <item><description>
///     Day fraction in [0, 1): where we are in the 16-hour Derethian day.
///   </description></item>
///   <item><description>
///     Hour name (Darktide, Dawnsong, Midsong, Warmtide, Evensong,
///     Gloaming and their "-and-Half" mid-hour variants).
///   </description></item>
///   <item><description>
///     Day / night flag for spawn rules + lighting.
///   </description></item>
///   <item><description>
///     Season / year / month for calendar UI.
///   </description></item>
/// </list>
/// </para>
///
/// <para>
/// Constants are retail-exact; changing them breaks the calendar panel's
/// "today is" display.
/// </para>
/// </summary>
public static class DerethDateTime
{
    public const int HoursInADay = 16;
    public const int DaysInAMonth = 30;
    public const int MonthsInAYear = 12;

    /// <summary>Ticks per Derethian day (matches retail GameTime.DayLength).</summary>
    public const double DayTicks = 7620.0;

    /// <summary>Ticks per Derethian hour (DayTicks / 16).</summary>
    public const double HourTicks = DayTicks / HoursInADay;     // 476.25

    /// <summary>Ticks per Derethian month.</summary>
    public const double MonthTicks = DayTicks * DaysInAMonth;    // 228,600

    /// <summary>Ticks per Derethian year.</summary>
    public const double YearTicks = MonthTicks * MonthsInAYear;  // 2,743,200

    /// <summary>
    /// Above this tick count the retail client crashes on connect.
    /// ~1.07 billion seconds = PY 401 Thistledown 2 Morntide-and-Half.
    /// </summary>
    public const double MaxTicks = 1_073_741_828.0;

    /// <summary>
    /// Base/anchor year for the Portal Year calendar (retail
    /// <c>GameTime.ZeroYear</c> = 10). Tick 0 corresponds to PY 10,
    /// Morningthaw 1, Morntide-and-Half. Retail's <c>TimeOfDay+0x64</c>
    /// ("absolute year") includes this offset, and <c>SkyDesc::PickCurrentDayGroup</c>
    /// (<c>FUN_00501990</c>) feeds the absolute year into its LCG seed.
    /// </summary>
    public const int ZeroYear = 10;

    /// <summary>
    /// The 16 named hour slots (r12 §1.2). Each one is half a Derethian
    /// hour; the "-and-Half" variants are the second half.
    /// </summary>
    public enum HourName
    {
        Darktide = 0,
        DarktideAndHalf,
        Foredawn,
        ForedawnAndHalf,
        Dawnsong,           // day starts here (hour 5)
        DawnsongAndHalf,
        Morntide,
        MorntideAndHalf,
        Midsong,
        MidsongAndHalf,
        Warmtide,
        WarmtideAndHalf,    // day ends here (hour 12)
        Evensong,
        EvensongAndHalf,
        Gloaming,
        GloamingAndHalf,
    }

    /// <summary>
    /// Derethian months in chronological order. Year-0 begins at month 0
    /// (<see cref="Morningthaw"/>) and progresses through the 12-month
    /// cycle. Names + order match retail's calendar display
    /// (<c>GameTime::CalcDayBegin</c> + <c>GetDateTimeString</c> at
    /// <c>0x005a6530</c>) and ACE's <c>DerethDateTime.cs</c>. Verified
    /// against retail's <c>@timestamp</c> output in 2026-04-27 dual-
    /// client comparison: at day-of-year 83, retail shows
    /// "Seedsow 24" — that fixes month index 2 = Seedsow.
    /// </summary>
    public enum MonthName
    {
        Morningthaw = 0,
        Solclaim,
        Seedsow,
        Leafdawning,
        Verdantine,
        Thistledown,
        Harvestgain,
        Leafcull,
        Frostfell,
        Snowreap,
        Coldeve,
        Wintersebb,
    }

    /// <summary>
    /// Default anchor ticks when no Region has been loaded: +7/16 of a
    /// full day (3333.75) per ACE's DerethDateTime comment "tick 0 =
    /// Morntide-and-Half". This is a fallback — retail reads
    /// <c>GameTime.ZeroTimeOfYear</c> from the Region dat (verified
    /// <c>3600</c> in Dereth, 2026-04-23 live dump) and uses that as
    /// the additive offset. We override <see cref="OriginOffsetTicks"/>
    /// once the dat loads; this constant is only for offline tests.
    /// </summary>
    public const double DayFractionOriginOffsetTicks = (7.0 / 16.0) * DayTicks; // 3333.75

    /// <summary>
    /// Additive tick offset applied before every calendar extraction
    /// (DayFraction / Year / DayOfYear / AbsoluteYear). Populated from
    /// the Region dat's <c>GameTime.ZeroTimeOfYear</c> via
    /// <see cref="SetOriginOffsetFromDat"/> at Region load. Defaults to
    /// <see cref="DayFractionOriginOffsetTicks"/> for offline tests and
    /// for the boot window before the dat parses.
    ///
    /// <para>
    /// Live Dereth dat value: <c>3600</c>. Retail's
    /// <c>GameTime::CalcDayBegin</c> at <c>0x005a6400</c> (decomp line
    /// 434549) computes <c>arg2 + zero_time_of_year</c> as the basis for
    /// year/day-of-year extraction, then derives <c>time_of_day_begin</c>
    /// such that <c>(arg2 - time_of_day_begin) / day_length</c> in
    /// <c>CalcTimeOfDay</c> gives <c>(arg2 + zero_time_of_year) mod day_length / day_length</c>.
    /// Net: the formula is ADD, not subtract — confirmed via the explicit
    /// add at line 434549. (A 2026-04-26 attempt to flip the sign over-
    /// corrected and broke DG selection; reverted in the same commit.)
    /// </para>
    /// </summary>
    public static double OriginOffsetTicks { get; private set; } = DayFractionOriginOffsetTicks;

    /// <summary>
    /// Adopt the Region dat's <c>GameTime.ZeroTimeOfYear</c> as the
    /// calendar-extraction offset. Idempotent; safe to call on every
    /// Region reload (though in practice Dereth is the only region).
    /// </summary>
    public static void SetOriginOffsetFromDat(double zeroTimeOfYear)
    {
        OriginOffsetTicks = zeroTimeOfYear;
    }

    /// <summary>
    /// Day fraction [0, 1): 0 = Darktide (midnight), 0.5 =
    /// Midsong-and-Half (noon-ish), 1.0 wraps to 0. Anchored by
    /// <see cref="OriginOffsetTicks"/> (dat's ZeroTimeOfYear).
    /// </summary>
    public static double DayFraction(double ticks)
    {
        if (ticks < 0) ticks = 0;
        double shifted = ticks + OriginOffsetTicks;
        double rem = shifted - Math.Floor(shifted / DayTicks) * DayTicks;
        return rem / DayTicks;
    }

    /// <summary>
    /// Hour name for the given tick count. Picked from
    /// <see cref="HourName"/> based on which named half-hour slot the
    /// current time falls in.
    /// </summary>
    public static HourName CurrentHour(double ticks)
    {
        double f = DayFraction(ticks);
        int slot = (int)Math.Floor(f * HoursInADay);
        if (slot < 0) slot = 0;
        if (slot > 15) slot = 15;
        return (HourName)slot;
    }

    /// <summary>
    /// True when the current time is "day" (Dawnsong through
    /// Warmtide-and-Half, hours 5–12 inclusive on the 16-hour scale).
    /// </summary>
    public static bool IsDaytime(double ticks)
    {
        int h = (int)CurrentHour(ticks);
        return h >= (int)HourName.Dawnsong && h <= (int)HourName.WarmtideAndHalf;
    }

    /// <summary>
    /// Derethian calendar breakdown: (year, month, day, hour).
    /// <see cref="Year"/> is the absolute Portal Year (= relative-year +
    /// <see cref="ZeroYear"/>) so the value matches retail's
    /// <c>@timestamp</c> output ("Date: &lt;Month&gt; &lt;Day&gt;,
    /// &lt;Year&gt; P.Y."). Day is 1-based within the month (1..30).
    /// </summary>
    public readonly record struct Calendar(int Year, MonthName Month, int Day, HourName Hour);

    public static Calendar ToCalendar(double ticks)
    {
        if (ticks < 0) ticks = 0;
        double shifted = ticks + OriginOffsetTicks;
        int relativeYear = (int)(shifted / YearTicks);
        double tYear = shifted - relativeYear * YearTicks;
        int monthIdx = (int)(tYear / MonthTicks);
        if (monthIdx > 11) monthIdx = 11;
        double tMonth = tYear - monthIdx * MonthTicks;
        int day = (int)(tMonth / DayTicks) + 1;
        if (day > DaysInAMonth) day = DaysInAMonth;

        // Absolute Portal Year for display: retail's @timestamp shows
        // PY-with-base (10 P.Y. == year 0 of the calendar epoch), so add
        // ZeroYear here. Matches AbsoluteYear() and the retail decomp at
        // FUN_005a7510:5300.
        return new Calendar(relativeYear + ZeroYear, (MonthName)monthIdx, day, CurrentHour(ticks));
    }

    /// <summary>
    /// Year offset from tick-0 (number of Derethian years elapsed since
    /// the PY <see cref="ZeroYear"/> anchor). Shifted by
    /// <see cref="OriginOffsetTicks"/> (dat's ZeroTimeOfYear) to match
    /// retail's TimeOfDay::OnTick extraction. Use for time-delta math.
    /// For calendar display and for the retail DayGroup LCG seed, use
    /// <see cref="AbsoluteYear"/> which adds the <c>ZeroYear</c> base.
    /// </summary>
    public static int Year(double ticks)
    {
        if (ticks < 0) ticks = 0;
        double shifted = ticks + OriginOffsetTicks;
        return (int)(shifted / YearTicks);
    }

    /// <summary>
    /// Absolute Portal Year (<see cref="ZeroYear"/> + relative year).
    /// Matches retail's <c>TimeOfDay + 0x64</c> field
    /// (<c>FUN_005a7510:5300</c>:
    /// <c>floor((worldTime+base)/secsPerYear) + baseYear</c>). This is
    /// the value the retail DayGroup picker (<c>FUN_00501990</c>) feeds
    /// into its LCG seed, so acdream must match for identical weather
    /// picks vs retail.
    /// </summary>
    public static int AbsoluteYear(double ticks) => Year(ticks) + ZeroYear;

    /// <summary>
    /// Day index within the current Derethian year in [0, 359] (360 days
    /// per year = 12 months × 30). Shifted by <see cref="OriginOffsetTicks"/>
    /// (dat's ZeroTimeOfYear) to match retail's <c>TimeOfDay + 0x68</c>
    /// field (<c>FUN_005a7510:5304</c>:
    /// <c>floor(withinYearSec / secsPerDay)</c>). Consumed by
    /// <c>SkyDesc.PickCurrentDayGroup</c> as part of the per-day seed.
    /// </summary>
    public static int DayOfYear(double ticks)
    {
        if (ticks < 0) ticks = 0;
        double shifted = ticks + OriginOffsetTicks;
        int year = (int)(shifted / YearTicks);
        double tYear = shifted - year * YearTicks;
        int d = (int)(tYear / DayTicks);
        if (d < 0) d = 0;
        if (d > 359) d = 359;
        return d;
    }
}