using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;

namespace CSharpGraphicsApp
{
    public class Task26_BouncingBallsPhysics : Form
    {
        private class Ball
        {
            public PointF Position { get; set; }
            public PointF Velocity { get; set; }
            public float Radius { get; set; }
            public Color Color { get; set; }
            public float Mass { get; set; }
            public const float Gravity = 0.3f;
            public const float Friction = 0.98f;
            public const float Elasticity = 0.85f;

            public void Update(int width, int height)
            {
                var vel = Velocity;
                var pos = Position;
                vel.Y += Gravity;
                vel.X *= Friction;
                vel.Y *= Friction;
                pos.X += vel.X;
                pos.Y += vel.Y;

                if (pos.X - Radius < 0 || pos.X + Radius > width)
                {
                    vel.X = -vel.X * Elasticity;
                    pos.X = pos.X - Radius < 0 ? Radius : width - Radius;
                }
                if (pos.Y - Radius < 0 || pos.Y + Radius > height)
                {
                    vel.Y = -vel.Y * Elasticity;
                    pos.Y = pos.Y - Radius < 0 ? Radius : height - Radius;
                }
                Position = pos; Velocity = vel;
            }

            public void Draw(Graphics g)
            {
                using (var brush = new SolidBrush(Color))
                using (var pen = new Pen(Color.Black, 2))
                {
                    g.FillEllipse(brush,
                        Position.X - Radius, Position.Y - Radius,
                        Radius * 2, Radius * 2);
                    g.DrawEllipse(pen,
                        Position.X - Radius, Position.Y - Radius,
                        Radius * 2, Radius * 2);
                }
            }

            public bool CollidesWith(Ball other)
            {
                float dx = Position.X - other.Position.X;
                float dy = Position.Y - other.Position.Y;
                return (float)Math.Sqrt(dx * dx + dy * dy) < Radius + other.Radius;
            }
        }

        private Ball[] balls;
        private Random rnd = new Random();
        private readonly Timer timer = new Timer();

        public Task26_BouncingBallsPhysics()
        {
            Text = "Задание 26 — Шары с физикой (Bouncing Balls Physics)";
            Size = new Size(900, 600);
            BackColor = Color.White;
            DoubleBuffered = true;
            Paint += Form_Paint;

            Color[] colors = { Color.Red, Color.Blue, Color.Green, Color.Yellow,
                               Color.Purple, Color.Orange, Color.Cyan, Color.Magenta };
            balls = new Ball[8];
            for (int i = 0; i < balls.Length; i++)
                balls[i] = new Ball
                {
                    Position = new PointF(rnd.Next(50, 850), rnd.Next(50, 200)),
                    Velocity = new PointF((float)(rnd.NextDouble() - 0.5) * 8,
                                          (float)(rnd.NextDouble() - 0.5) * 8),
                    Radius = rnd.Next(15, 35),
                    Color = colors[i], Mass = 1.0f
                };

            timer.Interval = 16;
            timer.Tick += Timer_Tick;
            timer.Start();
            FormClosed += (s, e) => timer.Stop();
        }

        private void Timer_Tick(object sender, EventArgs e)
        {
            foreach (var b in balls)
                b.Update(ClientSize.Width, ClientSize.Height);

            for (int i = 0; i < balls.Length; i++)
                for (int j = i + 1; j < balls.Length; j++)
                    if (balls[i].CollidesWith(balls[j]))
                        HandleCollision(balls[i], balls[j]);

            Invalidate();
        }

        private void HandleCollision(Ball a, Ball b)
        {
            float dx = b.Position.X - a.Position.X;
            float dy = b.Position.Y - a.Position.Y;
            float dist = (float)Math.Sqrt(dx * dx + dy * dy);
            if (dist == 0) dist = 0.01f;

            float nx = dx / dist, ny = dy / dist;
            float dvn = (b.Velocity.X - a.Velocity.X) * nx
                      + (b.Velocity.Y - a.Velocity.Y) * ny;

            if (dvn >= 0) return;

            float impulse = dvn / (a.Mass + b.Mass);

            var av = a.Velocity; av.X += impulse * a.Mass * nx; av.Y += impulse * a.Mass * ny; a.Velocity = av;
            var bv = b.Velocity; bv.X -= impulse * b.Mass * nx; bv.Y -= impulse * b.Mass * ny; b.Velocity = bv;

            float overlap = (a.Radius + b.Radius - dist) / 2;
            var ap = a.Position; ap.X -= overlap * nx; ap.Y -= overlap * ny; a.Position = ap;
            var bp = b.Position; bp.X += overlap * nx; bp.Y += overlap * ny; b.Position = bp;
        }

        private void Form_Paint(object sender, PaintEventArgs e)
        {
            e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
            foreach (var b in balls) b.Draw(e.Graphics);
            e.Graphics.DrawString($"Шаров: {balls.Length} | Физика: гравитация + столкновения",
                new Font("Segoe UI", 10, FontStyle.Bold), Brushes.Black, 10, 10);
        }
    }
}