4 files changed, 46 insertions, 1 deletions
diff --git a/src/wallet/api/wallet.cpp b/src/wallet/api/wallet.cpp
index 3a2074cc4..0afbda705 100644
--- a/src/wallet/api/wallet.cpp
+++ b/src/wallet/api/wallet.cpp
@@ -2365,6 +2365,11 @@ bool WalletImpl::parse_uri(const std::string &uri, std::string &address, std::st
     return m_wallet->parse_uri(uri, address, payment_id, amount, tx_description, recipient_name, unknown_parameters, error);
 }
 
+std::string WalletImpl::make_uri(const std::string &address, const std::string &payment_id, uint64_t amount, const std::string &tx_description, const std::string &recipient_name, std::string &error) const
+{
+    return m_wallet->make_uri(address, payment_id, amount, tx_description, recipient_name, error);
+}
+
 std::string WalletImpl::getDefaultDataDir() const
 {
  return tools::get_default_data_dir();
diff --git a/src/wallet/api/wallet.h b/src/wallet/api/wallet.h
index 011a94ec4..67fc2c08a 100644
--- a/src/wallet/api/wallet.h
+++ b/src/wallet/api/wallet.h
@@ -205,6 +205,7 @@ public:
     virtual void startRefresh() override;
     virtual void pauseRefresh() override;
     virtual bool parse_uri(const std::string &uri, std::string &address, std::string &payment_id, uint64_t &amount, std::string &tx_description, std::string &recipient_name, std::vector<std::string> &unknown_parameters, std::string &error) override;
+    virtual std::string make_uri(const std::string &address, const std::string &payment_id, uint64_t amount, const std::string &tx_description, const std::string &recipient_name, std::string &error) const override;
     virtual std::string getDefaultDataDir() const override;
     virtual bool lightWalletLogin(bool &isNewWallet) const override;
     virtual bool lightWalletImportWalletRequest(std::string &payment_id, uint64_t &fee, bool &new_request, bool &request_fulfilled, std::string &payment_address, std::string &status) override;
diff --git a/src/wallet/api/wallet2_api.h b/src/wallet/api/wallet2_api.h
index ed8c55d3b..f9c421a93 100644
--- a/src/wallet/api/wallet2_api.h
+++ b/src/wallet/api/wallet2_api.h
@@ -1020,6 +1020,7 @@ struct Wallet
     virtual bool verifyMessageWithPublicKey(const std::string &message, const std::string &publicKey, const std::string &signature) const = 0;
 
     virtual bool parse_uri(const std::string &uri, std::string &address, std::string &payment_id, uint64_t &amount, std::string &tx_description, std::string &recipient_name, std::vector<std::string> &unknown_parameters, std::string &error) = 0;
+    virtual std::string make_uri(const std::string &address, const std::string &payment_id, uint64_t amount, const std::string &tx_description, const std::string &recipient_name, std::string &error) const = 0;
 
     virtual std::string getDefaultDataDir() const = 0;
    
diff --git a/src/wallet/wallet2.cpp b/src/wallet/wallet2.cpp
index 217a22027..6e8d585dc 100644
--- a/src/wallet/wallet2.cpp
+++ b/src/wallet/wallet2.cpp
@@ -144,6 +144,9 @@ using namespace cryptonote;
 
 #define IGNORE_LONG_PAYMENT_ID_FROM_BLOCK_VERSION 12
 
+#define DEFAULT_UNLOCK_TIME (CRYPTONOTE_DEFAULT_TX_SPENDABLE_AGE * DIFFICULTY_TARGET_V2)
+#define RECENT_SPEND_WINDOW (50 * DIFFICULTY_TARGET_V2)
+
 static const std::string MULTISIG_SIGNATURE_MAGIC = "SigMultisigPkV1";
 static const std::string MULTISIG_EXTRA_INFO_MAGIC = "MultisigxV1";
 
@@ -1019,7 +1022,13 @@ gamma_picker::gamma_picker(const std::vector<uint64_t> &rct_offsets, double shap
   end = rct_offsets.data() + rct_offsets.size() - CRYPTONOTE_DEFAULT_TX_SPENDABLE_AGE;
   num_rct_outputs = *(end - 1);
   THROW_WALLET_EXCEPTION_IF(num_rct_outputs == 0, error::wallet_internal_error, "No rct outputs");
+  THROW_WALLET_EXCEPTION_IF(outputs_to_consider == 0, error::wallet_internal_error, "No rct outputs to consider");
   average_output_time = DIFFICULTY_TARGET_V2 * blocks_to_consider / outputs_to_consider; // this assumes constant target over the whole rct range
+  if (average_output_time == 0) {
+     // TODO: apply this to all cases; do so alongside a hard fork, where all clients will update at the same time, preventing anonymity puddle formation
+    average_output_time = DIFFICULTY_TARGET_V2 * blocks_to_consider / static_cast<double>(outputs_to_consider);
+  }
+  THROW_WALLET_EXCEPTION_IF(average_output_time == 0, error::wallet_internal_error, "Average seconds per output cannot be 0.");
 };
 
 gamma_picker::gamma_picker(const std::vector<uint64_t> &rct_offsets): gamma_picker(rct_offsets, GAMMA_SHAPE, GAMMA_SCALE) {}
@@ -1028,6 +1037,34 @@ uint64_t gamma_picker::pick()
 {
   double x = gamma(engine);
   x = exp(x);
+
+  if (x > DEFAULT_UNLOCK_TIME) 
+  {
+    // We are trying to select an output from the chain that appeared 'x' seconds before the
+    // current chain tip, where 'x' is selected from the gamma distribution recommended in Miller et al.
+    // (https://arxiv.org/pdf/1704.04299/).
+    // Our method is to get the average time delta between outputs in the recent past, estimate the number of
+    // outputs 'n' that would have appeared between 'chain_tip - x' and 'chain_tip', select the real output at
+    // 'current_num_outputs - n', then randomly select an output from the block where that output appears.
+    // Source code to paper: https://github.com/maltemoeser/moneropaper
+    //
+    // Due to the 'default spendable age' mechanic in Monero, 'current_num_outputs' only contains
+    // currently *unlocked* outputs, which means the earliest output that can be selected is not at the chain tip!
+    // Therefore, we must offset 'x' so it matches up with the timing of the outputs being considered. We do
+    // this by saying if 'x` equals the expected age of the first unlocked output (compared to the current
+    // chain tip - i.e. DEFAULT_UNLOCK_TIME), then select the first unlocked output.
+    x -= DEFAULT_UNLOCK_TIME;
+  }
+  else 
+  {
+    // If the spent time suggested by the gamma is less than the unlock time, that means the gamma is suggesting an output
+    // that is no longer feasible to be spent (possible since the gamma was constructed when consensus rules did not enforce the
+    // lock time). The assumption made in this code is that an output expected spent quicker than the unlock time would likely
+    // be spent within RECENT_SPEND_WINDOW after allowed. So it returns an output that falls between 0 and the RECENT_SPEND_WINDOW.
+    // The RECENT_SPEND_WINDOW was determined with empirical analysis of observed data.
+    x = crypto::rand_idx(static_cast<uint64_t>(RECENT_SPEND_WINDOW));
+  }
+
   uint64_t output_index = x / average_output_time;
   if (output_index >= num_rct_outputs)
     return std::numeric_limits<uint64_t>::max(); // bad pick
@@ -8659,7 +8696,8 @@ void wallet2::get_outs(std::vector<std::vector<tools::wallet2::get_outs_entry>>
         if (req.outputs[i].index == td.m_global_output_index)
           if (daemon_resp.outs[i].key == boost::get<txout_to_key>(td.m_tx.vout[td.m_internal_output_index].target).key)
             if (daemon_resp.outs[i].mask == mask)
-              real_out_found = true;
+              if (daemon_resp.outs[i].unlocked)
+                real_out_found = true;
       }
       THROW_WALLET_EXCEPTION_IF(!real_out_found, error::wallet_internal_error,
           "Daemon response did not include the requested real output");