From: Sunny King <sunnyking9999@gmail.com>
Date: Sun, 29 Jul 2012 20:13:33 +0000 (+0100)
Subject: PPCoin: Only immediately checkpoint proof-of-work block
X-Git-Tag: v0.4.0-unstable~120
X-Git-Url: https://git.novaco.in/?p=novacoin.git;a=commitdiff_plain;h=3f94e09d9ae6fdc0bcb608d6f1f62e1491e4d576

PPCoin: Only immediately checkpoint proof-of-work block
        Max 4 hours trailing checkpoint from latest block
---

diff --git a/src/checkpoints.cpp b/src/checkpoints.cpp
index bd4eba7..5bb9df6 100644
--- a/src/checkpoints.cpp
+++ b/src/checkpoints.cpp
@@ -163,7 +163,7 @@ namespace Checkpoints
                 {
                     txdb.TxnAbort();
                     hashInvalidCheckpoint = hashPendingCheckpoint;
-                    return error("ProcessSyncCheckpoint: Reorganize failed for sync checkpoint %s", hashPendingCheckpoint.ToString().c_str());
+                    return error("AcceptPendingSyncCheckpoint: Reorganize failed for sync checkpoint %s", hashPendingCheckpoint.ToString().c_str());
                 }
             }
             txdb.Close();
@@ -185,6 +185,21 @@ namespace Checkpoints
         return false;
     }
 
+    // Automatically select a suitable sync-checkpoint 
+    uint256 AutoSelectSyncCheckpoint()
+    {
+        // Proof-of-work blocks are immediately checkpointed
+        // to defend against 51% attack which rejects other miners block 
+
+        // Select the last proof-of-work block
+        const CBlockIndex *pindex = GetLastBlockIndex(pindexBest, false);
+        // Search forward for a block within max span and maturity window
+        while (pindex->pnext && (pindex->GetBlockTime() + CHECKPOINT_MAX_SPAN <= pindexBest->GetBlockTime() || pindex->nHeight + COINBASE_MATURITY <= pindexBest->nHeight))
+            pindex = pindex->pnext;
+        return pindex->GetBlockHash();
+    }
+
+    // Check against synchronized checkpoint
     bool CheckSync(const uint256& hashBlock, const CBlockIndex* pindexPrev)
     {
         if (fTestNet) return true; // Testnet has no checkpoints
diff --git a/src/checkpoints.h b/src/checkpoints.h
index cc4bebd..be18c79 100644
--- a/src/checkpoints.h
+++ b/src/checkpoints.h
@@ -10,6 +10,7 @@
 #include "util.h"
 
 #define STAKE_MIN_AGE (60 * 60 * 24)      // minimum age for coin age
+#define CHECKPOINT_MAX_SPAN (60 * 60 * 4) // max 4 hours before latest block
 
 class uint256;
 class CBlockIndex;
@@ -37,6 +38,7 @@ namespace Checkpoints
     CBlockIndex* GetLastSyncCheckpoint();
     bool WriteSyncCheckpoint(const uint256& hashCheckpoint);
     bool AcceptPendingSyncCheckpoint();
+    uint256 AutoSelectSyncCheckpoint();
     bool CheckSync(const uint256& hashBlock, const CBlockIndex* pindexPrev);
     bool WantedByPendingSyncCheckpoint(uint256 hashBlock);
     bool ResetSyncCheckpoint();
diff --git a/src/main.cpp b/src/main.cpp
index 96b05a2..cb4dd17 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -2094,7 +2094,7 @@ bool ProcessBlock(CNode* pfrom, CBlock* pblock)
 
     // ppcoin: if responsible for sync-checkpoint send it
     if (pfrom && !CSyncCheckpoint::strMasterPrivKey.empty())
-        Checkpoints::SendSyncCheckpoint(hashBestChain);
+        Checkpoints::SendSyncCheckpoint(Checkpoints::AutoSelectSyncCheckpoint());
 
     return true;
 }