Add ProfilingList & use it in Player.

Shared/Line.cs

@@ -4,7 +4,7 @@ using System.Collections.Generic;
 
 namespace SemiColinGames {
   public static class Line {
-    public static void Rasterize(Point p1, Point p2, List<Point> result) {
+    public static void Rasterize(Point p1, Point p2, IList<Point> result) {
       Line.Rasterize(p1.X, p1.Y, p2.X, p2.Y, result);
     }
 
@@ -14,7 +14,7 @@ namespace SemiColinGames {
     // https://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm
     // http://members.chello.at/~easyfilter/bresenham.html
     // http://members.chello.at/~easyfilter/Bresenham.pdf (section 1.6)
-    public static void Rasterize(int x1, int y1, int x2, int y2, List<Point> result) {
+    public static void Rasterize(int x1, int y1, int x2, int y2, IList<Point> result) {
       int dx = Math.Abs(x2 - x1);
       int stepX = x1 < x2 ? 1 : -1;
       int dy = -Math.Abs(y2 - y1);

Shared/Player.cs

@@ -36,9 +36,9 @@ namespace SemiColinGames {
     private float invincibilityTime = 0;
 
     // For passing into Line.Rasterize() during movement updates.
-    private readonly List<Point> movePoints = new List<Point>(100);
+    private readonly IList<Point> movePoints = new ProfilingList<Point>(32, "Player.movePoints");
     // Possible hitboxes for player <-> obstacles.
-    private readonly List<AABB> candidates = new List<AABB>(10);
+    private readonly IList<AABB> candidates = new ProfilingList<AABB>(8, "Player.candidates");
 
     public Player(Vector2 position, int facing) {
       this.position = position;

Shared/ProfilingList.cs

@@ -0,0 +1,81 @@
+using System.Collections;
+using System.Collections.Generic;
+
+namespace SemiColinGames {
+  // An IList<T>, backed by a List<T>, that prints out a debug message any time that the Capacity
+  // of the underlying List changes.
+  public class ProfilingList<T> : IList<T> {
+    private List<T> items;
+    private int previousCapacity;
+    private string name;
+
+    public ProfilingList(int capacity, string name) {
+      items = new List<T>(capacity);
+      previousCapacity = capacity;
+      this.name = name;
+    }
+
+    public void Add(T item) {
+      items.Add(item);
+      if (items.Capacity != previousCapacity) {
+        Debug.WriteLine($"{name} capacity: {previousCapacity} -> {items.Capacity}");
+        previousCapacity = items.Capacity;
+      }
+    }
+
+    public void Insert(int index, T item) {
+      items.Insert(index, item);
+      if (items.Capacity != previousCapacity) {
+        Debug.WriteLine($"{name} capacity: {previousCapacity} -> {items.Capacity}");
+        previousCapacity = items.Capacity;
+      }
+    }
+
+    public bool IsReadOnly {
+      get { return false; }
+    }
+
+    // Everything below this point is boilerplate delegation to the underlying list.
+
+    public void Clear() {
+      items.Clear();
+    }
+
+    public bool Contains(T item) {
+      return items.Contains(item);
+    }
+
+    public int Count {
+      get => items.Count;
+    }
+
+    public int IndexOf(T item) {
+      return items.IndexOf(item);
+    }
+
+    public bool Remove(T item) {
+      return items.Remove(item);
+    }
+
+    public void CopyTo(T[] array, int arrayIndex) {
+      items.CopyTo(array, arrayIndex);
+    }
+
+    public void RemoveAt(int index) {
+      items.RemoveAt(index);
+    }
+
+    public T this[int index] {
+      get { return items[index]; }
+      set { items[index] = value; }
+    }
+
+    public IEnumerator<T> GetEnumerator() {
+      return items.GetEnumerator();
+    }
+
+    IEnumerator IEnumerable.GetEnumerator() {
+      return items.GetEnumerator();
+    }
+  }
+}

Shared/Shared.projitems

@@ -13,6 +13,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)ExtensionMethods.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)FSM.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)NPC.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)ProfilingList.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)SoundEffects.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Sprites.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Strings.cs" />