add enemy ai to the game

the enemy can now track the player with a cast that uses a shape generated in code. the enemy moves using some fancy math that gets the sign of the distance between the ball and the enemy paddles y position to get a normalised number for velocity and the velocity is lerped to smooth out any janky movement.

BouncyMaterial.tres

@@ -0,0 +1,5 @@
+[gd_resource type="PhysicsMaterial" format=3 uid="uid://e05n66x8ug77"]
+
+[resource]
+friction = 0.0
+bounce = 1.25

Scenes/Enemy.tscn

@@ -0,0 +1,23 @@
+[gd_scene load_steps=5 format=3 uid="uid://krt6x241s3x6"]
+
+[ext_resource type="PhysicsMaterial" uid="uid://e05n66x8ug77" path="res://BouncyMaterial.tres" id="1_e3kk5"]
+[ext_resource type="Script" path="res://Scripts/Enemy.cs" id="1_fbrtv"]
+
+[sub_resource type="PlaceholderTexture2D" id="PlaceholderTexture2D_iw3nx"]
+size = Vector2(50, 75)
+
+[sub_resource type="RectangleShape2D" id="RectangleShape2D_dbik4"]
+size = Vector2(12, 150)
+
+[node name="Enemy" type="RigidBody2D"]
+physics_material_override = ExtResource("1_e3kk5")
+gravity_scale = 0.0
+lock_rotation = true
+script = ExtResource("1_fbrtv")
+
+[node name="Sprite" type="Sprite2D" parent="."]
+scale = Vector2(0.25, 2)
+texture = SubResource("PlaceholderTexture2D_iw3nx")
+
+[node name="CollisionShape2D" type="CollisionShape2D" parent="."]
+shape = SubResource("RectangleShape2D_dbik4")

Scenes/Paddle.tscn

@@ -1,11 +1,8 @@
 [gd_scene load_steps=5 format=3 uid="uid://bklo6torhapa0"]
 
+[ext_resource type="PhysicsMaterial" uid="uid://e05n66x8ug77" path="res://BouncyMaterial.tres" id="1_76uik"]
 [ext_resource type="Script" path="res://Scripts/Paddle.cs" id="1_uv7s3"]
 
-[sub_resource type="PhysicsMaterial" id="PhysicsMaterial_r5a55"]
-friction = 0.0
-bounce = 1.0
-
 [sub_resource type="PlaceholderTexture2D" id="PlaceholderTexture2D_iw3nx"]
 size = Vector2(50, 75)
 
@@ -13,7 +10,7 @@ size = Vector2(50, 75)
 size = Vector2(12, 150)
 
 [node name="Paddle" type="RigidBody2D"]
-physics_material_override = SubResource("PhysicsMaterial_r5a55")
+physics_material_override = ExtResource("1_76uik")
 gravity_scale = 0.0
 lock_rotation = true
 script = ExtResource("1_uv7s3")

Scenes/Pong.tscn

@@ -1,10 +1,13 @@
-[gd_scene load_steps=4 format=3 uid="uid://kmfgtiugs4m0"]
+[gd_scene load_steps=6 format=3 uid="uid://kmfgtiugs4m0"]
 
 [ext_resource type="PackedScene" uid="uid://bklo6torhapa0" path="res://Scenes/Paddle.tscn" id="1_5rs0o"]
+[ext_resource type="Script" path="res://Scripts/SceneManager.cs" id="1_m8437"]
 [ext_resource type="PackedScene" uid="uid://cggi01qnnlnwg" path="res://Scenes/Ball.tscn" id="2_u2ksv"]
 [ext_resource type="PackedScene" uid="uid://c5n541vsuvfk8" path="res://Scenes/Walls.tscn" id="3_jfis7"]
+[ext_resource type="PackedScene" uid="uid://krt6x241s3x6" path="res://Scenes/Enemy.tscn" id="4_uwvof"]
 
 [node name="Pong" type="Node2D"]
+script = ExtResource("1_m8437")
 
 [node name="Paddle" parent="." instance=ExtResource("1_5rs0o")]
 position = Vector2(-350, 0)
@@ -18,3 +21,7 @@ _ballSpeed = 50.0
 _maxRandomAngle = 0.42
 
 [node name="Walls" parent="." instance=ExtResource("3_jfis7")]
+
+[node name="Enemy" parent="." instance=ExtResource("4_uwvof")]
+position = Vector2(350, 0)
+_moveSpeed = 50.0

Scripts/Ball.cs

@@ -10,7 +10,7 @@ public partial class Ball : CharacterBody2D
 	private Vector2 _velocity;
 
 	/// <summary>
-	/// this property multiples the ballSpeed by the Meter constant.
+	/// this property multiples the ballSpeed by the <see cref="Constants.Meter">Meter</see> constant.
 	/// </summary>
 	private double BallSpeed => _ballSpeed * Constants.Meter;
 
@@ -32,7 +32,7 @@ public partial class Ball : CharacterBody2D
 	/// <summary>
 	/// this method moves the ball and bounces if it collides with something.
 	/// </summary>
-	/// <param name="delta">delta time from the _PhysicsProcess method.</param>
+	/// <param name="delta">delta time from the <see cref="_PhysicsProcess">_PhysicsProcess</see> method.</param>
 	private void CollisionCheck(double delta)
 	{
 		var collision = MoveAndCollide(_velocity * delta);
@@ -42,9 +42,10 @@ public partial class Ball : CharacterBody2D
 
 	/// <summary>
 	/// this method generates a random number between 0 and 1 and
-	/// either returns Vector2.Left or Vector2.Right based on that number.
+	/// either returns <see cref="Vector2.Left">Vector2.Left</see> or <see cref="Vector2.Right">Vector2.Right</see>
+	/// based on that number.
 	/// </summary>
-	private Vector2 GetRandomStartingDirection() {
+	private static Vector2 GetRandomStartingDirection() {
 		using var rng = new RandomNumberGenerator();
 		rng.Randomize();
 		var range = rng.RandiRange(0,1);

Scripts/Enemy.cs

@@ -0,0 +1,90 @@
+using System;
+using System.Collections.Generic;
+using Godot;
+using Godot.Collections;
+
+namespace Pong.Scripts;
+
+public partial class Enemy : RigidBody2D
+{
+	/// <summary>
+	/// draw shapes to the screen (like <see cref="_scanArea"/>).
+	/// </summary>
+	[Export] private bool _drawDebugShapes;
+
+	[Export] private double _moveSpeed;
+
+	private CollisionShape2D _collisionShape;
+	private Rect2 _scanArea;
+
+	public override void _Ready()
+	{
+		_collisionShape = GetNode<CollisionShape2D>("CollisionShape2D");
+		GenerateCastArea();
+	}
+
+	public override void _PhysicsProcess(double delta)
+	{
+		Scan(delta);
+	}
+
+	public override void _Draw()
+	{
+		if (!_drawDebugShapes) return;
+		DrawRect(_scanArea, Colors.Aqua);
+	}
+	
+	private void Scan(double delta)
+	{
+		var spaceState = GetWorld2D().DirectSpaceState;
+
+		var query = new PhysicsShapeQueryParameters2D
+		{
+			Shape = new RectangleShape2D { Size = _scanArea.Size },
+			Exclude = new Array<Rid>(new[] { GetRid() })
+		};
+
+		var result = spaceState.IntersectShape(query);
+
+		if (result.Count <= 0)
+		{
+			LinearVelocity = Vector2.Zero;
+			return;
+		}
+		
+		TrackBall(delta, result);
+	}
+
+	/// <summary>
+	/// track the distance between the ball and the enemy paddle on the y axis and move velocity accordingly.
+	/// </summary>
+	/// <param name="delta">how long it took to complete the last frame in seconds. this should be constant as
+	/// should be executed in the physics process method which should be separate from the main frame rate.</param>
+	/// <param name="result">a dictionary of objects that collided with the cast. this method only works if
+	/// theres an object that has the Ball class.</param>
+	private void TrackBall(double delta, IReadOnlyList<Dictionary> result)
+	{
+		// checks if the collider is a ball, if not, return.
+		if (result[0]["collider"].As<Ball>() is not { } ball) return;
+		
+		// gets the sign of the distance between the ball and the paddle on the y axis
+		var normalisedDistance = new Vector2(0, Mathf.Sign(ball.Position.Y - Position.Y));
+		var linearVelocity = normalisedDistance * _moveSpeed * Constants.Meter;
+		
+		// lerp the velocity to smooth out jerky movement.
+		LinearVelocity = LinearVelocity.Lerp(linearVelocity, delta);
+	}
+
+	/// <summary>
+	/// generate the shape that the cast uses to detect collisions.
+	/// </summary>
+	/// <exception cref="InvalidOperationException">if the collision shape is not a rectangle shape.</exception>
+	private void GenerateCastArea()
+	{
+		if (_collisionShape.Shape is not RectangleShape2D shape)
+			throw new InvalidOperationException("the collision shape needs to be a rectangle shape");
+		
+		// grow the area the enemy can see by around half of the screen area.
+		_scanArea = shape.GetRect().GrowSide(Side.Left, 400).GrowIndividual(0, 190, 0, 190);
+	}
+}

Scripts/Paddle.cs

@@ -9,7 +9,7 @@ public partial class Paddle : RigidBody2D
 	private double _verticalInput;
 
 	/// <summary>
-	/// property that multiples the moveSpeed by the Meter constant.
+	/// property that multiples the moveSpeed by the <see cref="Constants.Meter">Meter</see> constant.
 	/// </summary>
 	private double MoveSpeed => _moveSpeed * Constants.Meter;
 	

Scripts/SceneManager.cs

@@ -0,0 +1,14 @@
+using Godot;
+
+namespace Pong.Scripts;
+
+public partial class SceneManager : Node2D
+{
+	public override void _Process(double delta)
+	{
+		if (Input.IsActionJustPressed("scene_reload"))
+		{
+			GetTree().ReloadCurrentScene();
+		}
+	}
+}

project.godot

@@ -43,6 +43,11 @@ paddle_down={
 , Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":83,"key_label":0,"unicode":115,"echo":false,"script":null)
 ]
 }
+scene_reload={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":82,"key_label":0,"unicode":114,"echo":false,"script":null)
+]
+}
 
 [rendering]