blockchain: always select random outs using triangular distribution

It was only used by the older blockchain_storage.
We also move the code to the calling blockchain level, to avoid
replicating the code in every DB implementation. This also makes
the get_random_out method obsolete, and we delete it.

src/blockchain_db/berkeleydb/db_bdb.cpp

@@ -1264,19 +1264,6 @@ uint64_t BlockchainBDB::get_tx_block_height(const crypto::hash& h) const
     return (uint64_t)result - 1;
 }
 
-//FIXME: make sure the random method used here is appropriate
-uint64_t BlockchainBDB::get_random_output(const uint64_t& amount) const
-{
-    LOG_PRINT_L3("BlockchainBDB::" << __func__);
-    check_open();
-
-    uint64_t num_outputs = get_num_outputs(amount);
-    if (num_outputs == 0)
-        throw1(OUTPUT_DNE("Attempting to get a random output for an amount, but none exist"));
-
-    return crypto::rand<uint64_t>() % num_outputs;
-}
-
 uint64_t BlockchainBDB::get_num_outputs(const uint64_t& amount) const
 {
     LOG_PRINT_L3("BlockchainBDB::" << __func__);

src/blockchain_db/berkeleydb/db_bdb.h

@@ -295,8 +295,6 @@ public:
 
   virtual uint64_t get_tx_block_height(const crypto::hash& h) const;
 
-  virtual uint64_t get_random_output(const uint64_t& amount) const;
-
   virtual uint64_t get_num_outputs(const uint64_t& amount) const;
 
   virtual output_data_t get_output_key(const uint64_t& amount, const uint64_t& index);

src/blockchain_db/blockchain_db.h

@@ -104,7 +104,6 @@
  *   height      get_tx_block_height(hash)
  *
  * Outputs:
- *   index       get_random_output(amount)
  *   uint64_t    get_num_outputs(amount)
  *   pub_key     get_output_key(amount, index)
  *   tx_out      get_output(tx_hash, index)
@@ -463,9 +462,6 @@ public:
   // returns height of block that contains transaction with hash <h>
   virtual uint64_t get_tx_block_height(const crypto::hash& h) const = 0;
 
-  // return global output index of a random output of amount <amount>
-  virtual uint64_t get_random_output(const uint64_t& amount) const = 0;
-
   // returns the total number of outputs of amount <amount>
   virtual uint64_t get_num_outputs(const uint64_t& amount) const = 0;
 

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -1620,19 +1620,6 @@ uint64_t BlockchainLMDB::get_tx_block_height(const crypto::hash& h) const
   return *(const uint64_t*)result.mv_data;
 }
 
-//FIXME: make sure the random method used here is appropriate
-uint64_t BlockchainLMDB::get_random_output(const uint64_t& amount) const
-{
-  LOG_PRINT_L3("BlockchainLMDB::" << __func__);
-  check_open();
-
-  uint64_t num_outputs = get_num_outputs(amount);
-  if (num_outputs == 0)
-    throw1(OUTPUT_DNE("Attempting to get a random output for an amount, but none exist"));
-
-  return crypto::rand<uint64_t>() % num_outputs;
-}
-
 uint64_t BlockchainLMDB::get_num_outputs(const uint64_t& amount) const
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);

src/blockchain_db/lmdb/db_lmdb.h

@@ -157,8 +157,6 @@ public:
 
   virtual uint64_t get_tx_block_height(const crypto::hash& h) const;
 
-  virtual uint64_t get_random_output(const uint64_t& amount) const;
-
   virtual uint64_t get_num_outputs(const uint64_t& amount) const;
 
   virtual output_data_t get_output_key(const uint64_t& amount, const uint64_t& index);

src/cryptonote_core/blockchain.cpp

@@ -1511,7 +1511,15 @@ bool Blockchain::get_random_outs_for_amounts(const COMMAND_RPC_GET_RANDOM_OUTPUT
                 // get a random output index from the DB.  If we've already seen it,
                 // return to the top of the loop and try again, otherwise add it to the
                 // list of output indices we've seen.
-                uint64_t i = m_db->get_random_output(amount);
+
+                // 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));
+                uint64_t i = (uint64_t)(frac*num_outs);
+                // just in case rounding up to 1 occurs after sqrt
+                if (i == num_outs)
+                  --i;
+
                 if (seen_indices.count(i))
                 {
                     continue;