Sort mempool by min(feerate, ancestor_feerate)

This more closely approximates the desirability of a given transaction for
mining.

src/txmempool.h

@@ -266,27 +266,46 @@ class CompareTxMemPoolEntryByEntryTime
     }
 };
 
+/** \class CompareTxMemPoolEntryByAncestorScore
+ *
+ *  Sort an entry by min(score/size of entry's tx, score/size with all ancestors).
+ */
 class CompareTxMemPoolEntryByAncestorFee
 {
 public:
     bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b) const
     {
-        double aFees = a.GetModFeesWithAncestors();
-        double aSize = a.GetSizeWithAncestors();
+        double a_mod_fee, a_size, b_mod_fee, b_size;
 
-        double bFees = b.GetModFeesWithAncestors();
-        double bSize = b.GetSizeWithAncestors();
+        GetModFeeAndSize(a, a_mod_fee, a_size);
+        GetModFeeAndSize(b, b_mod_fee, b_size);
 
         // Avoid division by rewriting (a/b > c/d) as (a*d > c*b).
-        double f1 = aFees * bSize;
-        double f2 = aSize * bFees;
+        double f1 = a_mod_fee * b_size;
+        double f2 = a_size * b_mod_fee;
 
         if (f1 == f2) {
             return a.GetTx().GetHash() < b.GetTx().GetHash();
         }
-
         return f1 > f2;
     }
+
+    // Return the fee/size we're using for sorting this entry.
+    void GetModFeeAndSize(const CTxMemPoolEntry &a, double &mod_fee, double &size) const
+    {
+        // Compare feerate with ancestors to feerate of the transaction, and
+        // return the fee/size for the min.
+        double f1 = (double)a.GetModifiedFee() * a.GetSizeWithAncestors();
+        double f2 = (double)a.GetModFeesWithAncestors() * a.GetTxSize();
+
+        if (f1 > f2) {
+            mod_fee = a.GetModFeesWithAncestors();
+            size = a.GetSizeWithAncestors();
+        } else {
+            mod_fee = a.GetModifiedFee();
+            size = a.GetTxSize();
+        }
+    }
 };
 
 // Multi_index tag names