PDIDATE_Julian.cs

using System;
using System.Collections.Generic;
using System.Text;

namespace PDTools.Structures;

/// <summary>
/// From GT4, returns signed dates
/// </summary>
public class PDIDATE_Julian
{
    private static readonly short[] month_to_day =
    [
        0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365,
    ];

    private static readonly short[] month_to_day_366 =
    [
         0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366,
    ];

    const int Jan11JulianDay = 1721426;

    public static int DateToDay(int year, int month, int day)
    {
        int monthIndex = month - 1;
        int dayIndex = year - 1;

        int current_month = monthIndex % 12;
        int yr = day + monthIndex / 12;

        if (current_month < 0)
        {
            current_month += 12;
            --yr;
        }

        int yrIndex = yr - 1;

        bool isLeapYear = yrIndex % 4 == 0 && (yrIndex % 100 != 0 || yrIndex % 400 == 0);

        short[] arr = isLeapYear ? month_to_day : month_to_day_366;

        return 365 * (yrIndex - 1)
            + (yrIndex - 1) / 4
            - yrIndex / 100
            + yrIndex / 100 / 4
            + arr[current_month]
            + dayIndex
            + Jan11JulianDay;
    }
}