pong-game/PongGame/Hubs/PongGameState.cs

using System.Drawing;

namespace PongGame.Hubs;

/// <summary>
/// Pong game state saving the positions of the paddles and the ball.
/// The Pong board has aspect ratio 1:2 with height 500 and width 1000.
/// </summary>
public struct PongGameState {
    public const float HEIGHT = 500.0f;
    public const float WIDTH = 2 * HEIGHT;
    public const float PADDLE1_OFFSET = 50.0f;
    public const float PADDLE2_OFFSET = 1000 - PADDLE1_OFFSET;

    public static readonly PongGameState Initial = new() {
        BallState = PongBallState.Initial,
        Paddle1 = PongPaddleState.Initial,
        Paddle2 = PongPaddleState.Initial,
        Status = GameStatus.WaitingForPlayers
    };

    public PongPaddleState Paddle1;
    public PongPaddleState Paddle2;
    public PongBallState BallState;
    public GameStatus Status;

    public static void Update(ref PongGameState state) {
        if (state.Status is not GameStatus.InProgress) return;

        PongPaddleState.Update(ref state.Paddle1);
        PongPaddleState.Update(ref state.Paddle2);
        PongBallState.Update(ref state.BallState);

        Collide(in state.Paddle1, ref state.BallState, true);
        Collide(in state.Paddle2, ref state.BallState, false);
    }

    private static void Collide(in PongPaddleState paddle, ref PongBallState ballState, bool left) {
        var paddleX = left ? PADDLE1_OFFSET : PADDLE2_OFFSET;
        var intersection = RectangleF.Intersect(paddle.GetCollider(paddleX), ballState.GetCollider());
        if (intersection.IsEmpty) return;

        // TODO: continuous collision
        var ratio = (ballState.Y - paddle.Height + PongPaddleState.PADDLE_HALF_LENGTH) / PongPaddleState.PADDLE_LENGTH;
        var upAngle = left ? MathF.PI * 3 / 8 : MathF.PI * 5 / 8;
        var downAngle = -upAngle;

        ballState.BallAngle = ratio * downAngle + (1 - ratio) * upAngle;
    }

    public struct PongBallState {
        public const float BALL_SPEED = 8;
        public const float BALL_RADIUS = 2;

        public static readonly PongBallState Initial = new() {
            BallAngle = 0.0f,
            X = WIDTH / 2,
            Y = HEIGHT / 2
        };

        public float X;
        public float Y;
        public float BallAngle;

        public static void Update(ref PongBallState state) {
            var (dy, dx) = MathF.SinCos(state.BallAngle);
            state.X += BALL_SPEED * dx;
            state.Y -= BALL_SPEED * dy;
        }

        public RectangleF GetCollider() => new(X - BALL_RADIUS, Y - BALL_RADIUS, 2 * BALL_RADIUS, 2 * BALL_RADIUS);
    }

    public struct PongPaddleState {
        public const float PADDLE_LENGTH = HEIGHT / 10;
        public const float PADDLE_HALF_LENGTH = PADDLE_LENGTH / 2;
        public const float PADDLE_WIDTH = PADDLE_LENGTH / 6;
        public const float PADDLE_SPEED = 8;

        public static readonly PongPaddleState Initial = new() {
            Direction = PongPaddleDirection.Stop,
            Height = HEIGHT / 2
        };

        public float Height;
        public PongPaddleDirection Direction;

        public static void Update(ref PongPaddleState state) {
            state.Height = Math.Clamp(state.Height + ((int)state.Direction) * PADDLE_SPEED, PADDLE_HALF_LENGTH, HEIGHT - PADDLE_HALF_LENGTH);
        }

        public RectangleF GetCollider(float x) => new(x - PADDLE_HALF_LENGTH, Height - PADDLE_HALF_LENGTH, PADDLE_WIDTH, PADDLE_LENGTH);
    }
}