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

namespace HSchool.People;

/// <summary>One person looking for work at this school, with the hourly rate they named.</summary>
public sealed record Applicant(Person Person, float HourlyWageAsk);

/// <summary>
/// School-wide applicant list. Not the roster: a new candidate lives only here until hired.
/// A parent already in the roster may appear with the same id — that is the same person.
/// </summary>
public sealed record ApplicantPool(int Week, int NextIndex, IReadOnlyList<Applicant> Applicants)
{
    public static ApplicantPool Empty { get; } = new(0, 0, []);

    public static ApplicantPool Create(
        DefCatalog catalog,
        Roster roster,
        int schoolSeed,
        string countryId,
        DateTime asOf,
        string? nativeLanguage = null)
    {
        var week = ApplicantWeeks.Id(asOf);
        return Fill(catalog, roster, schoolSeed, countryId, asOf, new ApplicantPool(week, 0, []), nativeLanguage: nativeLanguage);
    }

    /// <summary>
    /// Walks each week from <see cref="Week"/> exclusive to <paramref name="asOf"/> inclusive.
    /// Same previous list, seed and week always yields the same stays and the same newcomers.
    /// </summary>
    public ApplicantPool Advance(
        DefCatalog catalog,
        Roster roster,
        int schoolSeed,
        string countryId,
        DateTime asOf,
        string? nativeLanguage = null)
    {
        var target = ApplicantWeeks.Id(asOf);
        if (target <= Week)
        {
            return this;
        }

        var pool = this;
        for (var week = Week + 1; week <= target; week++)
        {
            pool = Step(pool, catalog, roster, schoolSeed, countryId, asOf, week, nativeLanguage);
        }

        return pool;
    }

    public ApplicantPool Without(string personId) =>
        this with
        {
            Applicants = Applicants
                .Where(applicant => !applicant.Person.Id.Equals(personId, StringComparison.Ordinal))
                .ToArray(),
        };

    public static float HourlyAsk(DefCatalog catalog, Person person)
    {
        var rules = catalog.StaffingRules
            ?? throw new InvalidOperationException("The catalog has no StaffingDef.");

        var mean = 0d;
        if (person.Skills.Count > 0)
        {
            var total = 0;
            foreach (var value in person.Skills.Values)
            {
                total += value;
            }

            mean = total / (double)person.Skills.Count;
        }

        var trait = 0f;
        foreach (var name in person.Traits)
        {
            if (catalog.Traits.TryGetValue(name, out var def))
            {
                trait += def.WageAsk;
            }
        }

        var ask = rules.HourlyWageBase + (float)(rules.HourlyWagePerSkill * mean) + trait;
        return MathF.Round(MathF.Max(ask, 0f), 2);
    }

    private static ApplicantPool Step(
        ApplicantPool current,
        DefCatalog catalog,
        Roster roster,
        int schoolSeed,
        string countryId,
        DateTime asOf,
        int week,
        string? nativeLanguage)
    {
        var rules = RequireRules(catalog);
        var rng = new Random(Seed.Mix(schoolSeed, week, Seed.ApplicantSalt));
        var staying = new List<Applicant>(rules.PoolSize);
        foreach (var applicant in current.Applicants)
        {
            if (rng.NextDouble() < rules.StayChance)
            {
                staying.Add(applicant);
            }
        }

        return Fill(
            catalog,
            roster,
            schoolSeed,
            countryId,
            asOf,
            new ApplicantPool(week, current.NextIndex, staying),
            rng,
            nativeLanguage);
    }

    private static ApplicantPool Fill(
        DefCatalog catalog,
        Roster roster,
        int schoolSeed,
        string countryId,
        DateTime asOf,
        ApplicantPool current,
        Random? rng = null,
        string? nativeLanguage = null)
    {
        var rules = RequireRules(catalog);
        if (!catalog.TryGetCountryNames(countryId, out var names))
        {
            throw new ArgumentException($"Unknown country '{countryId}'.", nameof(countryId));
        }

        var native = NativeLanguages.Pick(names, schoolSeed, nativeLanguage, rollIfOmitted: false);

        rng ??= new Random(Seed.Mix(schoolSeed, current.Week, Seed.ApplicantSalt));
        var applicants = new List<Applicant>(rules.PoolSize);
        var taken = new HashSet<string>(StringComparer.Ordinal);
        foreach (var applicant in current.Applicants)
        {
            if (applicants.Count >= rules.PoolSize || !taken.Add(applicant.Person.Id))
            {
                continue;
            }

            applicants.Add(applicant);
        }

        var eligible = roster.People
            .Where(person => person.IsParent && !person.IsStaff && !taken.Contains(person.Id))
            .OrderBy(person => person.Id, StringComparer.Ordinal)
            .ToList();

        var nextIndex = current.NextIndex;
        while (applicants.Count < rules.PoolSize)
        {
            Applicant next;
            if (eligible.Count > 0 && rng.NextDouble() < rules.ParentChance)
            {
                var pick = rng.Next(eligible.Count);
                var parent = eligible[pick];
                eligible.RemoveAt(pick);
                taken.Add(parent.Id);
                next = new Applicant(parent, HourlyAsk(catalog, parent));
            }
            else
            {
                var (_, person) = FamilyFactory.CreateStaffOnly(catalog, names, schoolSeed, nextIndex, asOf, nativeLanguage: native);
                nextIndex++;
                taken.Add(person.Id);
                next = new Applicant(person, HourlyAsk(catalog, person));
            }

            applicants.Add(next);
        }

        return OrientationGenerator.AssignPool(
            catalog,
            new ApplicantPool(current.Week, nextIndex, applicants),
            schoolSeed);
    }

    private static StaffingDef RequireRules(DefCatalog catalog) =>
        catalog.StaffingRules
        ?? throw new InvalidOperationException("The catalog has no StaffingDef.");
}