wallet2: extend fake out picks away from the gamma pick

in order to unbias selection from blocks with few txes

1 files changed, 23 insertions, 2 deletions

diff --git a/src/wallet/wallet2.cpp b/src/wallet/wallet2.cpp
index 6b3a8533e..c5618375e 100644
--- a/src/wallet/wallet2.cpp
+++ b/src/wallet/wallet2.cpp
@@ -123,6 +123,8 @@ using namespace cryptonote;
 
 #define FIRST_REFRESH_GRANULARITY     1024
 
+#define GAMMA_PICK_HALF_WINDOW 5
+
 static const std::string MULTISIG_SIGNATURE_MAGIC = "SigMultisigPkV1";
 static const std::string MULTISIG_EXTRA_INFO_MAGIC = "MultisigxV1";
 
@@ -6795,10 +6797,29 @@ void wallet2::get_outs(std::vector<std::vector<tools::wallet2::get_outs_entry>>
           error::get_output_distribution, "Decreasing offsets in rct distribution: " +
           std::to_string(block_offset) + ": " + std::to_string(rct_offsets[block_offset]) + ", " +
           std::to_string(block_offset + 1) + ": " + std::to_string(rct_offsets[block_offset + 1]));
-      uint64_t n_rct = rct_offsets[block_offset + 1] - rct_offsets[block_offset];
+      uint64_t first_block_offset = block_offset, last_block_offset = block_offset;
+      for (size_t half_window = 0; half_window < GAMMA_PICK_HALF_WINDOW; ++half_window)
+      {
+        // end when we have a non empty block
+        uint64_t cum0 = first_block_offset > 0 ? rct_offsets[first_block_offset] - rct_offsets[first_block_offset - 1] : rct_offsets[0];
+        if (cum0 > 1)
+          break;
+        uint64_t cum1 = last_block_offset > 0 ? rct_offsets[last_block_offset] - rct_offsets[last_block_offset - 1] : rct_offsets[0];
+        if (cum1 > 1)
+          break;
+        if (first_block_offset == 0 && last_block_offset >= rct_offsets.size() - 2)
+          break;
+        // expand up to bounds
+        if (first_block_offset > 0)
+          --first_block_offset;
+        if (last_block_offset < rct_offsets.size() - 1)
+          ++last_block_offset;
+      }
+      const uint64_t n_rct = rct_offsets[last_block_offset] - (first_block_offset == 0 ? 0 : rct_offsets[first_block_offset - 1]);
       if (n_rct == 0)
         return rct_offsets[block_offset] ? rct_offsets[block_offset] - 1 : 0;
-      return rct_offsets[block_offset] + crypto::rand<uint64_t>() % n_rct;
+      MDEBUG("Picking 1/" << n_rct << " in " << (last_block_offset - first_block_offset + 1) << " blocks centered around " << block_offset);
+      return rct_offsets[first_block_offset] + crypto::rand<uint64_t>() % n_rct;
     };
 
     size_t num_selected_transfers = 0;