Implement MappedShuffle for tracking the permutation of an array

src/random.h

@@ -8,6 +8,7 @@
 
 #include "uint256.h"
 
+#include <functional>
 #include <stdint.h>
 
 /**
@@ -24,6 +25,29 @@ uint64_t GetRand(uint64_t nMax);
 int GetRandInt(int nMax);
 uint256 GetRandHash();
 
+/**
+ * Rearranges the elements in the range [first,first+len) randomly, assuming
+ * that gen is a uniform random number generator. Follows the same algorithm as
+ * std::shuffle in C++11 (a Durstenfeld shuffle).
+ *
+ * The elements in the range [mapFirst,mapFirst+len) are rearranged according to
+ * the same permutation, enabling the permutation to be tracked by the caller.
+ *
+ * gen takes an integer n and produces a uniform random output in [0,n).
+ */
+template <typename RandomAccessIterator, typename MapRandomAccessIterator>
+void MappedShuffle(RandomAccessIterator first,
+                   MapRandomAccessIterator mapFirst,
+                   size_t len,
+                   std::function<int(int)> gen)
+{
+    for (size_t i = len-1; i > 0; --i) {
+        auto r = gen(i+1);
+        std::swap(first[i], first[r]);
+        std::swap(mapFirst[i], mapFirst[r]);
+    }
+}
+
 /**
  * Seed insecure_rand using the random pool.
  * @param Deterministic Use a deterministic seed