Merge #13918: rpc: Replace median fee rate with feerate percentiles in getblockstats

4b7091a842 Replace median fee rate with feerate percentiles (Marcin Jachymiak)

Pull request description:

  Currently,  the `medianfeerate` statistic is calculated from the feerate of the middle transaction of a list of transactions sorted by feerate.

  This PR instead uses the value of the 50th percentile weight unit in the block, and also calculates the feerate at the 10th, 25th, 75th, and 90th percentiles.  This more accurately corresponds with what is generally meant by median feerate.

Tree-SHA512: 59255e243df90d7afbe69839408c58c9723884b8ab82c66dc24a769e89c6d539db1905374a3f025ff28272fb25a0b90e92d8101103e39a6d9c0d60423a596714

2 files changed, 60 insertions, 8 deletions

diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index 8bf6030833..51bc218d39 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -1640,6 +1640,35 @@ static T CalculateTruncatedMedian(std::vector<T>& scores)
     }
 }
 
+void CalculatePercentilesByWeight(CAmount result[NUM_GETBLOCKSTATS_PERCENTILES], std::vector<std::pair<CAmount, int64_t>>& scores, int64_t total_weight)
+{
+    if (scores.empty()) {
+        return;
+    }
+
+    std::sort(scores.begin(), scores.end());
+
+    // 10th, 25th, 50th, 75th, and 90th percentile weight units.
+    const double weights[NUM_GETBLOCKSTATS_PERCENTILES] = {
+        total_weight / 10.0, total_weight / 4.0, total_weight / 2.0, (total_weight * 3.0) / 4.0, (total_weight * 9.0) / 10.0
+    };
+
+    int64_t next_percentile_index = 0;
+    int64_t cumulative_weight = 0;
+    for (const auto& element : scores) {
+        cumulative_weight += element.second;
+        while (next_percentile_index < NUM_GETBLOCKSTATS_PERCENTILES && cumulative_weight >= weights[next_percentile_index]) {
+            result[next_percentile_index] = element.first;
+            ++next_percentile_index;
+        }
+    }
+
+    // Fill any remaining percentiles with the last value.
+    for (int64_t i = next_percentile_index; i < NUM_GETBLOCKSTATS_PERCENTILES; i++) {
+        result[i] = scores.back().first;
+    }
+}
+
 template<typename T>
 static inline bool SetHasKeys(const std::set<T>& set) {return false;}
 template<typename T, typename Tk, typename... Args>
@@ -1673,13 +1702,19 @@ static UniValue getblockstats(const JSONRPCRequest& request)
             "  \"avgfeerate\": xxxxx,      (numeric) Average feerate (in satoshis per virtual byte)\n"
             "  \"avgtxsize\": xxxxx,       (numeric) Average transaction size\n"
             "  \"blockhash\": xxxxx,       (string) The block hash (to check for potential reorgs)\n"
+            "  \"feerate_percentiles\": [  (array of numeric) Feerates at the 10th, 25th, 50th, 75th, and 90th percentile weight unit (in satoshis per virtual byte)\n"
+            "      \"10th_percentile_feerate\",      (numeric) The 10th percentile feerate\n"
+            "      \"25th_percentile_feerate\",      (numeric) The 25th percentile feerate\n"
+            "      \"50th_percentile_feerate\",      (numeric) The 50th percentile feerate\n"
+            "      \"75th_percentile_feerate\",      (numeric) The 75th percentile feerate\n"
+            "      \"90th_percentile_feerate\",      (numeric) The 90th percentile feerate\n"
+            "  ],\n"
             "  \"height\": xxxxx,          (numeric) The height of the block\n"
             "  \"ins\": xxxxx,             (numeric) The number of inputs (excluding coinbase)\n"
             "  \"maxfee\": xxxxx,          (numeric) Maximum fee in the block\n"
             "  \"maxfeerate\": xxxxx,      (numeric) Maximum feerate (in satoshis per virtual byte)\n"
             "  \"maxtxsize\": xxxxx,       (numeric) Maximum transaction size\n"
             "  \"medianfee\": xxxxx,       (numeric) Truncated median fee in the block\n"
-            "  \"medianfeerate\": xxxxx,   (numeric) Truncated median feerate (in satoshis per virtual byte)\n"
             "  \"mediantime\": xxxxx,      (numeric) The block median time past\n"
             "  \"mediantxsize\": xxxxx,    (numeric) Truncated median transaction size\n"
             "  \"minfee\": xxxxx,          (numeric) Minimum fee in the block\n"
@@ -1747,13 +1782,13 @@ static UniValue getblockstats(const JSONRPCRequest& request)
     const bool do_all = stats.size() == 0; // Calculate everything if nothing selected (default)
     const bool do_mediantxsize = do_all || stats.count("mediantxsize") != 0;
     const bool do_medianfee = do_all || stats.count("medianfee") != 0;
-    const bool do_medianfeerate = do_all || stats.count("medianfeerate") != 0;
-    const bool loop_inputs = do_all || do_medianfee || do_medianfeerate ||
+    const bool do_feerate_percentiles = do_all || stats.count("feerate_percentiles") != 0;
+    const bool loop_inputs = do_all || do_medianfee || do_feerate_percentiles ||
         SetHasKeys(stats, "utxo_size_inc", "totalfee", "avgfee", "avgfeerate", "minfee", "maxfee", "minfeerate", "maxfeerate");
     const bool loop_outputs = do_all || loop_inputs || stats.count("total_out");
     const bool do_calculate_size = do_mediantxsize ||
         SetHasKeys(stats, "total_size", "avgtxsize", "mintxsize", "maxtxsize", "swtotal_size");
-    const bool do_calculate_weight = do_all || SetHasKeys(stats, "total_weight", "avgfeerate", "swtotal_weight", "avgfeerate", "medianfeerate", "minfeerate", "maxfeerate");
+    const bool do_calculate_weight = do_all || SetHasKeys(stats, "total_weight", "avgfeerate", "swtotal_weight", "avgfeerate", "feerate_percentiles", "minfeerate", "maxfeerate");
     const bool do_calculate_sw = do_all || SetHasKeys(stats, "swtxs", "swtotal_size", "swtotal_weight");
 
     CAmount maxfee = 0;
@@ -1773,7 +1808,7 @@ static UniValue getblockstats(const JSONRPCRequest& request)
     int64_t total_weight = 0;
     int64_t utxo_size_inc = 0;
     std::vector<CAmount> fee_array;
-    std::vector<CAmount> feerate_array;
+    std::vector<std::pair<CAmount, int64_t>> feerate_array;
     std::vector<int64_t> txsize_array;
 
     for (const auto& tx : block.vtx) {
@@ -1848,26 +1883,34 @@ static UniValue getblockstats(const JSONRPCRequest& request)
 
             // New feerate uses satoshis per virtual byte instead of per serialized byte
             CAmount feerate = weight ? (txfee * WITNESS_SCALE_FACTOR) / weight : 0;
-            if (do_medianfeerate) {
-                feerate_array.push_back(feerate);
+            if (do_feerate_percentiles) {
+                feerate_array.emplace_back(std::make_pair(feerate, weight));
             }
             maxfeerate = std::max(maxfeerate, feerate);
             minfeerate = std::min(minfeerate, feerate);
         }
     }
 
+    CAmount feerate_percentiles[NUM_GETBLOCKSTATS_PERCENTILES] = { 0 };
+    CalculatePercentilesByWeight(feerate_percentiles, feerate_array, total_weight);
+
+    UniValue feerates_res(UniValue::VARR);
+    for (int64_t i = 0; i < NUM_GETBLOCKSTATS_PERCENTILES; i++) {
+        feerates_res.push_back(feerate_percentiles[i]);
+    }
+
     UniValue ret_all(UniValue::VOBJ);
     ret_all.pushKV("avgfee", (block.vtx.size() > 1) ? totalfee / (block.vtx.size() - 1) : 0);
     ret_all.pushKV("avgfeerate", total_weight ? (totalfee * WITNESS_SCALE_FACTOR) / total_weight : 0); // Unit: sat/vbyte
     ret_all.pushKV("avgtxsize", (block.vtx.size() > 1) ? total_size / (block.vtx.size() - 1) : 0);
     ret_all.pushKV("blockhash", pindex->GetBlockHash().GetHex());
+    ret_all.pushKV("feerate_percentiles", feerates_res);
     ret_all.pushKV("height", (int64_t)pindex->nHeight);
     ret_all.pushKV("ins", inputs);
     ret_all.pushKV("maxfee", maxfee);
     ret_all.pushKV("maxfeerate", maxfeerate);
     ret_all.pushKV("maxtxsize", maxtxsize);
     ret_all.pushKV("medianfee", CalculateTruncatedMedian(fee_array));
-    ret_all.pushKV("medianfeerate", CalculateTruncatedMedian(feerate_array));
     ret_all.pushKV("mediantime", pindex->GetMedianTimePast());
     ret_all.pushKV("mediantxsize", CalculateTruncatedMedian(txsize_array));
     ret_all.pushKV("minfee", (minfee == MAX_MONEY) ? 0 : minfee);
diff --git a/src/rpc/blockchain.h b/src/rpc/blockchain.h
index c664139ed3..544bc62c36 100644
--- a/src/rpc/blockchain.h
+++ b/src/rpc/blockchain.h
@@ -5,10 +5,16 @@
 #ifndef BITCOIN_RPC_BLOCKCHAIN_H
 #define BITCOIN_RPC_BLOCKCHAIN_H
 
+#include <vector>
+#include <stdint.h>
+#include <amount.h>
+
 class CBlock;
 class CBlockIndex;
 class UniValue;
 
+static constexpr int NUM_GETBLOCKSTATS_PERCENTILES = 5;
+
 /**
  * Get the difficulty of the net wrt to the given block index, or the chain tip if
  * not provided.
@@ -33,4 +39,7 @@ UniValue mempoolToJSON(bool fVerbose = false);
 /** Block header to JSON */
 UniValue blockheaderToJSON(const CBlockIndex* blockindex);
 
+/** Used by getblockstats to get feerates at different percentiles by weight  */
+void CalculatePercentilesByWeight(CAmount result[NUM_GETBLOCKSTATS_PERCENTILES], std::vector<std::pair<CAmount, int64_t>>& scores, int64_t total_weight);
+
 #endif