Fix uint256 comparison for stake modifier candidate selection

Upgrading to Bitcoin's latest uint256 type changes the backing storage
from an array of 32-bit integers to a byte array. Since the comparison
operator implementations changed as well, the less-than comparison for
sorting the vector of candidates can produce different block orderings
for the historical stake modifier than the original order from before.

To ensure that we reproduce the same stake modifier values out of this
candidate set, we adopt Peercoin's strategy for sorting the collection
that behaves like the old uint256 implementation.

peercoin/peercoin@4f8df68

src/kernel.cpp

@@ -206,7 +206,36 @@ bool ComputeNextStakeModifier(const CBlockIndex* pindexPrev, uint64_t& nStakeMod
         pindex = pindex->pprev;
     }
     int nHeightFirstCandidate = pindex ? (pindex->nHeight + 1) : 0;
-    std::sort(vSortedByTimestamp.begin(), vSortedByTimestamp.end());
+
+    // Shuffle before sort
+    for(int i = vSortedByTimestamp.size() - 1; i > 1; --i)
+    std::swap(vSortedByTimestamp[i], vSortedByTimestamp[GetRand(i)]);
+
+    // Upgrading to Bitcoin's latest uint256 type changes the backing storage
+    // from an array of 32-bit integers to a byte array. Since the comparison
+    // operator implementations changed as well, the less-than comparison for
+    // sorting the vector of candidates can produce different block orderings
+    // for the historical stake modifier than the original order from before.
+    //
+    // To ensure that we reproduce the same stake modifier values out of this
+    // candidate set, we adopt Peercoin's strategy for sorting the collection
+    // that behaves like the old uint256 implementation:
+    //
+    sort(vSortedByTimestamp.begin(), vSortedByTimestamp.end(), [] (const pair<int64_t, uint256> &a, const pair<int64_t, uint256> &b)
+    {
+        if (a.first != b.first)
+            return a.first < b.first;
+        // Timestamp equals - compare block hashes
+        const uint32_t *pa = a.second.GetDataPtr();
+        const uint32_t *pb = b.second.GetDataPtr();
+        int cnt = 256 / 32;
+        do {
+            --cnt;
+            if (pa[cnt] != pb[cnt])
+                return pa[cnt] < pb[cnt];
+        } while(cnt);
+            return false; // Elements are equal
+    });
 
     // Select 64 blocks from candidate blocks to generate stake modifier
     uint64_t nStakeModifierNew = 0;

src/uint256.h

@@ -51,6 +51,11 @@ class base_blob
     void SetHex(const std::string& str);
     std::string ToString() const;
 
+    const uint32_t *GetDataPtr() const
+    {
+        return (const uint32_t *)data;
+    }
+
     unsigned char* begin()
     {
         return &data[0];