Add basic FSM & use it from NPC.

Shared/FSM.cs

@@ -0,0 +1,40 @@
+using System;
+using System.Collections.Generic;
+
+namespace SemiColinGames {
+  public interface IState {
+    public void Enter();
+    public string? Update(NPC npc, float modelTime, AABB[] collisionTargets);
+  }
+
+  public class FSM {
+    float timeInState = 0f;
+    Dictionary<string, IState> states;
+    IState state;
+
+    public FSM(Dictionary<string, IState> states, string initial) {
+      this.states = states;
+      StateName = initial;
+      Transition(StateName);
+    }
+
+    public string StateName { get; private set; }
+
+    public void Update(NPC npc, float modelTime, AABB[] collisionTargets) {
+      timeInState += modelTime;
+      string? newState = state.Update(npc, modelTime, collisionTargets);
+      if (newState != null) {
+        Transition(newState);
+      }
+    }
+
+    void Transition(string state) {
+      Debug.WriteLine("{0} -> {1} @ {2}", StateName, state, timeInState);
+      timeInState = 0f;
+      StateName = state;
+      IState newState = states[state];
+      this.state = newState;
+      this.state.Enter();
+    }
+  }
+}

Shared/NPC.cs

@@ -1,25 +1,33 @@
 using Microsoft.Xna.Framework;
 using Microsoft.Xna.Framework.Graphics;
+using System.Collections.Generic;
 
 namespace SemiColinGames {
-  class NPC {
-    private Point position;
+  class IdleState : IState {
+    float timeInState = 0;
 
-    private const int spriteWidth = 96;
-    private const int spriteHeight = 81;
-    private const int spriteCenterYOffset = 3;
+    public void Enter() {
+      timeInState = 0;
+    }
 
-    public NPC(Point position) {
-      this.position = position;
+    public string? Update(NPC npc, float modelTime, AABB[] collisionTargets) {
+      timeInState += modelTime;
+      if (timeInState > 1.0f) {
+        npc.Facing *= -1;
+        return "run";
+      }
+      return null;
     }
+  }
 
-    public int Facing { get; private set; } = 1;
+  class RunState : IState {
+    public void Enter() {}
 
-    public void Update(float modelTime, AABB[] collisionTargets) {
+    public string? Update(NPC npc, float modelTime, AABB[] collisionTargets) {
       int moveSpeed = 120;
-      int desiredX = position.X + (int) (moveSpeed * Facing * modelTime);
+      int desiredX = npc.Position.X + (int) (moveSpeed * npc.Facing * modelTime);
       // TODO: define the box modularly & correctly.
-      AABB npcBox = new AABB(new Vector2(desiredX, position.Y), new Vector2(11, 33));
+      AABB npcBox = new AABB(new Vector2(desiredX, npc.Position.Y), new Vector2(1, 33));
       Debug.AddRect(npcBox, Color.Cyan);
       bool foundBox = false;
       foreach (AABB box in collisionTargets) {
@@ -30,20 +38,44 @@ namespace SemiColinGames {
       }
 
       if (!foundBox) {
-        Facing *= -1;
+        return "idle";
       }
-      position.X = desiredX;
+      npc.Position.X = desiredX;
+      return null;
+    }
+  }
+
+  public class NPC {
+    private const int spriteWidth = 96;
+    private const int spriteHeight = 81;
+    private const int spriteCenterYOffset = 2;
+
+    private FSM fsm;
+
+    public NPC(Point position) {
+      Position = position;
+      fsm = new FSM(new Dictionary<string, IState> {
+        { "idle", new IdleState() },
+        { "run", new RunState() }
+      }, "idle");
+    }
+
+    public int Facing = 1;
+    public Point Position;
+
+    public void Update(float modelTime, AABB[] collisionTargets) {
+      fsm.Update(this, modelTime, collisionTargets);
     }
 
     public void Draw(SpriteBatch spriteBatch) {
       Rectangle textureSource = Sprites.Executioner.GetTextureSource(
-          "run", (int) Clock.ModelTime.TotalMilliseconds);
+          fsm.StateName, (int) Clock.ModelTime.TotalMilliseconds);
       // TODO: move this into Sprite metadata.
       Vector2 spriteCenter = new Vector2(spriteWidth / 2, spriteHeight / 2 + spriteCenterYOffset);
       SpriteEffects effect = Facing == 1 ?
         SpriteEffects.None : SpriteEffects.FlipHorizontally;
       Color color = Color.White;
-      spriteBatch.Draw(Textures.Executioner.Get, position.ToVector2(), textureSource, color, 0f,
+      spriteBatch.Draw(Textures.Executioner.Get, Position.ToVector2(), textureSource, color, 0f,
         spriteCenter, Vector2.One, effect, 0f);
     }
   }

Shared/Shared.projitems

@@ -11,6 +11,7 @@
   <ItemGroup>
     <Compile Include="$(MSBuildThisFileDirectory)Camera.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ExtensionMethods.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)FSM.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)NPC.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)SoundEffects.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Sprites.cs" />