namespace HSchool.People;

internal readonly record struct FamilyPlan(int FamilyIndex, IReadOnlyList<PupilSeat> Seats);

/// <summary>
/// Walks seats in order and groups them into families of 1–3, taking a run of consecutive seats
/// per family. Callers hand in a shuffled seat list — a run of *adjacent* seats is one classroom,
/// which would put every sibling in the same class.
///
/// A family whose intended size does not fit the remainder takes one seat instead of shrinking,
/// so growing the seat list only disturbs the tail.
/// </summary>
internal static class FamilyPlanner
{
    /// <summary>
    /// One child per family. Used by the yearly intake: every seat there is a first-year seat, so
    /// grouping would hand a family two or three same-age children at once. An arriving pupil who
    /// does have an older sibling gets attached to that family instead.
    /// </summary>
    public static IReadOnlyList<FamilyPlan> Singletons(IReadOnlyList<PupilSeat> seats, int startIndex)
    {
        var plans = new FamilyPlan[seats.Count];
        for (var i = 0; i < seats.Count; i++)
        {
            plans[i] = new FamilyPlan(startIndex + i, [seats[i]]);
        }

        return plans;
    }

    public static IReadOnlyList<FamilyPlan> Plan(int schoolSeed, IReadOnlyList<PupilSeat> seats, int startIndex = 0)
    {
        var plans = new List<FamilyPlan>();
        var offset = 0;
        var index = startIndex;
        while (offset < seats.Count)
        {
            var remaining = seats.Count - offset;
            var want = ChildCount(schoolSeed, index);
            var take = want <= remaining ? want : 1;
            var slice = new PupilSeat[take];
            for (var i = 0; i < take; i++)
            {
                slice[i] = seats[offset + i];
            }

            plans.Add(new FamilyPlan(index, slice));
            offset += take;
            index++;
        }

        return plans;
    }

    public static int ChildCount(int schoolSeed, int familyIndex)
    {
        var rng = new Random(Seed.Mix(schoolSeed, familyIndex, Seed.ChildCountSalt));
        var roll = rng.Next(100);
        if (roll < 50)
        {
            return 1;
        }

        return roll < 85 ? 2 : 3;
    }
}