p2p: Add helper to compute reconciliation tx short ids and a cache of short ids to wtxids

src/node/txreconciliation.cpp

@@ -64,7 +64,28 @@ public:
      */
     std::unordered_set<Wtxid, SaltedTxidHasher> m_local_set;
 
+    /**
+     * Reconciliation sketches are computed over short transaction IDs.
+     * This is a cache of these IDs enabling faster lookups of full wtxids,
+     * useful when peer will ask for missing transactions by short IDs
+     * at the end of a reconciliation round.
+     * We also use this to keep track of short ID collisions. In case of a
+     * collision, both transactions should be fanout.
+     */
+    std::map<uint32_t, Wtxid> m_short_id_mapping;
+
     TxReconciliationState(bool we_initiate, uint64_t k0, uint64_t k1) : m_we_initiate(we_initiate), m_k0(k0), m_k1(k1) {}
+
+    /**
+     * Reconciliation sketches are computed over short transaction IDs.
+     * Short IDs are salted with a link-specific constant value.
+     */
+    uint32_t ComputeShortID(const uint256 wtxid) const
+    {
+        const uint64_t s = SipHashUint256(m_k0, m_k1, wtxid);
+        const uint32_t short_txid = 1 + (s & 0xFFFFFFFF);
+        return short_txid;
+    }
 };
 
 } // namespace