h-school/src/HSchool.People/DiseaseEffects.cs

using HSchool.Content;

namespace HSchool.People;

/// <summary>
/// Reads DiseaseDef stages for lesson gain, stay-home rolls and warmth decay.
/// Contagion rolls live in DiseaseSystem; this type only answers stage questions.
/// </summary>
public static class DiseaseEffects
{
    public static bool TryStage(DiseaseDef disease, float severity, out DiseaseStage stage)
    {
        ArgumentNullException.ThrowIfNull(disease);
        DiseaseStage? best = null;
        foreach (var candidate in disease.Stages)
        {
            if (severity + 1e-6f < candidate.MinSeverity)
            {
                continue;
            }

            if (best is null || candidate.MinSeverity >= best.MinSeverity)
            {
                best = candidate;
            }
        }

        if (best is null)
        {
            stage = null!;
            return false;
        }

        stage = best;
        return true;
    }

    public static bool IsIncubated(HealthCondition condition, DiseaseDef disease, DateTime now)
    {
        ArgumentNullException.ThrowIfNull(condition);
        ArgumentNullException.ThrowIfNull(disease);
        var elapsed = (DateTime.SpecifyKind(now, DateTimeKind.Utc) - condition.StartedAt).TotalDays;
        return elapsed >= disease.IncubationDays;
    }

    /// <summary>Product of active post-incubation lesson factors (1 when healthy).</summary>
    public static float LessonLearningFactor(Person person, DefCatalog catalog, DateTime now)
    {
        ArgumentNullException.ThrowIfNull(person);
        ArgumentNullException.ThrowIfNull(catalog);
        var list = person.Conditions;
        if (list is null || list.Count == 0)
        {
            return 1f;
        }

        var factor = 1f;
        foreach (var condition in list)
        {
            if (!catalog.Diseases.TryGetValue(condition.DefName, out var disease) || disease.Abstract)
            {
                continue;
            }

            if (!IsIncubated(condition, disease, now) || !TryStage(disease, condition.Severity, out var stage))
            {
                continue;
            }

            factor *= stage.LessonLearningFactor;
        }

        return factor;
    }

    /// <summary>Highest stay-home chance among incubated conditions.</summary>
    public static float StayHomeChance(Person person, DefCatalog catalog, DateTime now)
    {
        ArgumentNullException.ThrowIfNull(person);
        ArgumentNullException.ThrowIfNull(catalog);
        var list = person.Conditions;
        if (list is null || list.Count == 0)
        {
            return 0f;
        }

        var chance = 0f;
        foreach (var condition in list)
        {
            if (!catalog.Diseases.TryGetValue(condition.DefName, out var disease) || disease.Abstract)
            {
                continue;
            }

            if (!IsIncubated(condition, disease, now) || !TryStage(disease, condition.Severity, out var stage))
            {
                continue;
            }

            if (stage.StayHomeChance > chance)
            {
                chance = stage.StayHomeChance;
            }
        }

        return chance;
    }

    /// <summary>
    /// Deterministic per person+day: whether illness keeps them off campus today.
    /// </summary>
    public static bool ShouldStayHome(Person person, DefCatalog catalog, int peopleSeed, DateTime now)
    {
        var chance = StayHomeChance(person, catalog, now);
        if (chance <= 0f)
        {
            return false;
        }

        if (chance >= 1f)
        {
            return true;
        }

        var dayNumber = DateOnly.FromDateTime(DateTime.SpecifyKind(now, DateTimeKind.Utc)).DayNumber;
        var roll = Seed.Mix(peopleSeed, person.Id, dayNumber, Seed.DiseaseSalt);
        var unit = (roll & int.MaxValue) / (float)int.MaxValue;
        return unit < chance;
    }

    /// <summary>Max Warmth decay multiplier from incubated stages (1 when healthy).</summary>
    public static float WarmthDecayFactor(Person person, DefCatalog catalog, DateTime now)
    {
        ArgumentNullException.ThrowIfNull(person);
        ArgumentNullException.ThrowIfNull(catalog);
        var list = person.Conditions;
        if (list is null || list.Count == 0)
        {
            return 1f;
        }

        var factor = 1f;
        foreach (var condition in list)
        {
            if (!catalog.Diseases.TryGetValue(condition.DefName, out var disease) || disease.Abstract)
            {
                continue;
            }

            if (!IsIncubated(condition, disease, now) || !TryStage(disease, condition.Severity, out var stage))
            {
                continue;
            }

            if (stage.WarmthDecayFactor > factor)
            {
                factor = stage.WarmthDecayFactor;
            }
        }

        return factor;
    }
}