Optimize JSON-RPC getblockhash

- If the height is in the first half, start at the genesis block and go up, rather than at the top
- Cache the last lookup and use it as a reference point if it's close to the next request, to make linear lookups always fast

src/main.cpp

@@ -802,6 +802,24 @@ bool GetTransaction(const uint256 &hash, CTransaction &tx, uint256 &hashBlock)
 // CBlock and CBlockIndex
 //
 
+static CBlockIndex* pblockindexFBBHLast;
+CBlockIndex* FindBlockByHeight(int nHeight)
+{
+    CBlockIndex *pblockindex;
+    if (nHeight < nBestHeight / 2)
+        pblockindex = pindexGenesisBlock;
+    else
+        pblockindex = pindexBest;
+    if (pblockindexFBBHLast && abs(nHeight - pblockindex->nHeight) > abs(nHeight - pblockindexFBBHLast->nHeight))
+        pblockindex = pblockindexFBBHLast;
+    while (pblockindex->nHeight > nHeight)
+        pblockindex = pblockindex->pprev;
+    while (pblockindex->nHeight < nHeight)
+        pblockindex = pblockindex->pnext;
+    pblockindexFBBHLast = pblockindex;
+    return pblockindex;
+}
+
 bool CBlock::ReadFromDisk(const CBlockIndex* pindex, bool fReadTransactions)
 {
     if (!fReadTransactions)
@@ -1615,6 +1633,7 @@ bool CBlock::SetBestChain(CTxDB& txdb, CBlockIndex* pindexNew)
     // New best block
     hashBestChain = hash;
     pindexBest = pindexNew;
+    pblockindexFBBHLast = NULL;
     nBestHeight = pindexBest->nHeight;
     bnBestChainWork = pindexNew->bnChainWork;
     nTimeBestReceived = GetTime();