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diff --git a/src/index/txindex.h b/src/index/txindex.h
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+// Copyright (c) 2017-2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_INDEX_TXINDEX_H
+#define BITCOIN_INDEX_TXINDEX_H
+
+#include <primitives/block.h>
+#include <primitives/transaction.h>
+#include <threadinterrupt.h>
+#include <txdb.h>
+#include <uint256.h>
+#include <validationinterface.h>
+
+class CBlockIndex;
+
+/**
+ * TxIndex is used to look up transactions included in the blockchain by hash.
+ * The index is written to a LevelDB database and records the filesystem
+ * location of each transaction by transaction hash.
+ */
+class TxIndex final : public CValidationInterface
+{
+private:
+    const std::unique_ptr<TxIndexDB> m_db;
+
+    /// Whether the index is in sync with the main chain. The flag is flipped
+    /// from false to true once, after which point this starts processing
+    /// ValidationInterface notifications to stay in sync.
+    std::atomic<bool> m_synced;
+
+    /// The last block in the chain that the TxIndex is in sync with.
+    std::atomic<const CBlockIndex*> m_best_block_index;
+
+    std::thread m_thread_sync;
+    CThreadInterrupt m_interrupt;
+
+    /// Initialize internal state from the database and block index.
+    bool Init();
+
+    /// Sync the tx index with the block index starting from the current best
+    /// block. Intended to be run in its own thread, m_thread_sync, and can be
+    /// interrupted with m_interrupt. Once the txindex gets in sync, the
+    /// m_synced flag is set and the BlockConnected ValidationInterface callback
+    /// takes over and the sync thread exits.
+    void ThreadSync();
+
+    /// Write update index entries for a newly connected block.
+    bool WriteBlock(const CBlock& block, const CBlockIndex* pindex);
+
+    /// Write the current chain block locator to the DB.
+    bool WriteBestBlock(const CBlockIndex* block_index);
+
+protected:
+    void BlockConnected(const std::shared_ptr<const CBlock>& block, const CBlockIndex* pindex,
+                        const std::vector<CTransactionRef>& txn_conflicted) override;
+
+    void ChainStateFlushed(const CBlockLocator& locator) override;
+
+public:
+    /// Constructs the TxIndex, which becomes available to be queried.
+    explicit TxIndex(std::unique_ptr<TxIndexDB> db);
+
+    /// Destructor interrupts sync thread if running and blocks until it exits.
+    ~TxIndex();
+
+    /// Blocks the current thread until the transaction index is caught up to
+    /// the current state of the block chain. This only blocks if the index has gotten in sync once
+    /// and only needs to process blocks in the ValidationInterface queue. If the index is catching
+    /// up from far behind, this method does not block and immediately returns false.
+    bool BlockUntilSyncedToCurrentChain();
+
+    /// Look up a transaction by hash.
+    ///
+    /// @param[in]   tx_hash  The hash of the transaction to be returned.
+    /// @param[out]  block_hash  The hash of the block the transaction is found in.
+    /// @param[out]  tx  The transaction itself.
+    /// @return  true if transaction is found, false otherwise
+    bool FindTx(const uint256& tx_hash, uint256& block_hash, CTransactionRef& tx) const;
+
+    void Interrupt();
+
+    /// Start initializes the sync state and registers the instance as a
+    /// ValidationInterface so that it stays in sync with blockchain updates.
+    void Start();
+
+    /// Stops the instance from staying in sync with blockchain updates.
+    void Stop();
+};
+
+/// The global transaction index, used in GetTransaction. May be null.
+extern std::unique_ptr<TxIndex> g_txindex;
+
+#endif // BITCOIN_INDEX_TXINDEX_H