Merge bitcoin/bitcoin#21161: Fee estimation: extend bucket ranges consistently

a5e39d3 Fee estimation: extend bucket ranges consistently (Anthony Towns)

Pull request description:

  When calculating a median fee for a confirmation target at a particular threshold, we analyse buckets in ranges rather than individually in case some buckets have very little data. This patch ensures the breaks between ranges are independent of the the confirmation target.

  Fixes #20725

ACKs for top commit:
  ismaelsadeeq:
    Code review ACK a5e39d3
  glozow:
    btw what I meant by [this](bitcoin/bitcoin#21161 (review)) was ACK a5e39d3
  jonatack:
    Initial ACK a5e39d3

Tree-SHA512: 0edf4e56717c4ab8d4ab0bc0f1d7ab36a13b99de12f689e55c9142c6b81691367ffd8df2e8260c5e14335310b1a51770c6c22995db31109976239befcb558ef8

Author: glozow

src/policy/fees.cpp

@@ -260,6 +260,11 @@ double TxConfirmStats::EstimateMedianVal(int confTarget, double sufficientTxVal,
     unsigned int curFarBucket = maxbucketindex;
     unsigned int bestFarBucket = maxbucketindex;
 
+    // We'll always group buckets into sets that meet sufficientTxVal --
+    // this ensures that we're using consistent groups between different
+    // confirmation targets.
+    double partialNum = 0;
+
     bool foundAnswer = false;
     unsigned int bins = unconfTxs.size();
     bool newBucketRange = true;
@@ -275,6 +280,7 @@ double TxConfirmStats::EstimateMedianVal(int confTarget, double sufficientTxVal,
         }
         curFarBucket = bucket;
         nConf += confAvg[periodTarget - 1][bucket];
+        partialNum += txCtAvg[bucket];
         totalNum += txCtAvg[bucket];
         failNum += failAvg[periodTarget - 1][bucket];
         for (unsigned int confct = confTarget; confct < GetMaxConfirms(); confct++)
@@ -284,7 +290,14 @@ double TxConfirmStats::EstimateMedianVal(int confTarget, double sufficientTxVal,
         // we can test for success
         // (Only count the confirmed data points, so that each confirmation count
         // will be looking at the same amount of data and same bucket breaks)
-        if (totalNum >= sufficientTxVal / (1 - decay)) {
+
+        if (partialNum < sufficientTxVal / (1 - decay)) {
+            // the buckets we've added in this round aren't sufficient
+            // so keep adding
+            continue;
+        } else {
+            partialNum = 0; // reset for the next range we'll add
+
             double curPct = nConf / (totalNum + failNum + extraNum);
 
             // Check to see if we are no longer getting confirmed at the success rate