Implement MappedShuffle for tracking the permutation of an array

src/Makefile.gtest.include

@@ -17,6 +17,7 @@ zcash_gtest_SOURCES = \
 	gtest/test_noteencryption.cpp \
 	gtest/test_merkletree.cpp \
 	gtest/test_pow.cpp \
+	gtest/test_random.cpp \
 	gtest/test_rpc.cpp \
 	gtest/test_circuit.cpp \
 	gtest/test_txid.cpp \

src/gtest/test_random.cpp

@@ -0,0 +1,34 @@
+#include <gtest/gtest.h>
+
+#include "random.h"
+
+int GenZero(int n)
+{
+    return 0;
+}
+
+int GenMax(int n)
+{
+    return n-1;
+}
+
+TEST(Random, MappedShuffle) {
+    std::vector<int> a {8, 4, 6, 3, 5};
+    std::vector<int> m {0, 1, 2, 3, 4};
+
+    auto a1 = a;
+    auto m1 = m;
+    MappedShuffle(a1.begin(), m1.begin(), a1.size(), GenZero);
+    std::vector<int> ea1 {4, 6, 3, 5, 8};
+    std::vector<int> em1 {1, 2, 3, 4, 0};
+    EXPECT_EQ(ea1, a1);
+    EXPECT_EQ(em1, m1);
+
+    auto a2 = a;
+    auto m2 = m;
+    MappedShuffle(a2.begin(), m2.begin(), a2.size(), GenMax);
+    std::vector<int> ea2 {8, 4, 6, 3, 5};
+    std::vector<int> em2 {0, 1, 2, 3, 4};
+    EXPECT_EQ(ea2, a2);
+    EXPECT_EQ(em2, m2);
+}

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