Wallet2: calculate approximate blockchain height on offline creation

Wallet API: add approximateBlockChainHeight()

src/wallet/api/wallet.cpp

@@ -469,7 +469,10 @@ uint64_t WalletImpl::blockChainHeight() const
 {
     return m_wallet->get_blockchain_current_height();
 }
-
+uint64_t WalletImpl::approximateBlockChainHeight() const
+{
+    return m_wallet->get_approximate_blockchain_height();
+}
 uint64_t WalletImpl::daemonBlockChainHeight() const
 {
     std::string err;

src/wallet/api/wallet.h

@@ -76,6 +76,7 @@ public:
     uint64_t balance() const;
     uint64_t unlockedBalance() const;
     uint64_t blockChainHeight() const;
+    uint64_t approximateBlockChainHeight() const;
     uint64_t daemonBlockChainHeight() const;
     uint64_t daemonBlockChainTargetHeight() const;
     bool synchronized() const;

src/wallet/wallet2.cpp

@@ -2004,6 +2004,19 @@ crypto::secret_key wallet2::generate(const std::string& wallet_, const std::stri
   m_account_public_address = m_account.get_keys().m_account_address;
   m_watch_only = false;
 
+  if(m_refresh_from_block_height == 0 && !recover){
+    // Wallets created offline don't know blockchain height.
+    // Set blockchain height calculated from current date/time
+    // -1 month for fluctuations in block time and machine date/time setup.
+    // avg seconds per block
+    const int seconds_per_block = DIFFICULTY_TARGET_V2;
+    // ~num blocks per month
+    const uint64_t blocks_per_month = 60*60*24*30/seconds_per_block;
+    uint64_t approx_blockchain_height = get_approximate_blockchain_height();
+    if(approx_blockchain_height > 0) {
+      m_refresh_from_block_height = approx_blockchain_height - blocks_per_month;
+    }
+  }
   bool r = store_keys(m_keys_file, password, false);
   THROW_WALLET_EXCEPTION_IF(!r, error::file_save_error, m_keys_file);
 
@@ -4578,6 +4591,21 @@ uint64_t wallet2::get_daemon_blockchain_target_height(string &err)
   return resp_t.result.target_height;
 }
 
+uint64_t wallet2::get_approximate_blockchain_height() const
+{
+  if (m_testnet) return 0;
+  // time of v2 fork
+  const time_t fork_time = 1458748658;
+  // v2 fork block
+  const uint64_t fork_block = 1009827;
+  // avg seconds per block
+  const int seconds_per_block = DIFFICULTY_TARGET_V2;
+  // Calculated blockchain height
+  uint64_t approx_blockchain_height = fork_block + (time(NULL) - fork_time)/seconds_per_block;
+  LOG_PRINT_L2("Calculated blockchain height: " << approx_blockchain_height);
+  return approx_blockchain_height;
+}
+
 void wallet2::set_tx_note(const crypto::hash &txid, const std::string &note)
 {
   m_tx_notes[txid] = note;

src/wallet/wallet2.h

@@ -492,7 +492,10 @@ namespace tools
     std::string get_daemon_address() const;
     uint64_t get_daemon_blockchain_height(std::string& err);
     uint64_t get_daemon_blockchain_target_height(std::string& err);
-
+   /*!
+    * \brief Calculates the approximate blockchain height from current date/time.
+    */
+    uint64_t get_approximate_blockchain_height() const;
     std::vector<size_t> select_available_outputs_from_histogram(uint64_t count, bool atleast, bool unlocked, bool trusted_daemon);
     std::vector<size_t> select_available_outputs(const std::function<bool(const transfer_details &td)> &f);
     std::vector<size_t> select_available_unmixable_outputs(bool trusted_daemon);

src/wallet/wallet2_api.h

@@ -281,6 +281,12 @@ struct Wallet
      */
     virtual uint64_t blockChainHeight() const = 0;
 
+    /**
+    * @brief approximateBlockChainHeight - returns approximate blockchain height calculated from date/time
+    * @return
+    */
+    virtual uint64_t approximateBlockChainHeight() const = 0;
+
     /**
      * @brief daemonBlockChainHeight - returns daemon blockchain height
      * @return 0 - in case error communicating with the daemon.