Replace median fee rate with feerate percentiles

Removes medianfeerate result from getblockstats.
Adds feerate_percentiles which give the feerate of the 10th, 25th, 50th,
75th, and 90th percentile weight unit in the block.

3 files changed, 140 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
diff --git a/src/test/rpc_tests.cpp b/src/test/rpc_tests.cpp
index d0f6fba78d..a49796d6f4 100644
--- a/src/test/rpc_tests.cpp
+++ b/src/test/rpc_tests.cpp
@@ -16,6 +16,8 @@
 
 #include <univalue.h>
 
+#include <rpc/blockchain.h>
+
 UniValue CallRPC(std::string args)
 {
     std::vector<std::string> vArgs;
@@ -336,4 +338,82 @@ BOOST_AUTO_TEST_CASE(rpc_convert_values_generatetoaddress)
     BOOST_CHECK_EQUAL(result[2].get_int(), 9);
 }
 
+BOOST_AUTO_TEST_CASE(rpc_getblockstats_calculate_percentiles_by_weight)
+{
+    int64_t total_weight = 200;
+    std::vector<std::pair<CAmount, int64_t>> feerates;
+    CAmount result[NUM_GETBLOCKSTATS_PERCENTILES] = { 0 };
+
+    for (int64_t i = 0; i < 100; i++) {
+        feerates.emplace_back(std::make_pair(1 ,1));
+    }
+
+    for (int64_t i = 0; i < 100; i++) {
+        feerates.emplace_back(std::make_pair(2 ,1));
+    }
+
+    CalculatePercentilesByWeight(result, feerates, total_weight);
+    BOOST_CHECK_EQUAL(result[0], 1);
+    BOOST_CHECK_EQUAL(result[1], 1);
+    BOOST_CHECK_EQUAL(result[2], 1);
+    BOOST_CHECK_EQUAL(result[3], 2);
+    BOOST_CHECK_EQUAL(result[4], 2);
+
+    // Test with more pairs, and two pairs overlapping 2 percentiles.
+    total_weight = 100;
+    CAmount result2[NUM_GETBLOCKSTATS_PERCENTILES] = { 0 };
+    feerates.clear();
+
+    feerates.emplace_back(std::make_pair(1, 9));
+    feerates.emplace_back(std::make_pair(2 , 16)); //10th + 25th percentile
+    feerates.emplace_back(std::make_pair(4 ,50)); //50th + 75th percentile
+    feerates.emplace_back(std::make_pair(5 ,10));
+    feerates.emplace_back(std::make_pair(9 ,15));  // 90th percentile
+
+    CalculatePercentilesByWeight(result2, feerates, total_weight);
+
+    BOOST_CHECK_EQUAL(result2[0], 2);
+    BOOST_CHECK_EQUAL(result2[1], 2);
+    BOOST_CHECK_EQUAL(result2[2], 4);
+    BOOST_CHECK_EQUAL(result2[3], 4);
+    BOOST_CHECK_EQUAL(result2[4], 9);
+
+    // Same test as above, but one of the percentile-overlapping pairs is split in 2.
+    total_weight = 100;
+    CAmount result3[NUM_GETBLOCKSTATS_PERCENTILES] = { 0 };
+    feerates.clear();
+
+    feerates.emplace_back(std::make_pair(1, 9));
+    feerates.emplace_back(std::make_pair(2 , 11)); // 10th percentile
+    feerates.emplace_back(std::make_pair(2 , 5)); // 25th percentile
+    feerates.emplace_back(std::make_pair(4 ,50)); //50th + 75th percentile
+    feerates.emplace_back(std::make_pair(5 ,10));
+    feerates.emplace_back(std::make_pair(9 ,15)); // 90th percentile
+
+    CalculatePercentilesByWeight(result3, feerates, total_weight);
+
+    BOOST_CHECK_EQUAL(result3[0], 2);
+    BOOST_CHECK_EQUAL(result3[1], 2);
+    BOOST_CHECK_EQUAL(result3[2], 4);
+    BOOST_CHECK_EQUAL(result3[3], 4);
+    BOOST_CHECK_EQUAL(result3[4], 9);
+
+    // Test with one transaction spanning all percentiles.
+    total_weight = 104;
+    CAmount result4[NUM_GETBLOCKSTATS_PERCENTILES] = { 0 };
+    feerates.clear();
+
+    feerates.emplace_back(std::make_pair(1, 100));
+    feerates.emplace_back(std::make_pair(2, 1));
+    feerates.emplace_back(std::make_pair(3, 1));
+    feerates.emplace_back(std::make_pair(3, 1));
+    feerates.emplace_back(std::make_pair(999999, 1));
+
+    CalculatePercentilesByWeight(result4, feerates, total_weight);
+
+    for (int64_t i = 0; i < NUM_GETBLOCKSTATS_PERCENTILES; i++) {
+        BOOST_CHECK_EQUAL(result4[i], 1);
+    }
+}
+
 BOOST_AUTO_TEST_SUITE_END()