1 files changed, 50 insertions, 1 deletions
diff --git a/src/chain.cpp b/src/chain.cpp
index e13c047861..719256106e 100644
--- a/src/chain.cpp
+++ b/src/chain.cpp
@@ -1,5 +1,5 @@
 // Copyright (c) 2009-2010 Satoshi Nakamoto
-// Copyright (c) 2009-2014 The Bitcoin developers
+// Copyright (c) 2009-2014 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -57,3 +57,52 @@ const CBlockIndex *CChain::FindFork(const CBlockIndex *pindex) const {
         pindex = pindex->pprev;
     return pindex;
 }
+
+/** 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));
+}