Add a skiplist to the CBlockIndex structure.

This allows fast (O(log n)) access to far predecessor blocks.
Use it to speed up CChain::FindFork and CChain::GetLocator.

src/main.cpp

@@ -414,8 +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)
@@ -442,6 +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;
@@ -2151,6 +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);
@@ -2508,6 +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);
@@ -2858,6 +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

src/main.h

@@ -677,6 +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;
 
@@ -716,6 +719,7 @@ public:
     {
         phashBlock = NULL;
         pprev = NULL;
+        pskip = NULL;
         nHeight = 0;
         nFile = 0;
         nDataPos = 0;
@@ -737,6 +741,7 @@ public:
     {
         phashBlock = NULL;
         pprev = NULL;
+        pskip = NULL;
         nHeight = 0;
         nFile = 0;
         nDataPos = 0;
@@ -869,9 +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