Add GenIdentity, an identity function for MappedShuffle.

We use this function in z_sendmany as part of the fix for #1779.

src/gtest/test_random.cpp

@@ -24,4 +24,12 @@ TEST(Random, MappedShuffle) {
     std::vector<int> em2 {0, 1, 2, 3, 4};
     EXPECT_EQ(ea2, a2);
     EXPECT_EQ(em2, m2);
+
+    auto a3 = a;
+    auto m3 = m;
+    MappedShuffle(a3.begin(), m3.begin(), a3.size(), GenIdentity);
+    std::vector<int> ea3 {8, 4, 6, 3, 5};
+    std::vector<int> em3 {0, 1, 2, 3, 4};
+    EXPECT_EQ(ea3, a3);
+    EXPECT_EQ(em3, m3);
 }

src/primitives/transaction.cpp

@@ -57,10 +57,11 @@ JSDescription JSDescription::Randomized(
     // Randomize the order of the inputs and outputs
     inputMap = {0, 1};
     outputMap = {0, 1};
-    if (gen) {
-        MappedShuffle(inputs.begin(), inputMap.begin(), ZC_NUM_JS_INPUTS, gen);
-        MappedShuffle(outputs.begin(), outputMap.begin(), ZC_NUM_JS_OUTPUTS, gen);
-    }
+
+    assert(gen);
+
+    MappedShuffle(inputs.begin(), inputMap.begin(), ZC_NUM_JS_INPUTS, gen);
+    MappedShuffle(outputs.begin(), outputMap.begin(), ZC_NUM_JS_OUTPUTS, gen);
 
     return JSDescription(
         params, pubKeyHash, anchor, inputs, outputs,

src/random.cpp

@@ -137,3 +137,8 @@ void seed_insecure_rand(bool fDeterministic)
         insecure_rand_Rw = tmp;
     }
 }
+
+int GenIdentity(int n)
+{
+    return n-1;
+}

src/random.h

@@ -25,6 +25,11 @@ uint64_t GetRand(uint64_t nMax);
 int GetRandInt(int nMax);
 uint256 GetRandHash();
 
+/**
+ * Identity function for MappedShuffle, so that elements retain their original order.
+ */
+ int GenIdentity(int n);
+
 /**
  * Rearranges the elements in the range [first,first+len) randomly, assuming
  * that gen is a uniform random number generator. Follows the same algorithm as

src/wallet/asyncrpcoperation_sendmany.cpp

@@ -875,7 +875,6 @@ Object AsyncRPCOperation_sendmany::perform_joinsplit(
             {info.vjsout[0], info.vjsout[1]};
     boost::array<size_t, ZC_NUM_JS_INPUTS> inputMap;
     boost::array<size_t, ZC_NUM_JS_OUTPUTS> outputMap;
-    std::function<int(int)> emptyFunc;
     JSDescription jsdesc = JSDescription::Randomized(
             *pzcashParams,
             joinSplitPubKey_,
@@ -888,7 +887,7 @@ Object AsyncRPCOperation_sendmany::perform_joinsplit(
             info.vpub_new,
             !this->testmode,
             // Temporary fix for #1779 is to disable shuffling of inputs and outputs.
-            emptyFunc);
+            GenIdentity);
 
     if (!(jsdesc.Verify(*pzcashParams, joinSplitPubKey_))) {
         throw std::runtime_error("error verifying joinsplit");