Chessistics/chessistics-engine/Rules/CollisionResolver.cs

using Chessistics.Engine.Model;

namespace Chessistics.Engine.Rules;

public static class CollisionResolver
{
    /// <summary>
    /// Resolves collisions after movement. For each cell with 2+ pieces,
    /// the strongest piece survives and destroys the others.
    /// Priority: SocialStatus desc → Level desc → mutual destruction on exact tie.
    /// </summary>
    public static List<(PieceState? Survivor, List<PieceState> Destroyed, Coords Cell)> ResolveCollisions(
        IReadOnlyList<PieceState> pieces)
    {
        var results = new List<(PieceState? Survivor, List<PieceState> Destroyed, Coords Cell)>();
        var byCell = new Dictionary<Coords, List<PieceState>>();

        foreach (var piece in pieces)
        {
            if (!byCell.TryGetValue(piece.CurrentCell, out var list))
            {
                list = [];
                byCell[piece.CurrentCell] = list;
            }
            list.Add(piece);
        }

        foreach (var (cell, occupants) in byCell)
        {
            if (occupants.Count < 2) continue;

            // Sort by priority: highest status first, then highest level
            var sorted = occupants
                .OrderByDescending(p => p.SocialStatus)
                .ThenByDescending(p => p.Level)
                .ToList();

            var top = sorted[0];
            var second = sorted[1];

            // If top two have same status AND same level → mutual destruction
            if (top.SocialStatus == second.SocialStatus && top.Level == second.Level)
            {
                results.Add((null, sorted, cell));
            }
            else
            {
                var destroyed = sorted.Skip(1).ToList();
                results.Add((top, destroyed, cell));
            }
        }

        return results;
    }
}