[FOLD] Add SpanTip:

Use the SpanTip class to allow finding the ID of the ancestor of a tip
of ledger history. The Validations::getPreferred function can use this
to more directly check if the current ledger is the parent of the
preferred ledger.

src/ripple/consensus/LedgerTrie.h

@@ -28,7 +28,46 @@
 
 namespace ripple {
 
+/** The tip of a span of ledger ancestry
+*/
+template <class Ledger>
+class SpanTip
+{
+public:
+    using Seq = typename Ledger::Seq;
+    using ID = typename Ledger::ID;
+
+    SpanTip(Seq s, ID i, Ledger const lgr)
+        : seq{s}, id{i}, ledger{std::move(lgr)}
+    {
+    }
+
+    // The sequence number the tip ledger
+    Seq seq;
+    // The ID of the tip ledger
+    ID id;
+
+    /** Lookup the ID of an ancestor of the tip ledger
+
+        @param s The sequence number of the ancestor
+        @return The ID of the ancestor with that sequence number
+
+        @note s must be less than or equal to the sequence number of the
+              preferred ledger
+    */
+    ID
+    ancestor(Seq const& s) const
+    {
+        assert(s <= seq);
+        return ledger[s];
+    }
+
+private:
+    Ledger const ledger;
+};
+
 namespace ledger_trie_detail {
+
 // Represents a span of ancestry of a ledger
 template <class Ledger>
 class Span
@@ -101,12 +140,12 @@ class Span
         return clamp(mismatch(ledger_, o));
     }
 
-    //  The Seq and ID of the end of the span
-    std::pair<Seq, ID>
+    //  The tip of this span
+    SpanTip<Ledger>
     tip() const
     {
         Seq tipSeq{end_ - Seq{1}};
-        return {tipSeq, ledger_[tipSeq]};
+        return SpanTip<Ledger>{tipSeq, ledger_[tipSeq], ledger_};
     }
 
 private:
@@ -137,7 +176,7 @@ class Span
     friend std::ostream&
     operator<<(std::ostream& o, Span const& s)
     {
-        return o << s.tip().second << "[" << s.start_ << "," << s.end_ << ")";
+        return o << s.tip().id << "[" << s.start_ << "," << s.end_ << ")";
     }
 
     friend Span
@@ -203,8 +242,8 @@ struct Node
     getJson() const
     {
         Json::Value res;
-        res["id"] = to_string(span.tip().second);
-        res["seq"] = static_cast<std::uint32_t>(span.tip().first);
+        res["id"] = to_string(span.tip().id);
+        res["seq"] = static_cast<std::uint32_t>(span.tip().seq);
         res["tipSupport"] = tipSupport;
         res["branchSupport"] = branchSupport;
         if (!children.empty())
@@ -407,7 +446,6 @@ class LedgerTrie
         boost::optional<Span> oldSuffix = loc->span.from(diffSeq);
         boost::optional<Span> newSuffix = Span{ledger}.from(diffSeq);
 
-
         if (oldSuffix)
         {
             // Have
@@ -623,7 +661,7 @@ class LedgerTrie
                              issued by this node.
         @return Pair with the sequence number and ID of the preferred ledger
     */
-    std::pair<Seq,ID>
+    SpanTip<Ledger>
     getPreferred(Seq const largestIssued) const
     {
         Node* curr = root.get();

src/ripple/consensus/Validations.h

@@ -637,74 +637,61 @@ class Validations
         and is *not* an ancestor of the current working ledger; otherwise it
         remains the current working ledger.
 
-        @param currLedger The local node's current working ledger
+        @param curr The local node's current working ledger
 
         @return The sequence and id of the preferred working ledger,
                 or Seq{0},ID{0} if no trusted validations are available to
                 determine the preferred ledger.
     */
     std::pair<Seq, ID>
-    getPreferred(Ledger const& currLedger)
+    getPreferred(Ledger const& curr)
     {
-        Seq preferredSeq{0};
-        ID preferredID{0};
-
         ScopedLock lock{mutex_};
-        std::tie(preferredSeq, preferredID) =
+        SpanTip<Ledger> preferred =
             withTrie(lock, [this](LedgerTrie<Ledger>& trie) {
                 return trie.getPreferred(localSeqEnforcer_.largest());
             });
 
         // No trusted validations to determine branch
-        if (preferredSeq == Seq{0})
-            return std::make_pair(preferredSeq, preferredID);
-
-        Seq currSeq = currLedger.seq();
-        ID currID = currLedger.id();
+        if (preferred.seq == Seq{0})
+            return std::make_pair(preferred.seq, preferred.id);
 
         // If we are the parent of the preferred ledger, stick with our
         // current ledger since we might be about to generate it
-        if (preferredSeq == currSeq + Seq{1})
-        {
-            for (auto const& it : lastLedger_)
-            {
-                Ledger const& ledger = it.second;
-                if (ledger.seq() == preferredSeq &&
-                    ledger.id() == preferredID && ledger[currSeq] == currID)
-                    return std::make_pair(currSeq, currID);
-            }
-        }
+        if (preferred.seq == curr.seq() + Seq{1} &&
+            preferred.ancestor(curr.seq()) == curr.id())
+            return std::make_pair(curr.seq(), curr.id());
 
         // A ledger ahead of us is preferred regardless of whether it is
         // a descendant of our working ledger or it is on a different chain
-        if (preferredSeq > currSeq)
-            return std::make_pair(preferredSeq, preferredID);
+        if (preferred.seq > curr.seq())
+            return std::make_pair(preferred.seq, preferred.id);
 
         // Only switch to earlier or same sequence number
         // if it is a different chain.
-        if (currLedger[preferredSeq] != preferredID)
-            return std::make_pair(preferredSeq, preferredID);
+        if (curr[preferred.seq] != preferred.id)
+            return std::make_pair(preferred.seq, preferred.id);
 
         // Stick with current ledger
-        return std::make_pair(currSeq, currID);
+        return std::make_pair(curr.seq(), curr.id());
     }
 
     /** Get the ID of the preferred working ledger that exceeds a minimum valid
         ledger sequence number
 
-        @param currLedger Current working ledger
+        @param curr Current working ledger
         @param minValidSeq Minimum allowed sequence number
 
-        @return ID Of the preferred ledger, or currLedger if the preferred ledger
+        @return ID Of the preferred ledger, or curr if the preferred ledger
                    is not valid
     */
     ID
-    getPreferred(Ledger const& currLedger, Seq minValidSeq)
+    getPreferred(Ledger const& curr, Seq minValidSeq)
     {
-        std::pair<Seq, ID> preferred = getPreferred(currLedger);
+        std::pair<Seq, ID> preferred = getPreferred(curr);
         if(preferred.first >= minValidSeq && preferred.second != ID{0})
             return preferred.second;
-        return currLedger.id();
+        return curr.id();
 
     }