Minimax with Alpha-Beta (generic game interface)

using System;
using System.Collections.Generic;

public interface IGameState<TMove> {
    bool IsTerminal { get; }
    double Evaluate();                 // heuristic for non-terminal cutoffs
    IEnumerable<TMove> LegalMoves(bool maximizingPlayer);
    IGameState<TMove> Apply(TMove move);
}

public static class AlphaBeta {
    public static (double score, TMove best) Search<TMove>(IGameState<TMove> s, int depth, bool maxPlayer,
        double alpha = double.NegativeInfinity, double beta = double.PositiveInfinity)
    {
        if (depth == 0 || s.IsTerminal) return (s.Evaluate(), default!);

        TMove bestMove = default!;
        if (maxPlayer) {
            double best = double.NegativeInfinity;
            foreach (var m in s.LegalMoves(true)) {
                var (sc, _) = Search(s.Apply(m), depth-1, false, alpha, beta);
                if (sc > best) { best = sc; bestMove = m; }
                alpha = Math.Max(alpha, best);
                if (beta <= alpha) break;
            }
            return (best, bestMove);
        } else {
            double best = double.PositiveInfinity;
            foreach (var m in s.LegalMoves(false)) {
                var (sc, _) = Search(s.Apply(m), depth-1, true, alpha, beta);
                if (sc < best) { best = sc; bestMove = m; }
                beta = Math.Min(beta, best);
                if (beta <= alpha) break;
            }
            return (best, bestMove);
        }
    }
}