h-school/src/HSchool.Simulation/WeatherSampler.cs

using HSchool.Content;
using HSchool.People;

namespace HSchool.Simulation;

/// <summary>
/// Outdoor temperature and precipitation from a climate preset, the school seed, and the
/// calendar. Same inputs always produce the same street. Not a tick accumulator — sample it
/// when the clock label or warmth would move.
/// </summary>
public static class WeatherSampler
{
    private const int DaySalt = 0x57EA11;
    private const int HourSalt = 0x57EA12;

    public static OutdoorWeather Sample(ClimatePresetDef preset, int schoolSeed, DateTime time)
    {
        ArgumentNullException.ThrowIfNull(preset);

        var utc = DateTime.SpecifyKind(time, DateTimeKind.Utc);
        var month = utc.Month;
        var norm = MonthNorm(preset, month);
        var dayNumber = DateOnly.FromDateTime(utc).DayNumber;
        var dayNoise = SignedUnit(Seed.Mix(schoolSeed, dayNumber, DaySalt)) * preset.DaySpread;
        var hour = utc.Hour + utc.Minute / 60d + utc.Second / 3600d;
        var diurnal = -Math.Cos((hour - 3d) / 24d * 2d * Math.PI) * preset.HourSpread;
        var temperature = (float)(norm + dayNoise + diurnal);

        var wet = Unit(Seed.Mix(schoolSeed, dayNumber * 24 + utc.Hour, HourSalt)) < preset.PrecipitationChance;
        var precipitation = !wet
            ? Precipitation.None
            : temperature < 0f ? Precipitation.Snow : Precipitation.Rain;

        return new OutdoorWeather(temperature, precipitation);
    }

    private static float MonthNorm(ClimatePresetDef preset, int month)
    {
        if (preset.MonthlyNorms.Count != 12)
        {
            return 0f;
        }

        return preset.MonthlyNorms[month - 1];
    }

    private static double Unit(int mixed)
    {
        return (uint)mixed / (double)uint.MaxValue;
    }

    private static double SignedUnit(int mixed) => Unit(mixed) * 2d - 1d;
}