Implement `GetUnpaidActionCount` and `GetWeightRatio` for ZIP 317.

Signed-off-by: Daira Emma Hopwood <daira@jacaranda.org>

src/Makefile.am

@@ -244,6 +244,7 @@ BITCOIN_CORE_H = \
   httprpc.h \
   httpserver.h \
   init.h \
+  int128.h \
   key.h \
   key_constants.h \
   key_io.h \

src/int128.h

@@ -0,0 +1,25 @@
+// Copyright (c) 2023 The Zcash developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://www.opensource.org/licenses/mit-license.php .
+
+#ifndef ZCASH_INT128_H
+#define ZCASH_INT128_H
+
+#include <cstdint>
+
+// <cstdint> will define INT128_MAX iff (in some future world) it provides int128_t.
+// Otherwise use the __int128 extension which is supported in clang and gcc.
+#ifndef INT128_MAX
+typedef __int128 int128_t;
+#define INT128_MAX (std::numeric_limits<int128_t>::max())
+#define INT128_MIN (std::numeric_limits<int128_t>::min())
+#endif
+
+// <cstdint> will define UINT128_MAX iff (in some future world) it provides uint128_t.
+// Otherwise use the __uint128_t extension which is supported in clang and gcc.
+#ifndef UINT128_MAX
+typedef __uint128_t uint128_t;
+#define UINT128_MAX (std::numeric_limits<uint128_t>::max())
+#endif
+
+#endif // ZCASH_INT128_H

src/txmempool.cpp

@@ -17,6 +17,8 @@
 #include "util/moneystr.h"
 #include "validationinterface.h"
 #include "version.h"
+#include "mempool_limit.h"
+#include "zip317.h"
 
 #include <optional>
 
@@ -298,6 +300,27 @@ void CTxMemPoolEntry::SetDirty()
     nModFeesWithDescendants = GetModifiedFee();
 }
 
+size_t CTxMemPoolEntry::GetUnpaidActionCount() const
+{
+    if (tx->IsCoinBase()) {
+        return 0;
+    } else {
+        return std::max(
+            int64_t {0},
+            (int64_t) std::max(GRACE_ACTIONS, tx->GetLogicalActionCount()) - (GetModifiedFee() / MARGINAL_FEE));
+    }
+}
+
+// Return a fixed-point representation of the entry's weight ratio, where 1 is represented by WEIGHT_RATIO_SCALE.
+int128_t CTxMemPoolEntry::GetWeightRatio() const
+{
+    // ensure that the result will always be nonzero
+    static_assert(WEIGHT_RATIO_SCALE > MAX_MONEY);
+    return std::min(
+        (int128_t {WEIGHT_RATIO_SCALE} * std::max(CAmount {1}, GetModifiedFee())) / tx->GetConventionalFee(),
+        int128_t {WEIGHT_RATIO_SCALE} * WEIGHT_RATIO_CAP);
+}
+
 void CTxMemPoolEntry::UpdateState(int64_t modifySize, CAmount modifyFee, int64_t modifyCount)
 {
     if (!IsDirty()) {

src/txmempool.h

@@ -11,6 +11,7 @@
 #include <memory>
 #include <set>
 
+#include "int128.h"
 #include "amount.h"
 #include "coins.h"
 #include "mempool_limit.h"
@@ -87,6 +88,15 @@ public:
     const CTransaction& GetTx() const { return *this->tx; }
     std::shared_ptr<const CTransaction> GetSharedTx() const { return this->tx; }
     const CAmount& GetFee() const { return nFee; }
+
+    // Return the number of unpaid actions calculated according to ZIP 317.
+    // <https://zips.z.cash/zip-0317#recommended-algorithm-for-block-template-construction>
+    size_t GetUnpaidActionCount() const;
+
+    // Return a fixed-point representation of the entry's weight ratio according
+    // to ZIP 317, where 1 is represented by WEIGHT_RATIO_SCALE.
+    int128_t GetWeightRatio() const;
+
     size_t GetTxSize() const { return nTxSize; }
     int64_t GetTime() const { return nTime; }
     unsigned int GetHeight() const { return nHeight; }