Return correct target for TestNet blocks

Src/Autarkysoft.Bitcoin/Blockchain/Chain.cs

@@ -214,12 +214,36 @@ public Target GetNextTarget(BlockHeader first, BlockHeader last)
         }
 
         /// <inheritdoc/>
-        public Target GetNextTarget()
+        public Target GetNextTarget(in BlockHeader hdr)
         {
-            // TODO: difficulty calculation is different for testnet
             int height = headerList.Count;
             if (height % Constants.DifficultyAdjustmentInterval != 0)
             {
+                if (Consensus.AllowMinDifficultyBlocks)
+                {
+                    // Special difficulty rule for testnet:
+                    // If the new block's timestamp is more than 2* 10 minutes
+                    // then allow mining of a min-difficulty block.
+                    if (hdr.BlockTime > headerList[^1].BlockTime + TimeConstants.Seconds.TwentyMin)
+                    {
+                        var temp = new BigInteger(Consensus.PowLimit.ToByteArray());
+                        return new Target(temp);
+                    }
+                    else
+                    {
+                        var temp = new BigInteger(Consensus.PowLimit.ToByteArray());
+                        Target min = new Target(temp);
+
+                        // Return the last non-special-min-difficulty-rules-block
+                        height--;
+                        while (height > 0 && height % Constants.DifficultyAdjustmentInterval != 0 && headerList[height].NBits == min)
+                        {
+                            height--;
+                        }
+                        return headerList[height].NBits;
+                    }
+                }
+
                 return headerList[^1].NBits;
             }
             else
@@ -564,7 +588,7 @@ public bool ProcessHeaders(BlockHeader[] headers, INodeStatus nodeStatus)
                     int count = 0;
                     for (int i = arrIndex; i < headers.Length; i++)
                     {
-                        if (ProcessHeader(headers[i], headerList[^1], headerList.Count, GetNextTarget()))
+                        if (ProcessHeader(headers[i], headerList[^1], headerList.Count, GetNextTarget(headers[i])))
                         {
                             headerList.Add(headers[i]);
                             count++;

Src/Autarkysoft.Bitcoin/Blockchain/IChain.cs

@@ -44,7 +44,7 @@ public interface IChain
         /// Returns the next difficulty target based on the best stored chain.
         /// </summary>
         /// <returns>Next difficulty target</returns>
-        Target GetNextTarget();
+        Target GetNextTarget(in BlockHeader hdr);
 
         /// <summary>
         /// Processes all the blocks that this peer received in its <see cref="INodeStatus.DownloadedBlocks"/>

Src/Autarkysoft.Bitcoin/Constants.cs

@@ -3,6 +3,8 @@
 // Distributed under the MIT software license, see the accompanying
 // file LICENCE or http://www.opensource.org/licenses/mit-license.php.
 
+using System.ComponentModel;
+
 namespace Autarkysoft.Bitcoin
 {
     /// <summary>
@@ -227,6 +229,10 @@ public struct TimeConstants
         /// </summary>
         public struct Seconds
         {
+            /// <summary>
+            /// 20 minutes in seconds
+            /// </summary>
+            public const int TwentyMin = 20 * 60;
             /// <summary>
             /// One day or 24 hours in seconds
             /// </summary>

Src/Tests/Bitcoin/Blockchain/MockChain.cs

@@ -11,7 +11,6 @@
 using Autarkysoft.Bitcoin.P2PNetwork.Messages;
 using System;
 using System.Collections.Generic;
-using Xunit;
 
 namespace Tests.Bitcoin.Blockchain
 {
@@ -55,7 +54,7 @@ public int Height
         public Digest256 Tip => _tip;
 
         internal Target? targetToReturn;
-        public Target GetNextTarget()
+        public Target GetNextTarget(in BlockHeader hdr)
         {
             Assert.True(targetToReturn.HasValue, UnexpectedCall);
             return targetToReturn.Value;