Improve prioritized transaction detection algorithm

backend/src/api/audit.ts

@@ -2,6 +2,7 @@ import config from '../config';
 import logger from '../logger';
 import { MempoolTransactionExtended, MempoolBlockWithTransactions } from '../mempool.interfaces';
 import rbfCache from './rbf-cache';
+import transactionUtils from './transaction-utils';
 
 const PROPAGATION_MARGIN = 180; // in seconds, time since a transaction is first seen after which it is assumed to have propagated to all miners
 
@@ -15,7 +16,8 @@ class Audit {
     const matches: string[] = []; // present in both mined block and template
     const added: string[] = []; // present in mined block, not in template
     const unseen: string[] = []; // present in the mined block, not in our mempool
-    const prioritized: string[] = []; // higher in the block than would be expected by in-band feerate alone
+    let prioritized: string[] = []; // higher in the block than would be expected by in-band feerate alone
+    let deprioritized: string[] = []; // lower in the block than would be expected by in-band feerate alone
     const fresh: string[] = []; // missing, but firstSeen or lastBoosted within PROPAGATION_MARGIN
     const rbf: string[] = []; // either missing or present, and either part of a full-rbf replacement, or a conflict with the mined block
     const accelerated: string[] = []; // prioritized by the mempool accelerator
@@ -133,23 +135,7 @@ class Audit {
       totalWeight += tx.weight;
     }
 
-
-    // identify "prioritized" transactions
-    let lastEffectiveRate = 0;
-    // Iterate over the mined template from bottom to top (excluding the coinbase)
-    // Transactions should appear in ascending order of mining priority.
-    for (let i = transactions.length - 1; i > 0; i--) {
-      const blockTx = transactions[i];
-      // If a tx has a lower in-band effective fee rate than the previous tx,
-      // it must have been prioritized out-of-band (in order to have a higher mining priority)
-      // so exclude from the analysis.
-      if ((blockTx.effectiveFeePerVsize || 0) < lastEffectiveRate) {
-        prioritized.push(blockTx.txid);
-        // accelerated txs may or may not have their prioritized fee rate applied, so don't use them as a reference
-      } else if (!isAccelerated[blockTx.txid]) {
-        lastEffectiveRate = blockTx.effectiveFeePerVsize || 0;
-      }
-    }
+    ({ prioritized, deprioritized } = transactionUtils.identifyPrioritizedTransactions(transactions, 'effectiveFeePerVsize'));
 
     // transactions missing from near the end of our template are probably not being censored
     let overflowWeightRemaining = overflowWeight - (config.MEMPOOL.BLOCK_WEIGHT_UNITS - totalWeight);

backend/src/api/transaction-utils.ts

@@ -338,6 +338,87 @@ class TransactionUtils {
     const positionOfScript = hasAnnex ? witness.length - 3 : witness.length - 2;
     return witness[positionOfScript];
   }
+
+  // calculate the most parsimonious set of prioritizations given a list of block transactions
+  // (i.e. the most likely prioritizations and deprioritizations)
+  public identifyPrioritizedTransactions(transactions: any[], rateKey: string): { prioritized: string[], deprioritized: string[] } {
+    // find the longest increasing subsequence of transactions
+    // (adapted from https://en.wikipedia.org/wiki/Longest_increasing_subsequence#Efficient_algorithms)
+    // should be O(n log n)
+    const X = transactions.slice(1).reverse().map((tx) => ({ txid: tx.txid, rate: tx[rateKey] })); // standard block order is by *decreasing* effective fee rate, but we want to iterate in increasing order (and skip the coinbase)
+    if (X.length < 2) {
+      return { prioritized: [], deprioritized: [] };
+    }
+    const N = X.length;
+    const P: number[] = new Array(N);
+    const M: number[] = new Array(N + 1);
+    M[0] = -1; // undefined so can be set to any value
+
+    let L = 0;
+    for (let i = 0; i < N; i++) {
+      // Binary search for the smallest positive l ≤ L
+      // such that X[M[l]].effectiveFeePerVsize > X[i].effectiveFeePerVsize
+      let lo = 1;
+      let hi = L + 1;
+      while (lo < hi) {
+        const mid = lo + Math.floor((hi - lo) / 2); // lo <= mid < hi
+        if (X[M[mid]].rate > X[i].rate) {
+          hi = mid;
+        } else { // if X[M[mid]].effectiveFeePerVsize < X[i].effectiveFeePerVsize
+          lo = mid + 1;
+        }
+      }
+
+      // After searching, lo == hi is 1 greater than the
+      // length of the longest prefix of X[i]
+      const newL = lo;
+
+      // The predecessor of X[i] is the last index of
+      // the subsequence of length newL-1
+      P[i] = M[newL - 1];
+      M[newL] = i;
+
+      if (newL > L) {
+        // If we found a subsequence longer than any we've
+        // found yet, update L
+        L = newL;
+      }
+    }
+
+    // Reconstruct the longest increasing subsequence
+    // It consists of the values of X at the L indices:
+    // ..., P[P[M[L]]], P[M[L]], M[L]
+    const LIS: any[] = new Array(L);
+    let k = M[L];
+    for (let j = L - 1; j >= 0; j--) {
+      LIS[j] = X[k];
+      k = P[k];
+    }
+
+    const lisMap = new Map<string, number>();
+    LIS.forEach((tx, index) => lisMap.set(tx.txid, index));
+
+    const prioritized: string[] = [];
+    const deprioritized: string[] = [];
+
+    let lastRate = X[0].rate;
+
+    for (const tx of X) {
+      if (lisMap.has(tx.txid)) {
+        lastRate = tx.rate;
+      } else {
+        if (Math.abs(tx.rate - lastRate) < 0.1) {
+          // skip if the rate is almost the same as the previous transaction
+        } else if (tx.rate <= lastRate) {
+          prioritized.push(tx.txid);
+        } else {
+          deprioritized.push(tx.txid);
+        }
+      }
+    }
+
+    return { prioritized, deprioritized };
+  }
 }
 
 export default new TransactionUtils();