Merge pull request #4420 from sipa/skiplist

Add a skiplist to the CBlockIndex structure.

5 files changed, 163 insertions, 2 deletions

diff --git a/src/Makefile.test.include b/src/Makefile.test.include
index 4dab1773f4..8685452c7b 100644
--- a/src/Makefile.test.include
+++ b/src/Makefile.test.include
@@ -47,6 +47,7 @@ BITCOIN_TESTS =\
   test/script_tests.cpp \
   test/serialize_tests.cpp \
   test/sigopcount_tests.cpp \
+  test/skiplist_tests.cpp \
   test/test_bitcoin.cpp \
   test/transaction_tests.cpp \
   test/uint256_tests.cpp \
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)
         //
diff --git a/src/main.h b/src/main.h
index 0332db2163..1af86b065f 100644
--- a/src/main.h
+++ b/src/main.h
@@ -185,6 +185,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
 
 struct CNodeStateStats {
     int nMisbehavior;
+    int nSyncHeight;
 };
 
 struct CDiskBlockPos
@@ -676,6 +677,9 @@ public:
     // pointer to the index of the predecessor of this block
     CBlockIndex* pprev;
 
+    // pointer to the index of some further predecessor of this block
+    CBlockIndex* pskip;
+
     // height of the entry in the chain. The genesis block has height 0
     int nHeight;
 
@@ -715,6 +719,7 @@ public:
     {
         phashBlock = NULL;
         pprev = NULL;
+        pskip = NULL;
         nHeight = 0;
         nFile = 0;
         nDataPos = 0;
@@ -736,6 +741,7 @@ public:
     {
         phashBlock = NULL;
         pprev = NULL;
+        pskip = NULL;
         nHeight = 0;
         nFile = 0;
         nDataPos = 0;
@@ -868,9 +874,14 @@ public:
         }
         return false;
     }
-};
 
+    // Build the skiplist pointer for this entry.
+    void BuildSkip();
 
+    // Efficiently find an ancestor of this block.
+    CBlockIndex* GetAncestor(int height);
+    const CBlockIndex* GetAncestor(int height) const;
+};
 
 /** Used to marshal pointers into hashes for db storage. */
 class CDiskBlockIndex : public CBlockIndex
diff --git a/src/rpcnet.cpp b/src/rpcnet.cpp
index a54872ccc4..2d7abb2d58 100644
--- a/src/rpcnet.cpp
+++ b/src/rpcnet.cpp
@@ -134,6 +134,7 @@ Value getpeerinfo(const Array& params, bool fHelp)
         obj.push_back(Pair("startingheight", stats.nStartingHeight));
         if (fStateStats) {
             obj.push_back(Pair("banscore", statestats.nMisbehavior));
+            obj.push_back(Pair("syncheight", statestats.nSyncHeight));
         }
         obj.push_back(Pair("syncnode", stats.fSyncNode));
 
diff --git a/src/test/skiplist_tests.cpp b/src/test/skiplist_tests.cpp
new file mode 100644
index 0000000000..ea301685c9
--- /dev/null
+++ b/src/test/skiplist_tests.cpp
@@ -0,0 +1,45 @@
+// Copyright (c) 2014 The Bitcoin Core developers
+// Distributed under the MIT/X11 software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <boost/test/unit_test.hpp>
+#include <vector>
+#include "main.h"
+#include "util.h"
+
+
+#define SKIPLIST_LENGTH 300000
+
+BOOST_AUTO_TEST_SUITE(skiplist_tests)
+
+BOOST_AUTO_TEST_CASE(skiplist_test)
+{
+    std::vector<CBlockIndex> vIndex(SKIPLIST_LENGTH);
+
+    for (int i=0; i<SKIPLIST_LENGTH; i++) {
+        vIndex[i].nHeight = i;
+        vIndex[i].pprev = (i == 0) ? NULL : &vIndex[i - 1];
+        vIndex[i].BuildSkip();
+    }
+
+    for (int i=0; i<SKIPLIST_LENGTH; i++) {
+        if (i > 0) {
+            BOOST_CHECK(vIndex[i].pskip == &vIndex[vIndex[i].pskip->nHeight]);
+            BOOST_CHECK(vIndex[i].pskip->nHeight < i);
+        } else {
+            BOOST_CHECK(vIndex[i].pskip == NULL);
+        }
+    }
+
+    for (int i=0; i < 1000; i++) {
+        int from = insecure_rand() % (SKIPLIST_LENGTH - 1);
+        int to = insecure_rand() % (from + 1);
+
+        BOOST_CHECK(vIndex[SKIPLIST_LENGTH - 1].GetAncestor(from) == &vIndex[from]);
+        BOOST_CHECK(vIndex[from].GetAncestor(to) == &vIndex[to]);
+        BOOST_CHECK(vIndex[from].GetAncestor(0) == &vIndex[0]);
+    }
+}
+
+BOOST_AUTO_TEST_SUITE_END()
+