// Copyright (c) 2022-present 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_HEADERSSYNC_H

#define BITCOIN_HEADERSSYNC_H

#include <arith_uint256.h>

#include <chain.h>

#include <consensus/params.h>

#include <net.h>

#include <primitives/block.h>

#include <uint256.h>

#include <util/bitdeque.h>

#include <util/hasher.h>

#include <deque>

#include <stdexcept>

#include <vector>

// A compressed CBlockHeader, which leaves out the prevhash

struct CompressedHeader {

    // header

    int32_t nVersion{0};

    uint256 hashMerkleRoot;

    uint32_t nTime{0};

    uint32_t nBits{0};

    uint32_t nNonce{0};

    CompressedHeader()

    {

        hashMerkleRoot.SetNull();

    }

    explicit CompressedHeader(const CBlockHeader& header)

        : nVersion{header.nVersion},

          hashMerkleRoot{header.hashMerkleRoot},

          nTime{header.nTime},

          nBits{header.nBits},

          nNonce{header.nNonce}

    {

    }

    CBlockHeader GetFullHeader(const uint256& hash_prev_block) const

    {

        CBlockHeader ret;

        ret.nVersion = nVersion;

        ret.hashPrevBlock = hash_prev_block;

        ret.hashMerkleRoot = hashMerkleRoot;

        ret.nTime = nTime;

        ret.nBits = nBits;

        ret.nNonce = nNonce;

        return ret;

    };

};

/** HeadersSyncState:

 *

 * We wish to download a peer's headers chain in a DoS-resistant way.

 *

 * The Bitcoin protocol does not offer an easy way to determine the work on a

 * peer's chain. Currently, we can query a peer's headers by using a GETHEADERS

 * message, and our peer can return a set of up to 2000 headers that connect to

 * something we know. If a peer's chain has more than 2000 blocks, then we need

 * a way to verify that the chain actually has enough work on it to be useful to

 * us -- by being above our anti-DoS minimum-chain-work threshold -- before we

 * commit to storing those headers in memory. Otherwise, it would be cheap for

 * an attacker to waste all our memory by serving us low-work headers

 * (particularly for a new node coming online for the first time).

 *

 * To prevent memory-DoS with low-work headers, while still always being

 * able to reorg to whatever the most-work chain is, we require that a chain

 * meet a work threshold before committing it to memory. We can do this by

 * downloading a peer's headers twice, whenever we are not sure that the chain

 * has sufficient work:

 *

 * - In the first download phase, called pre-synchronization, we can calculate

 * the work on the chain as we go (just by checking the nBits value on each

 * header, and validating the proof-of-work).

 *

 * - Once we have reached a header where the cumulative chain work is

 * sufficient, we switch to downloading the headers a second time, this time

 * processing them fully, and possibly storing them in memory.

 *

 * To prevent an attacker from using (eg) the honest chain to convince us that

 * they have a high-work chain, but then feeding us an alternate set of

 * low-difficulty headers in the second phase, we store commitments to the

 * chain we see in the first download phase that we check in the second phase,

 * as follows:

 *

 * - In phase 1 (presync), store 1 bit (using a salted hash function) for every

 * N headers that we see. With a reasonable choice of N, this uses relatively

 * little memory even for a very long chain.

 *

 * - In phase 2 (redownload), keep a lookahead buffer and only accept headers

 * from that buffer into the block index (permanent memory usage) once they

 * have some target number of verified commitments on top of them. With this

 * parametrization, we can achieve a given security target for potential

 * permanent memory usage, while choosing N to minimize memory use during the

 * sync (temporary, per-peer storage).

 */

class HeadersSyncState

{

public:

    struct SystemClockError : std::runtime_error {

        using std::runtime_error::runtime_error;

    };

    ~HeadersSyncState() = default;

    enum class State {

        /** PRESYNC means the peer has not yet demonstrated their chain has

         * sufficient work and we're only building commitments to the chain they

         * serve us. */

        PRESYNC,

        /** REDOWNLOAD means the peer has given us a high-enough-work chain,

         * and now we're redownloading the headers we saw before and trying to

         * accept them */

        REDOWNLOAD,

        /** We're done syncing with this peer and can discard any remaining state */

        FINAL

    };

    /** Return the current state of our download */

    State GetState() const { return m_download_state; }

    /** Return the height reached during the PRESYNC phase */

    int64_t GetPresyncHeight() const { return m_current_height; }

    /** Return the block timestamp of the last header received during the PRESYNC phase. */

    uint32_t GetPresyncTime() const { return m_last_header_received.nTime; }

    /** Return the amount of work in the chain received during the PRESYNC phase. */

    arith_uint256 GetPresyncWork() const { return m_current_chain_work; }

    /** Construct a HeadersSyncState object representing a headers sync via this

     *  download-twice mechanism).

     *

     * id: node id (for logging)

     * consensus_params: parameters needed for difficulty adjustment validation

     * chain_start: best known fork point that the peer's headers branch from

     * minimum_required_work: amount of chain work required to accept the chain

     *

     * @throws SystemClockError if system clock is too far behind chain_start MTP.

     */

    HeadersSyncState(NodeId id, const Consensus::Params& consensus_params,

                     const HeadersSyncParams& params, const CBlockIndex& chain_start,

                     const arith_uint256& minimum_required_work);

    /** Result data structure for ProcessNextHeaders. */

    struct ProcessingResult {

        std::vector<CBlockHeader> pow_validated_headers;

        bool success{false};

        bool request_more{false};

    };

    /** Process a batch of headers, once a sync via this mechanism has started

     *

     * received_headers: headers that were received over the network for processing.

     *                   Assumes the caller has already verified the headers

     *                   are continuous, and has checked that each header

     *                   satisfies the proof-of-work target included in the

     *                   header (but not necessarily verified that the

     *                   proof-of-work target is correct and passes consensus

     *                   rules).

     * full_headers_message: true if the message was at max capacity,

     *                       indicating more headers may be available

     * ProcessingResult.pow_validated_headers: will be filled in with any

     *                       headers that the caller can fully process and

     *                       validate now (because these returned headers are

     *                       on a chain with sufficient work)

     * ProcessingResult.success: set to false if an error is detected and the sync is

     *                       aborted; true otherwise.

     * ProcessingResult.request_more: if true, the caller is suggested to call

     *                       NextHeadersRequestLocator and send a getheaders message using it.

     */

    ProcessingResult ProcessNextHeaders(std::span<const CBlockHeader>

            received_headers, bool full_headers_message);

    /** Issue the next GETHEADERS message to our peer.

     *

     * This will return a locator appropriate for the current sync object, to continue the

     * synchronization phase it is in.

     */

    CBlockLocator NextHeadersRequestLocator() const;

protected:

    /** The (secret) offset on the heights for which to create commitments.

     *

     * m_header_commitments entries are created at any height h for which

     * (h % m_params.commitment_period) == m_commit_offset. */

    const size_t m_commit_offset;

private:

    /** Clear out all download state that might be in progress (freeing any used

     * memory), and mark this object as no longer usable.

     */

    void Finalize();

    /**

     *  Only called in PRESYNC.

     *  Validate the work on the headers we received from the network, and

     *  store commitments for later. Update overall state with successfully

     *  processed headers.

     *  On failure, this invokes Finalize() and returns false.

     */

    bool ValidateAndStoreHeadersCommitments(std::span<const CBlockHeader> headers);

    /** In PRESYNC, process and update state for a single header */

    bool ValidateAndProcessSingleHeader(const CBlockHeader& current);

    /** In REDOWNLOAD, check a header's commitment (if applicable) and add to

     * buffer for later processing */

    bool ValidateAndStoreRedownloadedHeader(const CBlockHeader& header);

    /** Return a set of headers that satisfy our proof-of-work threshold */

    std::vector<CBlockHeader> PopHeadersReadyForAcceptance();

private:

    /** NodeId of the peer (used for log messages) **/

    const NodeId m_id;

    /** We use the consensus params in our anti-DoS calculations */

    const Consensus::Params& m_consensus_params;

    /** Parameters that impact memory usage for a given chain, especially when attacked. */

    const HeadersSyncParams m_params;

    /** Store the last block in our block index that the peer's chain builds from */

    const CBlockIndex& m_chain_start;

    /** Minimum work that we're looking for on this chain. */

    const arith_uint256 m_minimum_required_work;

    /** Work that we've seen so far on the peer's chain */

    arith_uint256 m_current_chain_work;

    /** m_hasher is a salted hasher for making our 1-bit commitments to headers we've seen. */

    const SaltedUint256Hasher m_hasher;

    /** A queue of commitment bits, created during the 1st phase, and verified during the 2nd. */

    bitdeque<> m_header_commitments;

    /** m_max_commitments is a bound we calculate on how long an honest peer's chain could be,

     * given the MTP rule.

     *

     * Any peer giving us more headers than this will have its sync aborted. This serves as a

     * memory bound on m_header_commitments. */

    uint64_t m_max_commitments{0};

    /** Store the latest header received while in PRESYNC (initialized to m_chain_start) */

    CBlockHeader m_last_header_received;

    /** Height of m_last_header_received */

    int64_t m_current_height{0};

    /** During phase 2 (REDOWNLOAD), we buffer redownloaded headers in memory

     *  until enough commitments have been verified; those are stored in

     *  m_redownloaded_headers */

    std::deque<CompressedHeader> m_redownloaded_headers;

    /** Height of last header in m_redownloaded_headers */

    int64_t m_redownload_buffer_last_height{0};

    /** Hash of last header in m_redownloaded_headers (initialized to

     * m_chain_start). We have to cache it because we don't have hashPrevBlock

     * available in a CompressedHeader.

     */

    uint256 m_redownload_buffer_last_hash;

    /** The hashPrevBlock entry for the first header in m_redownloaded_headers

     * We need this to reconstruct the full header when it's time for

     * processing.

     */

    uint256 m_redownload_buffer_first_prev_hash;

    /** The accumulated work on the redownloaded chain. */

    arith_uint256 m_redownload_chain_work;

    /** Set this to true once we encounter the target blockheader during phase

     * 2 (REDOWNLOAD). At this point, we can process and store all remaining

     * headers still in m_redownloaded_headers.

     */

    bool m_process_all_remaining_headers{false};

    /** Current state of our headers sync. */

    State m_download_state{State::PRESYNC};

};

#endif // BITCOIN_HEADERSSYNC_H