Api/DouyinApi.Services/Naming/BaziCalculator.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace DouyinApi.Services.Naming;

internal sealed class BaziCalculator
{
    private static readonly string[] HeavenlyStems =
    {
        "甲", "乙", "丙", "丁", "戊", "己", "庚", "辛", "壬", "癸"
    };

    private static readonly string[] EarthlyBranches =
    {
        "子", "丑", "寅", "卯", "辰", "巳", "午", "未", "申", "酉", "戌", "亥"
    };

    private static readonly int[] MonthNumberToBranchIndex =
    {
        2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 0, 1
    };

    private static readonly string[] ElementOrder = { "木", "火", "土", "金", "水" };

    private static readonly Dictionary<string, string> StemElements = new()
    {
        ["甲"] = "木",
        ["乙"] = "木",
        ["丙"] = "火",
        ["丁"] = "火",
        ["戊"] = "土",
        ["己"] = "土",
        ["庚"] = "金",
        ["辛"] = "金",
        ["壬"] = "水",
        ["癸"] = "水"
    };

    private static readonly Dictionary<string, string> BranchElements = new()
    {
        ["子"] = "水",
        ["丑"] = "土",
        ["寅"] = "木",
        ["卯"] = "木",
        ["辰"] = "土",
        ["巳"] = "火",
        ["午"] = "火",
        ["未"] = "土",
        ["申"] = "金",
        ["酉"] = "金",
        ["戌"] = "土",
        ["亥"] = "水"
    };

    private static readonly Dictionary<string, int> HourStemBaseIndex = new()
    {
        ["甲"] = 0,
        ["己"] = 0,
        ["乙"] = 2,
        ["庚"] = 2,
        ["丙"] = 4,
        ["辛"] = 4,
        ["丁"] = 6,
        ["壬"] = 6,
        ["戊"] = 8,
        ["癸"] = 8
    };

    private static readonly Dictionary<string, int> MonthStemBaseIndex = new()
    {
        ["甲"] = 2,
        ["己"] = 2,
        ["乙"] = 4,
        ["庚"] = 4,
        ["丙"] = 6,
        ["辛"] = 6,
        ["丁"] = 8,
        ["壬"] = 8,
        ["戊"] = 0,
        ["癸"] = 0
    };

    private static readonly DateTime DayBaseDate = new(1900, 1, 31);
    private const int DayBaseIndex = 0; // 1900-01-31 对应甲子日

    public BaziProfile Calculate(DateTime birthDateTime)
    {
        var solarYear = birthDateTime.Year;
        var currentYearSpring = new DateTime(birthDateTime.Year, 2, 4);
        if (birthDateTime < currentYearSpring)
        {
            solarYear -= 1;
        }

        var yearStemIndex = Mod(solarYear - 4, HeavenlyStems.Length);
        var yearBranchIndex = Mod(solarYear - 4, EarthlyBranches.Length);

        var monthNumber = ResolveMonthNumber(birthDateTime, solarYear);
        var monthStemBase = MonthStemBaseIndex[HeavenlyStems[yearStemIndex]];
        var monthStemIndex = Mod(monthStemBase + monthNumber - 1, HeavenlyStems.Length);
        var monthBranchIndex = MonthNumberToBranchIndex[monthNumber - 1];

        var dayCycleIndex = Mod(DayBaseIndex + (int)(birthDateTime.Date - DayBaseDate).TotalDays, 60);
        var dayStemIndex = Mod(dayCycleIndex, HeavenlyStems.Length);
        var dayBranchIndex = Mod(dayCycleIndex, EarthlyBranches.Length);

        var hourBranchIndex = GetHourBranchIndex(birthDateTime.Hour);
        var hourStemIndex = Mod(HourStemBaseIndex[HeavenlyStems[dayStemIndex]] + hourBranchIndex, HeavenlyStems.Length);

        var yearStem = HeavenlyStems[yearStemIndex];
        var yearBranch = EarthlyBranches[yearBranchIndex];
        var monthStem = HeavenlyStems[monthStemIndex];
        var monthBranch = EarthlyBranches[monthBranchIndex];
        var dayStem = HeavenlyStems[dayStemIndex];
        var dayBranch = EarthlyBranches[dayBranchIndex];
        var hourStem = HeavenlyStems[hourStemIndex];
        var hourBranch = EarthlyBranches[hourBranchIndex];

        var elementCounts = ElementOrder.ToDictionary(element => element, _ => 0);
        IncrementElement(elementCounts, StemElements[yearStem]);
        IncrementElement(elementCounts, BranchElements[yearBranch]);
        IncrementElement(elementCounts, StemElements[monthStem]);
        IncrementElement(elementCounts, BranchElements[monthBranch]);
        IncrementElement(elementCounts, StemElements[dayStem]);
        IncrementElement(elementCounts, BranchElements[dayBranch]);
        IncrementElement(elementCounts, StemElements[hourStem]);
        IncrementElement(elementCounts, BranchElements[hourBranch]);

        var counts = ElementOrder
            .Select(element => new KeyValuePair<string, int>(element, elementCounts[element]))
            .ToList();

        var max = counts.Max(pair => pair.Value);
        var min = counts.Min(pair => pair.Value);

        var isBalanced = max - min <= 1;
        var weakElements = isBalanced
            ? Array.Empty<string>()
            : counts.Where(pair => pair.Value == min).Select(pair => pair.Key).ToArray();
        var strongElements = counts.Where(pair => pair.Value == max).Select(pair => pair.Key).ToArray();

        return new BaziProfile(
            yearStem,
            yearBranch,
            monthStem,
            monthBranch,
            dayStem,
            dayBranch,
            hourStem,
            hourBranch,
            elementCounts,
            ElementOrder,
            weakElements,
            strongElements,
            isBalanced);
    }

    private static int ResolveMonthNumber(DateTime dt, int solarYear)
    {
        var springStart = new DateTime(solarYear, 2, 4);
        var jingZhe = new DateTime(solarYear, 3, 6);
        var qingMing = new DateTime(solarYear, 4, 5);
        var liXia = new DateTime(solarYear, 5, 5);
        var mangZhong = new DateTime(solarYear, 6, 6);
        var xiaoShu = new DateTime(solarYear, 7, 7);
        var liQiu = new DateTime(solarYear, 8, 8);
        var baiLu = new DateTime(solarYear, 9, 8);
        var hanLu = new DateTime(solarYear, 10, 8);
        var liDong = new DateTime(solarYear, 11, 7);
        var daXue = new DateTime(solarYear, 12, 7);
        var xiaoHan = new DateTime(solarYear + 1, 1, 6);
        var nextSpring = new DateTime(solarYear + 1, 2, 4);

        if (dt >= springStart && dt < jingZhe)
        {
            return 1;
        }
        if (dt >= jingZhe && dt < qingMing)
        {
            return 2;
        }
        if (dt >= qingMing && dt < liXia)
        {
            return 3;
        }
        if (dt >= liXia && dt < mangZhong)
        {
            return 4;
        }
        if (dt >= mangZhong && dt < xiaoShu)
        {
            return 5;
        }
        if (dt >= xiaoShu && dt < liQiu)
        {
            return 6;
        }
        if (dt >= liQiu && dt < baiLu)
        {
            return 7;
        }
        if (dt >= baiLu && dt < hanLu)
        {
            return 8;
        }
        if (dt >= hanLu && dt < liDong)
        {
            return 9;
        }
        if (dt >= liDong && dt < daXue)
        {
            return 10;
        }
        if (dt >= daXue && dt < xiaoHan)
        {
            return 11;
        }
        if (dt >= xiaoHan && dt < nextSpring)
        {
            return 12;
        }
        return 11;
    }

    private static int GetHourBranchIndex(int hour)
    {
        return ((hour + 1) / 2) % EarthlyBranches.Length;
    }

    private static void IncrementElement(IDictionary<string, int> container, string element)
    {
        if (container.TryGetValue(element, out var value))
        {
            container[element] = value + 1;
        }
    }

    private static int Mod(int value, int modulus)
    {
        var result = value % modulus;
        return result < 0 ? result + modulus : result;
    }
}

internal sealed class BaziProfile
{
    public BaziProfile(
        string yearStem,
        string yearBranch,
        string monthStem,
        string monthBranch,
        string dayStem,
        string dayBranch,
        string hourStem,
        string hourBranch,
        IReadOnlyDictionary<string, int> elementCounts,
        IReadOnlyList<string> elementOrder,
        IReadOnlyList<string> weakElements,
        IReadOnlyList<string> strongElements,
        bool isBalanced)
    {
        YearStem = yearStem;
        YearBranch = yearBranch;
        MonthStem = monthStem;
        MonthBranch = monthBranch;
        DayStem = dayStem;
        DayBranch = dayBranch;
        HourStem = hourStem;
        HourBranch = hourBranch;
        ElementCounts = elementCounts;
        ElementOrder = elementOrder;
        WeakElements = weakElements;
        StrongElements = strongElements;
        IsBalanced = isBalanced;
    }

    public string YearStem { get; }

    public string YearBranch { get; }

    public string MonthStem { get; }

    public string MonthBranch { get; }

    public string DayStem { get; }

    public string DayBranch { get; }

    public string HourStem { get; }

    public string HourBranch { get; }

    public IReadOnlyDictionary<string, int> ElementCounts { get; }

    public IReadOnlyList<string> ElementOrder { get; }

    public IReadOnlyList<string> WeakElements { get; }

    public IReadOnlyList<string> StrongElements { get; }

    public bool IsBalanced { get; }
}