Introduce recent header cache (to speed up header sync)

Author: dangershony

src/Blockcore/Consensus/LeveldbChainStore.cs

@@ -16,7 +16,12 @@ public class LeveldbChainStore : IChainStore, IDisposable
         /// <summary>
         /// Headers that are close to the tip
         /// </summary>
-        private readonly MemoryCountCache<uint256, BlockHeader> headers;
+        private readonly MemoryCountCache<uint256, BlockHeader> nearTipHeaders;
+
+        /// <summary>
+        /// Headers that are close to the tip
+        /// </summary>
+        private readonly MemoryCountCache<uint256, BlockHeader> recentHeaders;
 
         private readonly DB leveldb;
 
@@ -27,7 +32,8 @@ public LeveldbChainStore(Network network, DataFolder dataFolder, ChainIndexer ch
             this.network = network;
             this.ChainIndexer = chainIndexer;
             // this.headers = new Dictionary<uint256, BlockHeader>();
-            this.headers = new MemoryCountCache<uint256, BlockHeader>(601);
+            this.nearTipHeaders = new MemoryCountCache<uint256, BlockHeader>(601);
+            this.recentHeaders = new MemoryCountCache<uint256, BlockHeader>(100);
             this.locker = new object();
 
             // Open a connection to a new DB and create if not found
@@ -39,7 +45,12 @@ public LeveldbChainStore(Network network, DataFolder dataFolder, ChainIndexer ch
 
         public BlockHeader GetHeader(ChainedHeader chainedHeader, uint256 hash)
         {
-            if (this.headers.TryGetValue(hash, out BlockHeader blockHeader))
+            if (this.nearTipHeaders.TryGetValue(hash, out BlockHeader blockHeader))
+            {
+                return blockHeader;
+            }
+
+            if (this.recentHeaders.TryGetValue(hash, out blockHeader))
             {
                 return blockHeader;
             }
@@ -61,7 +72,13 @@ public BlockHeader GetHeader(ChainedHeader chainedHeader, uint256 hash)
 
             // If the header is 500 blocks behind tip or 100 blocks ahead cache it.
             if ((chainedHeader.Height > this.ChainIndexer.Height - 500) && (chainedHeader.Height <= this.ChainIndexer.Height + 100))
-                this.headers.AddOrUpdate(hash, blockHeader);
+            {
+                this.nearTipHeaders.AddOrUpdate(hash, blockHeader);
+            }
+            else
+            {
+                this.recentHeaders.AddOrUpdate(hash, blockHeader);
+            }
 
             return blockHeader;
         }