Pong/Assets/Scripts/Game/AIPlayer.cs

using System.Linq;
using UnityEngine;

namespace Game {

	public class AIPlayer : Player {
		private const float SmoothTime = 0.2f;

		private float currentSmoothV;

		private bool isApproaching;
		private float lastDirection;

		public Difficulty Difficulty { get; set; }

		private float FutureSeconds => (float) Difficulty * 0.5f;

		private float IdlePosition => Difficulty >= Difficulty.Medium ? 0 : X;

		private float DistortAmount => 1 - 1 / ((float) Difficulty + 1);

		private void FixedUpdate() {
			var dt = Time.fixedDeltaTime;
			var target = GetTargetPosition();
			var h = Mathf.Max(Speed * dt, Width / 2);
			goingLeft = target < X - h;
			goingRight = target > X + h;
			if (goingLeft || goingRight) {
				isApproaching = false;
				lastDirection = goingLeft ? -1 : 1;
				TryLinearMove(dt);
			} else {
				if (!isApproaching) {
					isApproaching = true;
					currentSmoothV = Speed * lastDirection;
				}

				ApproachPosition(target);
			}

		}

		// True if ball y velocity points towards player
		private bool BallApproaches(Ball ball) {
			var ballVy = ball.Rb.velocity.y;
			return ballVy * transform.up.y < 0;
		}

		// Not manhattan, only Y direction
		private float YDistanceToBall(Ball ball) {
			return Mathf.Abs(ball.Rb.position.y - Y);
		}

		// o: Origin, e: End
		public static bool Intersect(Vector2 o1, Vector2 e1, Vector2 o2, Vector2 e2, out Vector2 point, out Vector2 rs) {
			point = Vector2.zero;
			rs = Vector2.zero;

			var m1 = e1 - o1;
			var m2 = e2 - o2;

			/*
			 *
			 * o1 + r * m1 = 02 + s * m2 
			 * 
			 * o1x + r * m1x = o2x + s * m2x
			 * o1y + r * m1y = o2y + s * m2y
			 *
			 * Solve for r and s:
			 * Formulas below achieved by reordering
			 * Order is irrelevant, but if m1x == 0 then first approach results in division by zero ->
			 * Invert order then, and if division by zero is still a problem, both lines are parallel anyway
			 */

			float r, s;
			if (m1.x != 0) {
				s = (o1.y + o2.x * m1.y / m1.x - o1.x * m1.y / m1.x - o2.y) / (m2.y - m2.x * m1.y / m1.x);
				r = (o2.x + s * m2.x - o1.x) / m1.x;
			} else if (m2.x != 0) {
				r = (o2.y + o1.x * m2.y / m2.x - o2.x * m2.y / m2.x - o1.y) / (m1.y - m1.x * m2.y / m2.x);
				s = (o1.x + r * m1.x - o2.x) / m2.x;
			} else {
				return false;
			}

			if (s is > 0 and < 1 && r is > 0 and < 1) {
				point = o1 + r * m1;
				rs = new Vector2(r, s);
				return true;
			}

			return false;
		}

		// TODO Also must include fact that players have a height, maybe check the incoming angle
		// angle: 0 -> perpendicular, from -90 to 90
		private bool IsPositionReachableInTime(float futurePosition, float seconds, float radius, float angle) {
			var requiredDistance = Mathf.Abs(futurePosition - X) - Width / 2 - radius;
			if (requiredDistance < 0)
				return true;
			if (Mathf.Abs(futurePosition) > Border)
				return false;
			return Speed * seconds > requiredDistance;
		}

		private float Simulate(Vector2 origin, Vector2 velocity, float radius, float secondsLeft, float secondsUsed, out bool ignore) {
			ignore = false;
			var r = new Vector2(radius, radius);
			var validAreaOrigin = -Dimensions.Singleton.PlaySizeBoards / 2 + r;
			var validAreaSize = Dimensions.Singleton.PlaySizeBoards - r * 2;
			var area = new Rect(validAreaOrigin, validAreaSize);

			// Try to follow this line from origin -> end
			var end = origin + velocity * secondsLeft;

			// Line ends in playground
			if (area.Contains(end))
				return end.x;

			var playerY = Side == Side.Bottom ? area.yMin : area.yMax;
			var playerLeft = new Vector2(area.xMin, playerY);
			var playerRight = new Vector2(area.xMax, playerY);

			// Horizontal line (player line) -> stop simulating
			if (Intersect(origin, end, playerLeft, playerRight, out var point, out var rs)) {
				secondsUsed += secondsLeft * rs.x;
				if (!IsPositionReachableInTime(point.x, secondsUsed, radius, Vector2.Angle(velocity, transform.up)))
					ignore = true;
				return point.x;
			}

			var borderHit = false;
			var borderHitPoint = Vector2.zero;
			var borderRs = Vector2.zero;

			// Left vertical border
			if (Intersect(origin, end, new Vector2(area.xMin, area.yMin), new Vector2(area.xMin, area.yMax), out point, out rs)) {
				borderHit = true;
				borderHitPoint = point;
				borderRs = rs;
			}

			// Right vertical border
			if (Intersect(origin, end, new Vector2(area.xMax, area.yMin), new Vector2(area.xMax, area.yMax), out point, out rs)) {
				borderHit = true;
				borderHitPoint = point;
				borderRs = rs;
			}

			// Any border -> invert x velocity and simulate again from there
			if (borderHit) {
				var secondsUsedHere = borderRs.x * secondsLeft;
				secondsLeft -= secondsUsedHere;
				secondsUsed += secondsUsedHere;
				velocity = new Vector2(-velocity.x, velocity.y);
				origin = borderHitPoint;
				return Simulate(origin, velocity, radius, secondsLeft, secondsUsed, out ignore);
			}

			// No intersection -> Ball outside of field -> ignore
			ignore = true;
			return 0;
		}

		private float Distort(float target) {
			var max = Width / 3;
			var distortionOffset = DistortAmount * max;
			distortionOffset *= target > X ? -1 : 1;
			return target + distortionOffset;
		}

		private float GetTargetPosition() {
			var approaching = GameManager.Singleton.Balls.Where(BallApproaches).ToList();

			while (approaching.Count > 0) {

				// Nearest by Y-Distance
				var ball = approaching.Aggregate(approaching[0], (prev, current) => YDistanceToBall(current) < YDistanceToBall(prev) ? current : prev);

				var target = Simulate(ball.Rb.position, ball.Rb.velocity, ball.Radius, FutureSeconds, 0, out var ignore);
				if (!ignore)
					return Distort(target);

				approaching.Remove(ball);
				// This ball was marked "ignore" by simulation for unknown reasons, try next one
			}

			return IdlePosition;
		}

		private void ApproachPosition(float pos) {
			var result = Mathf.SmoothDamp(X, pos, ref currentSmoothV, SmoothTime, Speed);
			transform.position = new Vector2(result, Y);
			ClampInsideBorders();
		}
	}
}