1 files changed, 104 insertions, 1 deletions

diff --git a/src/main.cpp b/src/main.cpp
index 336a5a9a86..41125cb577 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -207,6 +207,10 @@ struct CNodeState {
     std::string name;
     // List of asynchronously-determined block rejections to notify this peer about.
     std::vector<CBlockReject> rejects;
+    // The best known block we know this peer has announced.
+    CBlockIndex *pindexBestKnownBlock;
+    // The hash of the last unknown block this peer has announced.
+    uint256 hashLastUnknownBlock;
     list<QueuedBlock> vBlocksInFlight;
     int nBlocksInFlight;
     list<uint256> vBlocksToDownload;
@@ -217,6 +221,8 @@ struct CNodeState {
     CNodeState() {
         nMisbehavior = 0;
         fShouldBan = false;
+        pindexBestKnownBlock = NULL;
+        hashLastUnknownBlock = uint256(0);
         nBlocksToDownload = 0;
         nBlocksInFlight = 0;
         nLastBlockReceive = 0;
@@ -313,6 +319,39 @@ void MarkBlockAsInFlight(NodeId nodeid, const uint256 &hash) {
     mapBlocksInFlight[hash] = std::make_pair(nodeid, it);
 }
 
+/** Check whether the last unknown block a peer advertized is not yet known. */
+void ProcessBlockAvailability(NodeId nodeid) {
+    CNodeState *state = State(nodeid);
+    assert(state != NULL);
+
+    if (state->hashLastUnknownBlock != 0) {
+        map<uint256, CBlockIndex*>::iterator itOld = mapBlockIndex.find(state->hashLastUnknownBlock);
+        if (itOld != mapBlockIndex.end() && itOld->second->nChainWork > 0) {
+            if (state->pindexBestKnownBlock == NULL || itOld->second->nChainWork >= state->pindexBestKnownBlock->nChainWork)
+                state->pindexBestKnownBlock = itOld->second;
+            state->hashLastUnknownBlock = uint256(0);
+        }
+    }
+}
+
+/** Update tracking information about which blocks a peer is assumed to have. */
+void UpdateBlockAvailability(NodeId nodeid, const uint256 &hash) {
+    CNodeState *state = State(nodeid);
+    assert(state != NULL);
+
+    ProcessBlockAvailability(nodeid);
+
+    map<uint256, CBlockIndex*>::iterator it = mapBlockIndex.find(hash);
+    if (it != mapBlockIndex.end() && it->second->nChainWork > 0) {
+        // An actually better block was announced.
+        if (state->pindexBestKnownBlock == NULL || it->second->nChainWork >= state->pindexBestKnownBlock->nChainWork)
+            state->pindexBestKnownBlock = it->second;
+    } else {
+        // An unknown block was announced; just assume that the latest one is the best one.
+        state->hashLastUnknownBlock = hash;
+    }
+}
+
 } // anon namespace
 
 bool GetNodeStateStats(NodeId nodeid, CNodeStateStats &stats) {
@@ -321,6 +360,7 @@ bool GetNodeStateStats(NodeId nodeid, CNodeStateStats &stats) {
     if (state == NULL)
         return false;
     stats.nMisbehavior = state->nMisbehavior;
+    stats.nSyncHeight = state->pindexBestKnownBlock ? state->pindexBestKnownBlock->nHeight : -1;
     return true;
 }
 
@@ -374,8 +414,11 @@ CBlockLocator CChain::GetLocator(const CBlockIndex *pindex) const {
             break;
         // Exponentially larger steps back, plus the genesis block.
         int nHeight = std::max(pindex->nHeight - nStep, 0);
+        // Jump back quickly to the same height as the chain.
+        if (pindex->nHeight > nHeight)
+            pindex = pindex->GetAncestor(nHeight);
         // In case pindex is not in this chain, iterate pindex->pprev to find blocks.
-        while (pindex->nHeight > nHeight && !Contains(pindex))
+        while (!Contains(pindex))
             pindex = pindex->pprev;
         // If pindex is in this chain, use direct height-based access.
         if (pindex->nHeight > nHeight)
@@ -402,6 +445,8 @@ CBlockIndex *CChain::FindFork(const CBlockLocator &locator) const {
 }
 
 CBlockIndex *CChain::FindFork(CBlockIndex *pindex) const {
+    if (pindex->nHeight > Height())
+        pindex = pindex->GetAncestor(Height());
     while (pindex && !Contains(pindex))
         pindex = pindex->pprev;
     return pindex;
@@ -2111,6 +2156,7 @@ CBlockIndex* AddToBlockIndex(CBlockHeader& block)
     {
         pindexNew->pprev = (*miPrev).second;
         pindexNew->nHeight = pindexNew->pprev->nHeight + 1;
+        pindexNew->BuildSkip();
     }
     pindexNew->nChainWork = (pindexNew->pprev ? pindexNew->pprev->nChainWork : 0) + pindexNew->GetBlockWork();
     pindexNew->RaiseValidity(BLOCK_VALID_TREE);
@@ -2468,6 +2514,55 @@ bool CBlockIndex::IsSuperMajority(int minVersion, const CBlockIndex* pstart, uns
     return (nFound >= nRequired);
 }
 
+/** Turn the lowest '1' bit in the binary representation of a number into a '0'. */
+int static inline InvertLowestOne(int n) { return n & (n - 1); }
+
+/** Compute what height to jump back to with the CBlockIndex::pskip pointer. */
+int static inline GetSkipHeight(int height) {
+    if (height < 2)
+        return 0;
+
+    // Determine which height to jump back to. Any number strictly lower than height is acceptable,
+    // but the following expression seems to perform well in simulations (max 110 steps to go back
+    // up to 2**18 blocks).
+    return (height & 1) ? InvertLowestOne(InvertLowestOne(height - 1)) + 1 : InvertLowestOne(height);
+}
+
+CBlockIndex* CBlockIndex::GetAncestor(int height)
+{
+    if (height > nHeight || height < 0)
+        return NULL;
+
+    CBlockIndex* pindexWalk = this;
+    int heightWalk = nHeight;
+    while (heightWalk > height) {
+        int heightSkip = GetSkipHeight(heightWalk);
+        int heightSkipPrev = GetSkipHeight(heightWalk - 1);
+        if (heightSkip == height ||
+            (heightSkip > height && !(heightSkipPrev < heightSkip - 2 &&
+                                      heightSkipPrev >= height))) {
+            // Only follow pskip if pprev->pskip isn't better than pskip->pprev.
+            pindexWalk = pindexWalk->pskip;
+            heightWalk = heightSkip;
+        } else {
+            pindexWalk = pindexWalk->pprev;
+            heightWalk--;
+        }
+    }
+    return pindexWalk;
+}
+
+const CBlockIndex* CBlockIndex::GetAncestor(int height) const
+{
+    return const_cast<CBlockIndex*>(this)->GetAncestor(height);
+}
+
+void CBlockIndex::BuildSkip()
+{
+    if (pprev)
+        pskip = pprev->GetAncestor(GetSkipHeight(nHeight));
+}
+
 void PushGetBlocks(CNode* pnode, CBlockIndex* pindexBegin, uint256 hashEnd)
 {
     AssertLockHeld(cs_main);
@@ -2818,6 +2913,8 @@ bool static LoadBlockIndexDB()
             setBlockIndexValid.insert(pindex);
         if (pindex->nStatus & BLOCK_FAILED_MASK && (!pindexBestInvalid || pindex->nChainWork > pindexBestInvalid->nChainWork))
             pindexBestInvalid = pindex;
+        if (pindex->pprev)
+            pindex->BuildSkip();
     }
 
     // Load block file info
@@ -3613,6 +3710,9 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
                 PushGetBlocks(pfrom, chainActive.Tip(), GetOrphanRoot(inv.hash));
             }
 
+            if (inv.type == MSG_BLOCK)
+                UpdateBlockAvailability(pfrom->GetId(), inv.hash);
+
             // Track requests for our stuff
             g_signals.Inventory(inv.hash);
         }
@@ -4359,6 +4459,9 @@ bool SendMessages(CNode* pto, bool fSendTrickle)
             pto->fDisconnect = true;
         }
 
+        // Update knowledge of peer's block availability.
+        ProcessBlockAvailability(pto->GetId());
+
         //
         // Message: getdata (blocks)
         //