namespace HSchool.Ai;

/// <summary>
/// One person's place on the graph. <see cref="NodeId"/> is null when they are off campus.
/// There is no "on an edge" state — remaining minutes are spent occupying the current node.
/// </summary>
public readonly record struct Presence(
    string? NodeId,
    float RemainingMinutes,
    string? DestinationId,
    bool HeadingHome,
    string[] Path)
{
    public static Presence OffCampus { get; } = new(null, 0f, null, false, []);

    public bool IsOnCampus => NodeId is not null;
}

/// <summary>
/// Hot-path movement in the library so tests can prove travel time without a world. Simulation
/// copies the same loop onto components and does not call this every tick.
/// </summary>
public static class PresenceStepper
{
    public static Presence Advance(Presence presence, WalkGraph walks, float minutes)
    {
        ArgumentNullException.ThrowIfNull(walks);
        if (presence.NodeId is null || minutes <= 0)
        {
            return presence;
        }

        var remaining = presence.RemainingMinutes - minutes;
        var node = presence.NodeId;
        var path = presence.Path;
        var index = 0;
        while (remaining <= 0 && index < path.Length)
        {
            node = path[index];
            index++;
            remaining += walks.TravelMinutes(node);
        }

        if (remaining < 0)
        {
            remaining = 0;
        }

        var leftover = index >= path.Length ? [] : path[index..];
        return presence with { NodeId = node, RemainingMinutes = remaining, Path = leftover };
    }

    public static Presence StartWalk(Presence presence, WalkGraph walks, string destination, bool headingHome)
    {
        ArgumentNullException.ThrowIfNull(walks);
        if (presence.NodeId is null)
        {
            var path = walks.Path(walks.TerritoryId, destination);
            return new Presence(walks.TerritoryId, 0f, destination, headingHome, [.. path]);
        }

        var hops = walks.Path(presence.NodeId, destination);
        return presence with
        {
            DestinationId = destination,
            HeadingHome = headingHome,
            Path = [.. hops],
        };
    }

    public static Presence ArriveOffCampus(Presence presence) =>
        presence.NodeId is not null
        && presence.HeadingHome
        && presence.Path.Length == 0
        && presence.RemainingMinutes <= 0
        && presence.NodeId.Equals(presence.DestinationId, StringComparison.Ordinal)
            ? Presence.OffCampus
            : presence;
}