// Copyright (c) 2022-present The Bitcoin Core developers

// Distributed under the MIT software license, see the accompanying

// file COPYING or https://opensource.org/license/mit.

#include <arith_uint256.h>

#include <chain.h>

#include <chainparams.h>

#include <headerssync.h>

#include <test/fuzz/fuzz.h>

#include <test/fuzz/util.h>

#include <test/util/setup_common.h>

#include <uint256.h>

#include <util/chaintype.h>

#include <util/time.h>

#include <validation.h>

#include <iterator>

#include <vector>

static void initialize_headers_sync_state_fuzz()

{

    static const auto testing_setup = MakeNoLogFileContext<>(

        /*chain_type=*/ChainType::MAIN);

}

void MakeHeadersContinuous(

    const CBlockHeader& genesis_header,

    const std::vector<CBlockHeader>& all_headers,

    std::vector<CBlockHeader>& new_headers)

{

    Assume(!new_headers.empty());

    const CBlockHeader* prev_header{

        all_headers.empty() ? &genesis_header : &all_headers.back()};

    for (auto& header : new_headers) {

        header.hashPrevBlock = prev_header->GetHash();

        prev_header = &header;

    }

}

class FuzzedHeadersSyncState : public HeadersSyncState

{

public:

    FuzzedHeadersSyncState(const unsigned commit_offset, const CBlockIndex* chain_start, const arith_uint256& minimum_required_work)

        : HeadersSyncState(/*id=*/0, Params().GetConsensus(), chain_start, minimum_required_work)

    {

        const_cast<unsigned&>(m_commit_offset) = commit_offset;

    }

};

FUZZ_TARGET(headers_sync_state, .init = initialize_headers_sync_state_fuzz)

{

    SeedRandomStateForTest(SeedRand::ZEROS);

    FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());

    auto mock_time{ConsumeTime(fuzzed_data_provider)};

    CBlockHeader genesis_header{Params().GenesisBlock()};

    CBlockIndex start_index(genesis_header);

    if (mock_time < start_index.GetMedianTimePast()) return;

    SetMockTime(mock_time);

    const uint256 genesis_hash = genesis_header.GetHash();

    start_index.phashBlock = &genesis_hash;

    arith_uint256 min_work{UintToArith256(ConsumeUInt256(fuzzed_data_provider))};

    FuzzedHeadersSyncState headers_sync(

        /*commit_offset=*/fuzzed_data_provider.ConsumeIntegralInRange<unsigned>(1, 1024),

        /*chain_start=*/&start_index,

        /*minimum_required_work=*/min_work);

    // Store headers for potential redownload phase.

    std::vector<CBlockHeader> all_headers;

    std::vector<CBlockHeader>::const_iterator redownloaded_it;

    bool presync{true};

    bool requested_more{true};

    while (requested_more) {

        std::vector<CBlockHeader> headers;

        // Consume headers from fuzzer or maybe replay headers if we got to the

        // redownload phase.

        if (presync || fuzzed_data_provider.ConsumeBool()) {

            auto deser_headers = ConsumeDeserializable<std::vector<CBlockHeader>>(fuzzed_data_provider);

            if (!deser_headers || deser_headers->empty()) return;

            if (fuzzed_data_provider.ConsumeBool()) {

                MakeHeadersContinuous(genesis_header, all_headers, *deser_headers);

            }

            headers.swap(*deser_headers);

        } else if (auto num_headers_left{std::distance(redownloaded_it, all_headers.cend())}; num_headers_left > 0) {

            // Consume some headers from the redownload buffer (At least one

            // header is consumed).

            auto begin_it{redownloaded_it};

            std::advance(redownloaded_it, fuzzed_data_provider.ConsumeIntegralInRange<int>(1, num_headers_left));

            headers.insert(headers.cend(), begin_it, redownloaded_it);

        }

        if (headers.empty()) return;

        auto result = headers_sync.ProcessNextHeaders(headers, fuzzed_data_provider.ConsumeBool());

        requested_more = result.request_more;

        if (result.request_more) {

            if (presync) {

                all_headers.insert(all_headers.cend(), headers.cbegin(), headers.cend());

                if (headers_sync.GetState() == HeadersSyncState::State::REDOWNLOAD) {

                    presync = false;

                    redownloaded_it = all_headers.cbegin();

                    // If we get to redownloading, the presynced headers need

                    // to have the min amount of work on them.

                    assert(CalculateClaimedHeadersWork(all_headers) >= min_work);

                }

            }

            (void)headers_sync.NextHeadersRequestLocator();

        }

    }

}