Merge bitcoin/bitcoin#32799: mempool: use FeeFrac for ancestor/descendant score comparators

922adf6 mempool: use `FeeFrac` for calculating regular score (Sebastian Falbesoner)
3322b3a mempool: use `FeeFrac` for calculating ancestor score (Sebastian Falbesoner)
ac9c113 mempool: use `FeeFrac` for calculating descendant score (Sebastian Falbesoner)

Pull request description:

  Rather than determining fee-rates for the mempool index scores and comparators manually in a rather tedious way (even involving floating-points), use the `FeeFrac` class [1] to simplify and deduplicate the code. Note that though this is intended to be a refactoring PR, there might be subtle differences in behaviour due to floating-point arithmetic involved in the original code (to avoid overflows at the cost of precision loss), but these shouldn't matter.

  [1] introduced in PR #29242, commit ce8e225

ACKs for top commit:
  ismaelsadeeq:
    Code review ACK 922adf6
  glozow:
    ACK 922adf6

Tree-SHA512: 6c3a9436f2be668aa8561b40c1b93efa7dc97b4ef354e98233ac3d3286a88804668164a55f2fcce4239fee5830e4e70f520e6285b667b87baa65c7cec09159cf

Author: glozow

src/txmempool.h

@@ -93,36 +93,24 @@ class CompareTxMemPoolEntryByDescendantScore
 public:
     bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b) const
     {
-        double a_mod_fee, a_size, b_mod_fee, b_size;
+        FeeFrac f1 = GetModFeeAndSize(a);
+        FeeFrac f2 = GetModFeeAndSize(b);
 
-        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 = a_mod_fee * b_size;
-        double f2 = a_size * b_mod_fee;
-
-        if (f1 == f2) {
+        if (FeeRateCompare(f1, f2) == 0) {
             return a.GetTime() >= b.GetTime();
         }
         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
+    FeeFrac GetModFeeAndSize(const CTxMemPoolEntry &a) const
     {
         // Compare feerate with descendants to feerate of the transaction, and
         // return the fee/size for the max.
-        double f1 = (double)a.GetModifiedFee() * a.GetSizeWithDescendants();
-        double f2 = (double)a.GetModFeesWithDescendants() * a.GetTxSize();
-
-        if (f2 > f1) {
-            mod_fee = a.GetModFeesWithDescendants();
-            size = a.GetSizeWithDescendants();
-        } else {
-            mod_fee = a.GetModifiedFee();
-            size = a.GetTxSize();
-        }
+        return std::max<FeeFrac>(
+            FeeFrac(a.GetModFeesWithDescendants(), a.GetSizeWithDescendants()),
+            FeeFrac(a.GetModifiedFee(), a.GetTxSize())
+        );
     }
 };
 
@@ -138,9 +126,9 @@ class CompareTxMemPoolEntryByScore
 public:
     bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b) const
     {
-        double f1 = (double)a.GetFee() * b.GetTxSize();
-        double f2 = (double)b.GetFee() * a.GetTxSize();
-        if (f1 == f2) {
+        FeeFrac f1(a.GetFee(), a.GetTxSize());
+        FeeFrac f2(b.GetFee(), b.GetTxSize());
+        if (FeeRateCompare(f1, f2) == 0) {
             return b.GetTx().GetHash() < a.GetTx().GetHash();
         }
         return f1 > f2;
@@ -166,37 +154,25 @@ class CompareTxMemPoolEntryByAncestorFee
     template<typename T>
     bool operator()(const T& a, const T& b) const
     {
-        double a_mod_fee, a_size, b_mod_fee, b_size;
+        FeeFrac f1 = GetModFeeAndSize(a);
+        FeeFrac f2 = GetModFeeAndSize(b);
 
-        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 = a_mod_fee * b_size;
-        double f2 = a_size * b_mod_fee;
-
-        if (f1 == f2) {
+        if (FeeRateCompare(f1, f2) == 0) {
             return a.GetTx().GetHash() < b.GetTx().GetHash();
         }
         return f1 > f2;
     }
 
     // Return the fee/size we're using for sorting this entry.
     template <typename T>
-    void GetModFeeAndSize(const T &a, double &mod_fee, double &size) const
+    FeeFrac GetModFeeAndSize(const T &a) 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();
-        }
+        return std::min<FeeFrac>(
+            FeeFrac(a.GetModFeesWithAncestors(), a.GetSizeWithAncestors()),
+            FeeFrac(a.GetModifiedFee(), a.GetTxSize())
+        );
     }
 };