Introduce src/reverse_iterator.hpp and include it...

...where it will be needed

Taken from https://gist.github.com/arvidsson/7231973 with small
modifications to fit the bitcoin core project

src/Makefile.am

@@ -124,6 +124,7 @@ BITCOIN_CORE_H = \
   pow.h \
   protocol.h \
   random.h \
+  reverse_iterator.h \
   reverselock.h \
   rpc/blockchain.h \
   rpc/client.h \

src/checkpoints.cpp

@@ -6,6 +6,7 @@
 
 #include "chain.h"
 #include "chainparams.h"
+#include "reverse_iterator.h"
 #include "validation.h"
 #include "uint256.h"
 

src/net_processing.cpp

@@ -22,6 +22,7 @@
 #include "primitives/block.h"
 #include "primitives/transaction.h"
 #include "random.h"
+#include "reverse_iterator.h"
 #include "tinyformat.h"
 #include "txmempool.h"
 #include "ui_interface.h"

src/reverse_iterator.h

@@ -0,0 +1,39 @@
+// Taken from https://gist.github.com/arvidsson/7231973
+
+#ifndef BITCOIN_REVERSE_ITERATOR_HPP
+#define BITCOIN_REVERSE_ITERATOR_HPP
+
+/**
+ * Template used for reverse iteration in C++11 range-based for loops.
+ * 
+ *   std::vector<int> v = {1, 2, 3, 4, 5};
+ *   for (auto x : reverse_iterate(v))
+ *       std::cout << x << " ";
+ */
+
+template <typename T>
+class reverse_range
+{
+    T &x;
+    
+public:
+    reverse_range(T &x) : x(x) {}
+    
+    auto begin() const -> decltype(this->x.rbegin())
+    {
+        return x.rbegin();
+    }
+    
+    auto end() const -> decltype(this->x.rend())
+    {
+        return x.rend();
+    }
+};
+ 
+template <typename T>
+reverse_range<T> reverse_iterate(T &x)
+{
+    return reverse_range<T>(x);
+}
+
+#endif // BITCOIN_REVERSE_ITERATOR_HPP

src/test/prevector_tests.cpp

@@ -5,6 +5,7 @@
 #include <vector>
 #include "prevector.h"
 
+#include "reverse_iterator.h"
 #include "serialize.h"
 #include "streams.h"
 

src/txmempool.cpp

@@ -11,6 +11,7 @@
 #include "validation.h"
 #include "policy/policy.h"
 #include "policy/fees.h"
+#include "reverse_iterator.h"
 #include "streams.h"
 #include "timedata.h"
 #include "util.h"

src/validation.cpp

@@ -23,6 +23,7 @@
 #include "primitives/block.h"
 #include "primitives/transaction.h"
 #include "random.h"
+#include "reverse_iterator.h"
 #include "script/script.h"
 #include "script/sigcache.h"
 #include "script/standard.h"