Mimic Core's ordering for equal-score transactions

backend/src/api/mempool.ts

@@ -89,6 +89,9 @@ class Mempool {
       if (this.mempoolCache[txid].sigops == null || this.mempoolCache[txid].effectiveFeePerVsize == null) {
         this.mempoolCache[txid] = transactionUtils.extendMempoolTransaction(this.mempoolCache[txid]);
       }
+      if (this.mempoolCache[txid].order == null) {
+        this.mempoolCache[txid].order = transactionUtils.txidToOrdering(txid);
+      }
       count++;
     }
     if (this.mempoolChangedCallback) {

backend/src/api/transaction-utils.ts

@@ -76,6 +76,7 @@ class TransactionUtils {
     const adjustedFeePerVsize = Math.max(Common.isLiquid() ? 0.1 : 1,
       (transaction.fee || 0) / adjustedVsize);
     const transactionExtended: MempoolTransactionExtended = Object.assign(transaction, {
+      order: this.txidToOrdering(transaction.txid),
       vsize: Math.round(transaction.weight / 4),
       adjustedVsize,
       sigops,
@@ -154,6 +155,11 @@ class TransactionUtils {
 
     return sigops;
   }
+
+  // returns the most significant 4 bytes of the txid as an integer
+  public txidToOrdering(txid: string): number {
+    return parseInt(txid.slice(56).match(/../g)?.reverse().join('') as string, 16);
+  }
 }
 
 export default new TransactionUtils();