core: dynamic fee algorithm from ArticMine

The fee will vary based on the base reward and the current
block size limit:

fee = (R/R0) * (M0/M) * F0

R: base reward
R0: reference base reward (10 monero)
M: block size limit
M0: minimum block size limit (60000)
F0: 0.002 monero

Starts applying at v4

3 files changed, 80 insertions, 5 deletions

diff --git a/src/cryptonote_core/blockchain.cpp b/src/cryptonote_core/blockchain.cpp
index 9ea023a4c..c92ee6c9b 100644
--- a/src/cryptonote_core/blockchain.cpp
+++ b/src/cryptonote_core/blockchain.cpp
@@ -46,6 +46,7 @@
 #include "misc_language.h"
 #include "profile_tools.h"
 #include "file_io_utils.h"
+#include "common/int-util.h"
 #include "common/boost_serialization_helper.h"
 #include "warnings.h"
 #include "crypto/hash.h"
@@ -2709,6 +2710,57 @@ void Blockchain::check_ring_signature(const crypto::hash &tx_prefix_hash, const
 }
 
 //------------------------------------------------------------------
+uint64_t Blockchain::get_dynamic_per_kb_fee(uint64_t block_reward, size_t median_block_size)
+{
+  if (median_block_size < CRYPTONOTE_BLOCK_GRANTED_FULL_REWARD_ZONE_V2)
+    median_block_size = CRYPTONOTE_BLOCK_GRANTED_FULL_REWARD_ZONE_V2;
+
+  uint64_t unscaled_fee_per_kb = (DYNAMIC_FEE_PER_KB_BASE_FEE * CRYPTONOTE_BLOCK_GRANTED_FULL_REWARD_ZONE_V2 / median_block_size);
+  uint64_t hi, lo = mul128(unscaled_fee_per_kb, block_reward, &hi);
+  static_assert(DYNAMIC_FEE_PER_KB_BASE_BLOCK_REWARD % 1000000 == 0, "DYNAMIC_FEE_PER_KB_BASE_BLOCK_REWARD must be divisible by 1000000");
+  static_assert(DYNAMIC_FEE_PER_KB_BASE_BLOCK_REWARD / 1000000 <= std::numeric_limits<uint32_t>::max(), "DYNAMIC_FEE_PER_KB_BASE_BLOCK_REWARD is too large");
+  // divide in two steps, since the divisor must be 32 bits, but DYNAMIC_FEE_PER_KB_BASE_BLOCK_REWARD isn't
+  div128_32(hi, lo, DYNAMIC_FEE_PER_KB_BASE_BLOCK_REWARD / 1000000, &hi, &lo);
+  div128_32(hi, lo, 1000000, &hi, &lo);
+  assert(hi == 0);
+
+  return lo;
+}
+
+//------------------------------------------------------------------
+bool Blockchain::check_fee(size_t blob_size, uint64_t fee) const
+{
+  const uint8_t version = get_current_hard_fork_version();
+
+  uint64_t fee_per_kb;
+  if (version < HF_VERSION_DYNAMIC_FEE)
+  {
+    fee_per_kb = FEE_PER_KB;
+  }
+  else
+  {
+    uint64_t median = m_current_block_cumul_sz_limit / 2;
+    uint64_t already_generated_coins = m_db->height() ? m_db->get_block_already_generated_coins(m_db->height() - 1) : 0;
+    uint64_t base_reward;
+    if (!get_block_reward(median, 1, already_generated_coins, base_reward, version))
+      return false;
+    fee_per_kb = get_dynamic_per_kb_fee(base_reward, median);
+  }
+  LOG_PRINT_L2("Using " << print_money(fee) << "/kB fee");
+
+  uint64_t needed_fee = blob_size / 1024;
+  needed_fee += (blob_size % 1024) ? 1 : 0;
+  needed_fee *= fee_per_kb;
+
+  if (fee < needed_fee)
+  {
+    LOG_PRINT_L1("transaction fee is not enough: " << print_money(fee) << ", minimum fee: " << print_money(needed_fee));
+    return false;
+  }
+  return true;
+}
+
+//------------------------------------------------------------------
 // This function checks to see if a tx is unlocked.  unlock_time is either
 // a block index or a unix time.
 bool Blockchain::is_tx_spendtime_unlocked(uint64_t unlock_time) const
diff --git a/src/cryptonote_core/blockchain.h b/src/cryptonote_core/blockchain.h
index 262c2952b..3311798f0 100644
--- a/src/cryptonote_core/blockchain.h
+++ b/src/cryptonote_core/blockchain.h
@@ -513,6 +513,33 @@ namespace cryptonote
     bool check_tx_inputs(transaction& tx, uint64_t& pmax_used_block_height, crypto::hash& max_used_block_id, tx_verification_context &tvc, bool kept_by_block = false);
 
     /**
+     * @brief get dynamic per kB fee for a given block size
+     *
+     * The dynamic fee is based on the block size in a past window, and
+     * the current block reward. It is expressed by kB.
+     *
+     * @param block_reward the current block reward
+     * @param median_block_size the median blob's size in the past window
+     *
+     * @return the per kB fee
+     */
+    static uint64_t get_dynamic_per_kb_fee(uint64_t block_reward, size_t median_block_size);
+
+    /**
+     * @brief validate a transaction's fee
+     *
+     * This function validates the fee is enough for the transaction.
+     * This is based on the size of the transaction blob, and, after a
+     * height threshold, on the average size of transaction in a past window
+     *
+     * @param blob_size the transaction blob's size
+     * @param fee the fee
+     *
+     * @return true if the fee is enough, false otherwise
+     */
+    bool check_fee(size_t blob_size, uint64_t fee) const;
+
+    /**
      * @brief check that a transaction's outputs conform to current standards
      *
      * This function checks, for example at the time of this writing, that
diff --git a/src/cryptonote_core/tx_pool.cpp b/src/cryptonote_core/tx_pool.cpp
index dba05a539..e72a592ca 100644
--- a/src/cryptonote_core/tx_pool.cpp
+++ b/src/cryptonote_core/tx_pool.cpp
@@ -133,12 +133,8 @@ namespace cryptonote
       fee = tx.rct_signatures.txnFee;
     }
 
-    uint64_t needed_fee = blob_size / 1024;
-    needed_fee += (blob_size % 1024) ? 1 : 0;
-    needed_fee *= FEE_PER_KB;
-    if (!kept_by_block && fee < needed_fee)
+    if (!kept_by_block && !m_blockchain.check_fee(blob_size, fee))
     {
-      LOG_PRINT_L1("transaction fee is not enough: " << print_money(fee) << ", minimum fee: " << print_money(needed_fee));
       tvc.m_verifivation_failed = true;
       tvc.m_fee_too_low = true;
       return false;