From a8b95ce6ed5e84d34748ecdd0ff1db4d03377cf0 Mon Sep 17 00:00:00 2001
From: "Wladimir J. van der Laan" <laanwj@gmail.com>
Date: Wed, 28 Sep 2011 21:35:58 +0200
Subject: [PATCH] use median filter for peer-reported reported number of blocks
 - fixes problem that one misconfigured or malicious node can mess up progress
 bar - implementation in src/util.h - testcase in src/test/util_tests.cpp

---
 src/main.cpp            | 16 +++++----------
 src/test/util_tests.cpp | 36 +++++++++++++++++++++++++++++++++
 src/util.h              | 44 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 85 insertions(+), 11 deletions(-)
 create mode 100644 src/test/util_tests.cpp

diff --git a/src/main.cpp b/src/main.cpp
index e732ddcf..5c0dd7d7 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -32,7 +32,6 @@ uint256 hashGenesisBlock("0x000000000019d6689c085ae165831e934ff763ae46a2a6c172b3
 static CBigNum bnProofOfWorkLimit(~uint256(0) >> 32);
 const int nTotalBlocksEstimate = 140700; // Conservative estimate of total nr of blocks on main chain
 const int nInitialBlockThreshold = 120; // Regard blocks up until N-threshold as "initial download"
-int nMaxBlocksOfPeers = 0; // Amount of blocks that other nodes claim to have
 CBlockIndex* pindexGenesisBlock = NULL;
 int nBestHeight = -1;
 CBigNum bnBestChainWork = 0;
@@ -41,6 +40,8 @@ uint256 hashBestChain = 0;
 CBlockIndex* pindexBest = NULL;
 int64 nTimeBestReceived = 0;
 
+CMedianFilter<int> cPeerBlockCounts(5, 0); // Amount of blocks that other nodes claim to have
+
 map<uint256, CBlock*> mapOrphanBlocks;
 multimap<uint256, CBlock*> mapOrphanBlocksByPrev;
 
@@ -65,11 +66,6 @@ int fUseUPnP = false;
 #endif
 
 
-
-
-
-
-
 //////////////////////////////////////////////////////////////////////////////
 //
 // dispatching functions
@@ -730,7 +726,7 @@ int GetTotalBlocksEstimate()
 // Return maximum amount of blocks that other nodes claim to have
 int GetNumBlocksOfPeers()
 {
-    return std::max(nMaxBlocksOfPeers, GetTotalBlocksEstimate());
+    return std::max(cPeerBlockCounts.median(), GetTotalBlocksEstimate());
 }
 
 bool IsInitialBlockDownload()
@@ -1847,10 +1843,8 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
         pfrom->fSuccessfullyConnected = true;
 
         printf("version message: version %d, blocks=%d\n", pfrom->nVersion, pfrom->nStartingHeight);
-        if(pfrom->nStartingHeight > nMaxBlocksOfPeers)
-        {
-            nMaxBlocksOfPeers = pfrom->nStartingHeight;
-        }
+
+        cPeerBlockCounts.input(pfrom->nStartingHeight);
     }
 
 
diff --git a/src/test/util_tests.cpp b/src/test/util_tests.cpp
new file mode 100644
index 00000000..337d9012
--- /dev/null
+++ b/src/test/util_tests.cpp
@@ -0,0 +1,36 @@
+#include <vector>
+#include <boost/test/unit_test.hpp>
+#include <boost/foreach.hpp>
+
+#include "../util.h"
+
+using namespace std;
+
+BOOST_AUTO_TEST_SUITE(util_tests)
+
+BOOST_AUTO_TEST_CASE(util_MedianFilter)
+{    
+    CMedianFilter<int> filter(5, 15);
+
+    BOOST_CHECK(filter.median() == 15);
+
+    filter.input(20); // [15 20]
+    BOOST_CHECK(filter.median() == 17);
+
+    filter.input(30); // [15 20 30]
+    BOOST_CHECK(filter.median() == 20);
+
+    filter.input(3); // [3 15 20 30]
+    BOOST_CHECK(filter.median() == 17);
+
+    filter.input(7); // [3 7 15 20 30]
+    BOOST_CHECK(filter.median() == 15);
+
+    filter.input(18); // [3 7 18 20 30]
+    BOOST_CHECK(filter.median() == 18);
+
+    filter.input(0); // [0 3 7 18 30]
+    BOOST_CHECK(filter.median() == 7);
+}
+
+BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/util.h b/src/util.h
index fabcaf93..290195cb 100644
--- a/src/util.h
+++ b/src/util.h
@@ -565,6 +565,50 @@ inline uint160 Hash160(const std::vector<unsigned char>& vch)
 }
 
 
+// Median filter over a stream of values
+// Returns the median of the last N numbers
+template <typename T> class CMedianFilter
+{
+private:
+    std::vector<T> vValues;
+    std::vector<T> vSorted;
+    int nSize;
+public:
+    CMedianFilter(int size, T initial_value):
+        nSize(size)
+    {
+        vValues.reserve(size);
+        vValues.push_back(initial_value);
+        vSorted = vValues;
+    }
+    
+    void input(T value)
+    {
+        if(vValues.size() == nSize)
+        {
+            vValues.erase(vValues.begin());
+        }
+        vValues.push_back(value);
+
+        vSorted.resize(vValues.size());
+        std::copy(vValues.begin(), vValues.end(), vSorted.begin());
+        std::sort(vSorted.begin(), vSorted.end());
+    }
+
+    T median() const
+    {
+        int size = vSorted.size();
+        if(size & 1) // Odd number of elements
+        {
+            return vSorted[size/2];
+        }
+        else // Even number of elements
+        {
+            return (vSorted[size/2-1] + vSorted[size/2]) / 2;
+        }
+    }
+};
+