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.

1 files changed, 52 insertions, 0 deletions

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