triangular distribution to choose recent outputs more often for mixins

src/cryptonote_core/blockchain_storage.cpp

@@ -30,6 +30,7 @@
 
 #include <algorithm>
 #include <cstdio>
+#include <cmath>
 #include <boost/archive/binary_oarchive.hpp>
 #include <boost/archive/binary_iarchive.hpp>
 
@@ -1081,7 +1082,10 @@ bool blockchain_storage::get_random_outs_for_amounts(const COMMAND_RPC_GET_RANDO
       size_t try_count = 0;
       for(uint64_t j = 0; j != req.outs_count && try_count < up_index_limit;)
       {
-        size_t i = crypto::rand<size_t>()%up_index_limit;
+	// triangular distribution over [a,b) with a=0, mode c=b=up_index_limit
+        uint64_t r = crypto::rand<uint64_t>() % ((uint64_t)1 << 53);
+	double frac = std::sqrt((double)r / ((uint64_t)1 << 53));
+	size_t i = (size_t)(frac*up_index_limit);
         if(used.count(i))
           continue;
         bool added = add_out_to_get_random_outs(amount_outs, result_outs, amount, i);