mapNextTx: use pointer as key, simplify value

Saves about 10% of application memory usage once the mempool warms up. Since the
mempool is DynamicUsage-regulated, this will translate to a larger mempool in
the same amount of space.

Map value type: eliminate the vin index; no users of the map need to know which
input of the transaction is spending the prevout.

Map key type: replace the COutPoint with a pointer to a COutPoint. A COutPoint
is 36 bytes, but each COutPoint is accessible from the same map entry's value.
A trivial DereferencingComparator functor allows indirect map keys, but the
resulting syntax is misleading: `map.find(&outpoint)`. Implement an indirectmap
that acts as a wrapper to a map that uses a DereferencingComparator, supporting
a syntax that accurately reflect the container's semantics: inserts and
iterators use pointers since they store pointers and need them to remain
constant and dereferenceable, but lookup functions take const references.

6 files changed, 96 insertions, 41 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index 3c056386fa..ad61e4f108 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -94,6 +94,7 @@ BITCOIN_CORE_H = \
   core_memusage.h \
   httprpc.h \
   httpserver.h \
+  indirectmap.h \
   init.h \
   key.h \
   keystore.h \
diff --git a/src/indirectmap.h b/src/indirectmap.h
new file mode 100644
index 0000000000..28e1e8dedd
--- /dev/null
+++ b/src/indirectmap.h
@@ -0,0 +1,52 @@
+#ifndef BITCOIN_INDIRECTMAP_H
+#define BITCOIN_INDIRECTMAP_H
+
+template <class T>
+struct DereferencingComparator { bool operator()(const T a, const T b) const { return *a < *b; } };
+
+/* Map whose keys are pointers, but are compared by their dereferenced values.
+ *
+ * Differs from a plain std::map<const K*, T, DereferencingComparator<K*> > in
+ * that methods that take a key for comparison take a K rather than taking a K*
+ * (taking a K* would be confusing, since it's the value rather than the address
+ * of the object for comparison that matters due to the dereferencing comparator).
+ *
+ * Objects pointed to by keys must not be modified in any way that changes the
+ * result of DereferencingComparator.
+ */
+template <class K, class T>
+class indirectmap {
+private:
+    typedef std::map<const K*, T, DereferencingComparator<const K*> > base;
+    base m;
+public:
+    typedef typename base::iterator iterator;
+    typedef typename base::const_iterator const_iterator;
+    typedef typename base::size_type size_type;
+    typedef typename base::value_type value_type;
+
+    // passthrough (pointer interface)
+    std::pair<iterator, bool> insert(const value_type& value) { return m.insert(value); }
+
+    // pass address (value interface)
+    iterator find(const K& key)                     { return m.find(&key); }
+    const_iterator find(const K& key) const         { return m.find(&key); }
+    iterator lower_bound(const K& key)              { return m.lower_bound(&key); }
+    const_iterator lower_bound(const K& key) const  { return m.lower_bound(&key); }
+    size_type erase(const K& key)                   { return m.erase(&key); }
+    size_type count(const K& key) const             { return m.count(&key); }
+
+    // passthrough
+    bool empty() const              { return m.empty(); }
+    size_type size() const          { return m.size(); }
+    size_type max_size() const      { return m.max_size(); }
+    void clear()                    { m.clear(); }
+    iterator begin()                { return m.begin(); }
+    iterator end()                  { return m.end(); }
+    const_iterator begin() const    { return m.begin(); }
+    const_iterator end() const      { return m.end(); }
+    const_iterator cbegin() const   { return m.cbegin(); }
+    const_iterator cend() const     { return m.cend(); }
+};
+
+#endif // BITCOIN_INDIRECTMAP_H
diff --git a/src/main.cpp b/src/main.cpp
index 11ccab253e..13da2137e5 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1049,9 +1049,10 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState& state, const C
     LOCK(pool.cs); // protect pool.mapNextTx
     BOOST_FOREACH(const CTxIn &txin, tx.vin)
     {
-        if (pool.mapNextTx.count(txin.prevout))
+        auto itConflicting = pool.mapNextTx.find(txin.prevout);
+        if (itConflicting != pool.mapNextTx.end())
         {
-            const CTransaction *ptxConflicting = pool.mapNextTx[txin.prevout].ptx;
+            const CTransaction *ptxConflicting = itConflicting->second;
             if (!setConflicts.count(ptxConflicting->GetHash()))
             {
                 // Allow opt-out of transaction replacement by setting
diff --git a/src/memusage.h b/src/memusage.h
index 49760e64c7..9c98e5c2cf 100644
--- a/src/memusage.h
+++ b/src/memusage.h
@@ -5,6 +5,8 @@
 #ifndef BITCOIN_MEMUSAGE_H
 #define BITCOIN_MEMUSAGE_H
 
+#include "indirectmap.h"
+
 #include <stdlib.h>
 
 #include <map>
@@ -106,6 +108,20 @@ static inline size_t IncrementalDynamicUsage(const std::map<X, Y, Z>& m)
     return MallocUsage(sizeof(stl_tree_node<std::pair<const X, Y> >));
 }
 
+// indirectmap has underlying map with pointer as key
+
+template<typename X, typename Y>
+static inline size_t DynamicUsage(const indirectmap<X, Y>& m)
+{
+    return MallocUsage(sizeof(stl_tree_node<std::pair<const X*, Y> >)) * m.size();
+}
+
+template<typename X, typename Y>
+static inline size_t IncrementalDynamicUsage(const indirectmap<X, Y>& m)
+{
+    return MallocUsage(sizeof(stl_tree_node<std::pair<const X*, Y> >));
+}
+
 // Boost data structures
 
 template<typename X>
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 52c7793118..d7fb77661b 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -147,11 +147,11 @@ void CTxMemPool::UpdateTransactionsFromBlock(const std::vector<uint256> &vHashes
         if (it == mapTx.end()) {
             continue;
         }
-        std::map<COutPoint, CInPoint>::iterator iter = mapNextTx.lower_bound(COutPoint(hash, 0));
+        auto iter = mapNextTx.lower_bound(COutPoint(hash, 0));
         // First calculate the children, and update setMemPoolChildren to
         // include them, and update their setMemPoolParents to include this tx.
-        for (; iter != mapNextTx.end() && iter->first.hash == hash; ++iter) {
-            const uint256 &childHash = iter->second.ptx->GetHash();
+        for (; iter != mapNextTx.end() && iter->first->hash == hash; ++iter) {
+            const uint256 &childHash = iter->second->GetHash();
             txiter childIter = mapTx.find(childHash);
             assert(childIter != mapTx.end());
             // We can skip updating entries we've encountered before or that
@@ -365,11 +365,11 @@ void CTxMemPool::pruneSpent(const uint256 &hashTx, CCoins &coins)
 {
     LOCK(cs);
 
-    std::map<COutPoint, CInPoint>::iterator it = mapNextTx.lower_bound(COutPoint(hashTx, 0));
+    auto it = mapNextTx.lower_bound(COutPoint(hashTx, 0));
 
     // iterate over all COutPoints in mapNextTx whose hash equals the provided hashTx
-    while (it != mapNextTx.end() && it->first.hash == hashTx) {
-        coins.Spend(it->first.n); // and remove those outputs from coins
+    while (it != mapNextTx.end() && it->first->hash == hashTx) {
+        coins.Spend(it->first->n); // and remove those outputs from coins
         it++;
     }
 }
@@ -414,7 +414,7 @@ bool CTxMemPool::addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry,
     const CTransaction& tx = newit->GetTx();
     std::set<uint256> setParentTransactions;
     for (unsigned int i = 0; i < tx.vin.size(); i++) {
-        mapNextTx[tx.vin[i].prevout] = CInPoint(&tx, i);
+        mapNextTx.insert(std::make_pair(&tx.vin[i].prevout, &tx));
         setParentTransactions.insert(tx.vin[i].prevout.hash);
     }
     // Don't bother worrying about child transactions of this one.
@@ -500,10 +500,10 @@ void CTxMemPool::removeRecursive(const CTransaction &origTx, std::list<CTransact
             // happen during chain re-orgs if origTx isn't re-accepted into
             // the mempool for any reason.
             for (unsigned int i = 0; i < origTx.vout.size(); i++) {
-                std::map<COutPoint, CInPoint>::iterator it = mapNextTx.find(COutPoint(origTx.GetHash(), i));
+                auto it = mapNextTx.find(COutPoint(origTx.GetHash(), i));
                 if (it == mapNextTx.end())
                     continue;
-                txiter nextit = mapTx.find(it->second.ptx->GetHash());
+                txiter nextit = mapTx.find(it->second->GetHash());
                 assert(nextit != mapTx.end());
                 txToRemove.insert(nextit);
             }
@@ -561,9 +561,9 @@ void CTxMemPool::removeConflicts(const CTransaction &tx, std::list<CTransaction>
     list<CTransaction> result;
     LOCK(cs);
     BOOST_FOREACH(const CTxIn &txin, tx.vin) {
-        std::map<COutPoint, CInPoint>::iterator it = mapNextTx.find(txin.prevout);
+        auto it = mapNextTx.find(txin.prevout);
         if (it != mapNextTx.end()) {
-            const CTransaction &txConflict = *it->second.ptx;
+            const CTransaction &txConflict = *it->second;
             if (txConflict != tx)
             {
                 removeRecursive(txConflict, removed);
@@ -671,10 +671,10 @@ void CTxMemPool::check(const CCoinsViewCache *pcoins) const
                 assert(coins && coins->IsAvailable(txin.prevout.n));
             }
             // Check whether its inputs are marked in mapNextTx.
-            std::map<COutPoint, CInPoint>::const_iterator it3 = mapNextTx.find(txin.prevout);
+            auto it3 = mapNextTx.find(txin.prevout);
             assert(it3 != mapNextTx.end());
-            assert(it3->second.ptx == &tx);
-            assert(it3->second.n == i);
+            assert(it3->first == &txin.prevout);
+            assert(it3->second == &tx);
             i++;
         }
         assert(setParentCheck == GetMemPoolParents(it));
@@ -701,10 +701,10 @@ void CTxMemPool::check(const CCoinsViewCache *pcoins) const
 
         // Check children against mapNextTx
         CTxMemPool::setEntries setChildrenCheck;
-        std::map<COutPoint, CInPoint>::const_iterator iter = mapNextTx.lower_bound(COutPoint(it->GetTx().GetHash(), 0));
+        auto iter = mapNextTx.lower_bound(COutPoint(it->GetTx().GetHash(), 0));
         int64_t childSizes = 0;
-        for (; iter != mapNextTx.end() && iter->first.hash == it->GetTx().GetHash(); ++iter) {
-            txiter childit = mapTx.find(iter->second.ptx->GetHash());
+        for (; iter != mapNextTx.end() && iter->first->hash == it->GetTx().GetHash(); ++iter) {
+            txiter childit = mapTx.find(iter->second->GetHash());
             assert(childit != mapTx.end()); // mapNextTx points to in-mempool transactions
             if (setChildrenCheck.insert(childit).second) {
                 childSizes += childit->GetTxSize();
@@ -738,14 +738,12 @@ void CTxMemPool::check(const CCoinsViewCache *pcoins) const
             stepsSinceLastRemove = 0;
         }
     }
-    for (std::map<COutPoint, CInPoint>::const_iterator it = mapNextTx.begin(); it != mapNextTx.end(); it++) {
-        uint256 hash = it->second.ptx->GetHash();
+    for (auto it = mapNextTx.cbegin(); it != mapNextTx.cend(); it++) {
+        uint256 hash = it->second->GetHash();
         indexed_transaction_set::const_iterator it2 = mapTx.find(hash);
         const CTransaction& tx = it2->GetTx();
         assert(it2 != mapTx.end());
-        assert(&tx == it->second.ptx);
-        assert(tx.vin.size() > it->second.n);
-        assert(it->first == it->second.ptx->vin[it->second.n].prevout);
+        assert(&tx == it->second);
     }
 
     assert(totalTxSize == checkTotal);
@@ -1044,8 +1042,8 @@ void CTxMemPool::TrimToSize(size_t sizelimit, std::vector<uint256>* pvNoSpendsRe
                 BOOST_FOREACH(const CTxIn& txin, tx.vin) {
                     if (exists(txin.prevout.hash))
                         continue;
-                    std::map<COutPoint, CInPoint>::iterator it = mapNextTx.lower_bound(COutPoint(txin.prevout.hash, 0));
-                    if (it == mapNextTx.end() || it->first.hash != txin.prevout.hash)
+                    auto it = mapNextTx.lower_bound(COutPoint(txin.prevout.hash, 0));
+                    if (it == mapNextTx.end() || it->first->hash != txin.prevout.hash)
                         pvNoSpendsRemaining->push_back(txin.prevout.hash);
                 }
             }
diff --git a/src/txmempool.h b/src/txmempool.h
index de4ba0b371..7ad5579a06 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -11,6 +11,7 @@
 
 #include "amount.h"
 #include "coins.h"
+#include "indirectmap.h"
 #include "primitives/transaction.h"
 #include "sync.h"
 
@@ -305,20 +306,6 @@ struct ancestor_score {};
 
 class CBlockPolicyEstimator;
 
-/** An inpoint - a combination of a transaction and an index n into its vin */
-class CInPoint
-{
-public:
-    const CTransaction* ptx;
-    uint32_t n;
-
-    CInPoint() { SetNull(); }
-    CInPoint(const CTransaction* ptxIn, uint32_t nIn) { ptx = ptxIn; n = nIn; }
-    void SetNull() { ptx = NULL; n = (uint32_t) -1; }
-    bool IsNull() const { return (ptx == NULL && n == (uint32_t) -1); }
-    size_t DynamicMemoryUsage() const { return 0; }
-};
-
 /**
  * CTxMemPool stores valid-according-to-the-current-best-chain
  * transactions that may be included in the next block.
@@ -477,7 +464,7 @@ private:
     void UpdateChild(txiter entry, txiter child, bool add);
 
 public:
-    std::map<COutPoint, CInPoint> mapNextTx;
+    indirectmap<COutPoint, const CTransaction*> mapNextTx;
     std::map<uint256, std::pair<double, CAmount> > mapDeltas;
 
     /** Create a new CTxMemPool.