card-game-engine/RuleEngine/AI/RandomMahjongAI.cs

namespace RuleEngine.AI;

using RuleEngine.Core;
using RuleEngine.Phase;

public class RandomMahjongAI
{
    public string Name { get; }
    private readonly Random _rng;

    public RandomMahjongAI(string name, Random? rng = null)
    {
        Name = name;
        _rng = rng ?? Random.Shared;
    }

    public (string action, int? tile) Decide(List<ActionOption> legalActions, MahjongGameState state)
    {
        if (legalActions.Count == 0)
            return ("pass", null);

        // Priority: win > kong > pung > chi > discard
        var win = legalActions.FirstOrDefault(a => a.Action == "win");
        if (win != null) return ("win", null);

        var mingKong = legalActions.FirstOrDefault(a => a.Action == "ming_kong");
        if (mingKong != null) return ("ming_kong", null);

        var anKong = legalActions.FirstOrDefault(a => a.Action == "an_kong");
        if (anKong != null) return ("an_kong", null);

        var pung = legalActions.FirstOrDefault(a => a.Action == "pung");
        if (pung != null) return ("pung", null);

        var chi = legalActions.FirstOrDefault(a => a.Action == "chi");
        if (chi != null) return ("chi", null);

        // Discard: random non-wildcard tile
        var discards = legalActions.Where(a => a.Action == "discard").ToList();
        if (discards.Count > 0)
        {
            var hand = state.Hands.GetValueOrDefault(Name, new List<int>());
            var nonWildTiles = hand.Where(t => !MahjongTile.IsWildcard(t)).ToList();
            if (nonWildTiles.Count > 0)
                return ("discard", nonWildTiles[_rng.Next(nonWildTiles.Count)]);
            // Only wildcards left — discard one
            if (hand.Count > 0)
                return ("discard", hand[_rng.Next(hand.Count)]);
        }

        return ("pass", null);
    }
}