Speed up getchaintips.

src/rpc/blockchain.cpp

@@ -760,17 +760,30 @@ UniValue getchaintips(const UniValue& params, bool fHelp)
 
     LOCK(cs_main);
 
-    /* Build up a list of chain tips.  We start with the list of all
-       known blocks, and successively remove blocks that appear as pprev
-       of another block.  */
+    /*
+     * Idea:  the set of chain tips is chainActive.tip, plus orphan blocks which do not have another orphan building off of them. 
+     * Algorithm:
+     *  - Make one pass through mapBlockIndex, picking out the orphan blocks, and also storing a set of the orphan block's pprev pointers.
+     *  - Iterate through the orphan blocks. If the block isn't pointed to by another orphan, it is a chain tip.
+     *  - add chainActive.Tip()
+     */
     std::set<const CBlockIndex*, CompareBlocksByHeight> setTips;
-    BOOST_FOREACH(const PAIRTYPE(const uint256, CBlockIndex*)& item, mapBlockIndex)
-        setTips.insert(item.second);
+    std::set<const CBlockIndex*> setOrphans;
+    std::set<const CBlockIndex*> setPrevs;
+
     BOOST_FOREACH(const PAIRTYPE(const uint256, CBlockIndex*)& item, mapBlockIndex)
     {
-        const CBlockIndex* pprev = item.second->pprev;
-        if (pprev)
-            setTips.erase(pprev);
+        if (!chainActive.Contains(item.second)) {
+            setOrphans.insert(item.second);
+            setPrevs.insert(item.second->pprev);
+        }
+    }
+
+    for (std::set<const CBlockIndex*>::iterator it = setOrphans.begin(); it != setOrphans.end(); ++it)
+    {
+        if (setPrevs.erase(*it) == 0) {
+            setTips.insert(*it);
+        }
     }
 
     // Always report the currently active tip.