Chessistics/chessistics-engine/Rules/CollisionDetector.cs

using Chessistics.Engine.Model;

namespace Chessistics.Engine.Rules;

public static class CollisionDetector
{
    public static IReadOnlyList<(int PieceIdA, int PieceIdB, Coords Cell)> DetectCollisions(
        IReadOnlyList<PieceState> pieces)
    {
        var collisions = new List<(int, int, Coords)>();
        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;

            for (int i = 0; i < occupants.Count; i++)
                for (int j = i + 1; j < occupants.Count; j++)
                    collisions.Add((occupants[i].Id, occupants[j].Id, cell));
        }

        return collisions;
    }
}