Add SwiftSync aggregate class

Author: rustaceanrob

src/CMakeLists.txt

@@ -260,6 +260,7 @@ add_library(bitcoin_node STATIC EXCLUDE_FROM_ALL
   rpc/txoutproof.cpp
   script/sigcache.cpp
   signet.cpp
+  swiftsync.cpp
   torcontrol.cpp
   txdb.cpp
   txgraph.cpp

src/swiftsync.cpp

@@ -0,0 +1,39 @@
+#include "primitives/transaction.h"
+#include <array>
+#include <cstddef>
+#include <random.h>
+#include <swiftsync.h>
+
+Aggregate::Aggregate() : limb0(0), limb1(0), limb2(0), limb3(0)
+{
+    std::array<std::byte, 32> salt;
+    FastRandomContext{}.fillrand(salt);
+    HashWriter m_salted_hasher;
+    m_salted_hasher.write(salt);
+}
+
+void Aggregate::Add(const COutPoint& outpoint)
+{
+    auto hash = (HashWriter(m_salted_hasher) << outpoint).GetSHA256();
+    auto a0 = hash.GetUint64(0);
+    auto a1 = hash.GetUint64(1);
+    auto a2 = hash.GetUint64(2);
+    auto a3 = hash.GetUint64(3);
+    limb0 += a0;
+    limb1 += a1;
+    limb2 += a2;
+    limb3 += a3;
+}
+
+void Aggregate::Spend(const COutPoint& outpoint)
+{
+    auto hash = (HashWriter(m_salted_hasher) << outpoint).GetSHA256();
+    auto a0 = hash.GetUint64(0);
+    auto a1 = hash.GetUint64(1);
+    auto a2 = hash.GetUint64(2);
+    auto a3 = hash.GetUint64(3);
+    limb0 -= a0;
+    limb1 -= a1;
+    limb2 -= a2;
+    limb3 -= a3;
+}

src/swiftsync.h

@@ -0,0 +1,24 @@
+#include "primitives/transaction.h"
+#include <hash.h>
+#include <cstdint>
+
+/** An aggregate for the SwiftSync protocol.
+ *This class is intentionally left opaque, as internal changes may occur,
+ * but all aggregates will have the concept of "adding" and "spending" an
+ * outpoint.
+ *
+ * The current implementation uses a salted SHA-256 hash, and updates 4
+ * independent 32-bit limbs by dividing the hash into 4 parts and adding
+ * or subtracting accordingly.
+ * */
+class Aggregate
+{
+private:
+    uint64_t limb0, limb1, limb2, limb3;
+    HashWriter m_salted_hasher;
+public:
+    Aggregate();
+    bool IsZero() const { return limb0 == 0 && limb1 == 0 && limb2 == 0 && limb3 == 0; }
+    void Add(const COutPoint& outpoint);
+    void Spend(const COutPoint& outpoint);
+};