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/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;
+        }
+    }
+};
+