sorted coords variant

Assets/Mirror/Core/LagCompensation/HistoryBounds.cs

@@ -4,15 +4,54 @@
 // standalone C# implementation to be engine (and language) agnostic.
 
 using System.Collections.Generic;
+using System.Runtime.CompilerServices;
 using UnityEngine;
 
 namespace Mirror
 {
+    // by default, SortedList is sorted from smallest to largest.
+    // for max coordinates, we need to sort from largest to smallest.
+    class DescendingComparer : IComparer<float>
+    {
+        public int Compare(float a, float b) =>
+            -Comparer<float>.Default.Compare(a, b);
+    }
+
     public class HistoryBounds
     {
         // history of bounds
-        readonly Queue<Bounds> history;
-        public int Count => history.Count;
+        // readonly Queue<Bounds> history;
+        // public int Count => history.Count;
+
+        // instead of keeping a history of Queue<Bounds> for each timestamp,
+        // we split it into xmin, xmax, ymin, ymax, ... sorted lists.
+        // each entry has (value, timestamp).
+        // this way when removing, we can easily walk the sorted list.
+        //
+        // for example:
+        //  xmin: (1, t0), (3, t2), (4, t1)
+        //  when it's time to remove oldest=t0, we remove (1, t0)
+        //
+        //  xmin: (3, t2), (4, t1)
+        //  when it's time to remove oldest=t1, we check (3, t2) which is too new.
+        //  we remove nothing
+        //
+        //  when it's time to remove oldest=t2, we check (3, t2) which is ready to be removed.
+        //  we also check the next one: (4, t1) which is also older.
+        //  we keep checking until one is newer.
+        //
+        // for total bounds, we always use xmin.peek(), xmax.peek(), etc.
+        readonly SortedList<float, int> xmin, xmax;
+        readonly SortedList<float, int> ymin, ymax;
+        readonly SortedList<float, int> zmin, zmax;
+
+        // current insertion timestamp.
+        // TODO what if overflows?
+        int timestamp = 0;
+
+        // keep a custom count since lists may be larger than actual count while
+        // we're still waiting for the oldest to be ready to be removed.
+        public int Count { get; private set; }
 
         // history limit. oldest bounds will be removed.
         public readonly int limit;
@@ -32,7 +71,104 @@ public HistoryBounds(int limit, int recalculateEveryNth)
             // +1 because it makes the code easier if we insert first, and then remove.
             this.limit = limit;
             this.recalculateEveryNth = recalculateEveryNth;
-            history = new Queue<Bounds>(limit + 1);
+            xmin = new SortedList<float, int>(limit + 1);
+            ymin = new SortedList<float, int>(limit + 1);
+            zmin = new SortedList<float, int>(limit + 1);
+            xmax = new SortedList<float, int>(limit + 1, new DescendingComparer());
+            ymax = new SortedList<float, int>(limit + 1, new DescendingComparer());
+            zmax = new SortedList<float, int>(limit + 1, new DescendingComparer());
+        }
+
+        // enqueue a value into a sorted list.
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        static void AddOrReplace(SortedList<float, int> list, float value, int timestamp)
+        {
+            // players may revist the same coordinates multiple times.
+            // inserting the same key twice would throw.
+            // instead, overwrite the key's value timestamp with the newer one.
+            if (list.ContainsKey(value))
+            {
+                list[value] = timestamp;
+            }
+            else
+            {
+                list.Add(value, timestamp);
+            }
+        }
+
+        // split bounds and add to sorted lists
+        void Enqueue(Bounds bounds)
+        {
+            // insert individual coordinates into the sorted lists
+            AddOrReplace(xmin, bounds.min.x, timestamp);
+            AddOrReplace(ymin, bounds.min.y, timestamp);
+            AddOrReplace(zmin, bounds.min.z, timestamp);
+            AddOrReplace(xmax, bounds.max.x, timestamp);
+            AddOrReplace(ymax, bounds.max.y, timestamp);
+            AddOrReplace(zmax, bounds.max.z, timestamp);
+
+            // timestamp always increases, never decreases.
+            timestamp += 1;
+
+            // keep count of intended amount of entries.
+            // decreases when dequeueing.
+            Count += 1;
+        }
+
+        // walk the sorted lists to remove all old entries up to timestamp.
+        // the lists are sorted by value, not by timestamp:
+        // (2, t0), (3, t2), (4, t1), ...
+        // sometimes there's nothing to remove.
+        // sometimes there are one more values to remove.
+        static void RemoveOldest(SortedList<float, int> list, int old)
+        {
+            // find out until which index we need to remove.
+            for (int i = 0; i < list.Count; ++i)
+            {
+                // get the timestamp at [index]
+                int timestamp = list.Values[i];
+
+                // is this older than the timestamp limit?
+                if (timestamp < old)
+                {
+                    list.RemoveAt(i);
+                    --i;
+                }
+                // otherwise stop for now
+                else break;
+            }
+        }
+
+        // walk the sorted lists to remove all old entries up to timestamp
+        void RemoveOldest()
+        {
+            // calculate the target timestamp to remove up to.
+            // we are now at 'timestamp'.
+            // we allow 'limit' entries.
+            // so we want to remove everything older than 'timestamp - limit'.
+            int old = timestamp - limit;
+
+            // remove oldest in ascending order from min lists
+            RemoveOldest(xmin, old);
+            RemoveOldest(ymin, old);
+            RemoveOldest(zmin, old);
+
+            // remove oldest in descending order from max lists
+            RemoveOldest(xmax, old);
+            RemoveOldest(ymax, old);
+            RemoveOldest(zmax, old);
+
+            // even though not all list's counts may change just yet,
+            // we still decrease the true count.
+            Count -= 1;
+        }
+
+        // build total bounds from the first of each sorted list
+        void RecalculateTotal()
+        {
+            Vector3 min = new Vector3(xmin.Keys[0], ymin.Keys[0], zmin.Keys[0]);
+            Vector3 max = new Vector3(xmax.Keys[0], ymax.Keys[0], zmax.Keys[0]);
+            total.SetMinMax(min, max);
         }
 
         // insert new bounds into history. calculates new total bounds.
@@ -41,18 +177,18 @@ public void Insert(Bounds bounds)
         {
             // initialize 'total' if not initialized yet.
             // we don't want to call (0,0).Encapsulate(bounds).
-            if (history.Count == 0)
+            if (Count == 0)
                 total = bounds;
 
             // insert and encapsulate the new bounds
-            history.Enqueue(bounds);
+            Enqueue(bounds);
             total.Encapsulate(bounds);
 
             // ensure history stays within limit
-            if (history.Count > limit)
+            if (Count > limit)
             {
                 // remove oldest
-                history.Dequeue();
+                RemoveOldest();
 
                 // optimization: only recalculate every n-th removal.
                 // accurate enough, and N times faster.
@@ -64,15 +200,20 @@ public void Insert(Bounds bounds)
 
                 // recalculate total bounds
                 // (only needed after removing the oldest)
-                total = bounds;
-                foreach (Bounds b in history)
-                    total.Encapsulate(b);
+                RecalculateTotal();
             }
         }
 
         public void Reset()
         {
-            history.Clear();
+            xmin.Clear();
+            xmax.Clear();
+            ymin.Clear();
+            ymax.Clear();
+            zmin.Clear();
+            zmax.Clear();
+            Count = 0;
+            timestamp = 0;
             total = new Bounds();
         }
     }

Assets/Mirror/Tests/Editor/LagCompensation/HistoryBoundsTests.cs

@@ -25,6 +25,7 @@ public static Bounds MinMax(float min, float max) =>
         // Unity 2021.3 LTS, release mode: 100k; limit=8
         //   O(N) Queue<Bounds> implementation:     183 ms
         //   O(N) Queue and recalculate every 2nd:  108 ms
+        //   sorted coords variant:                1275 ms
         [Test]
         [TestCase(100_000, 8, 1)]
         [TestCase(100_000, 8, 2)]