// Copyright (c) 2019-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.

#include <signet.h>

#include <consensus/merkle.h>

#include <consensus/params.h>

#include <consensus/validation.h>

#include <primitives/block.h>

#include <primitives/transaction.h>

#include <script/interpreter.h>

#include <script/script.h>

#include <script/verify_flags.h>

#include <streams.h>

#include <uint256.h>

#include <util/log.h>

#include <algorithm>

#include <cstddef>

#include <cstdint>

#include <exception>

#include <span>

#include <utility>

#include <vector>

static constexpr uint8_t SIGNET_HEADER[4] = {0xec, 0xc7, 0xda, 0xa2};

static constexpr script_verify_flags BLOCK_SCRIPT_VERIFY_FLAGS = SCRIPT_VERIFY_P2SH | SCRIPT_VERIFY_WITNESS | SCRIPT_VERIFY_DERSIG | SCRIPT_VERIFY_NULLDUMMY;

static bool FetchAndClearCommitmentSection(const std::span<const uint8_t> header, CScript& witness_commitment, std::vector<uint8_t>& result)

{

    CScript replacement;

    bool found_header = false;

    result.clear();

    opcodetype opcode;

    CScript::const_iterator pc = witness_commitment.begin();

    std::vector<uint8_t> pushdata;

    while (witness_commitment.GetOp(pc, opcode, pushdata)) {

        if (pushdata.size() > 0) {

            if (!found_header && pushdata.size() > header.size() && std::ranges::equal(std::span{pushdata}.first(header.size()), header)) {

                // pushdata only counts if it has the header _and_ some data

                result.insert(result.end(), pushdata.begin() + header.size(), pushdata.end());

                pushdata.erase(pushdata.begin() + header.size(), pushdata.end());

                found_header = true;

            }

            replacement << pushdata;

        } else {

            replacement << opcode;

        }

    }

    if (found_header) witness_commitment = replacement;

    return found_header;

}

static uint256 ComputeModifiedMerkleRoot(const CMutableTransaction& cb, const CBlock& block)

{

    std::vector<uint256> leaves;

    leaves.reserve((block.vtx.size() + 1) & ~1ULL); // capacity rounded up to even

    leaves.push_back(cb.GetHash().ToUint256());

    for (size_t s = 1; s < block.vtx.size(); ++s) {

        leaves.push_back(block.vtx[s]->GetHash().ToUint256());

    }

    return ComputeMerkleRoot(std::move(leaves));

}

std::optional<SignetTxs> SignetTxs::Create(const CBlock& block, const CScript& challenge)

{

    CMutableTransaction tx_to_spend;

    tx_to_spend.version = 0;

    tx_to_spend.nLockTime = 0;

    tx_to_spend.vin.emplace_back(COutPoint(), CScript(OP_0), 0);

    tx_to_spend.vout.emplace_back(0, challenge);

    CMutableTransaction tx_spending;

    tx_spending.version = 0;

    tx_spending.nLockTime = 0;

    tx_spending.vin.emplace_back(COutPoint(), CScript(), 0);

    tx_spending.vout.emplace_back(0, CScript(OP_RETURN));

    // can't fill any other fields before extracting signet

    // responses from block coinbase tx

    // find and delete signet signature

    if (block.vtx.empty()) return std::nullopt; // no coinbase tx in block; invalid

    CMutableTransaction modified_cb(*block.vtx.at(0));

    const int cidx = GetWitnessCommitmentIndex(block);

    if (cidx == NO_WITNESS_COMMITMENT) {

        return std::nullopt; // require a witness commitment

    }

    CScript& witness_commitment = modified_cb.vout.at(cidx).scriptPubKey;

    std::vector<uint8_t> signet_solution;

    if (!FetchAndClearCommitmentSection(SIGNET_HEADER, witness_commitment, signet_solution)) {

        // no signet solution -- allow this to support OP_TRUE as trivial block challenge

    } else {

        try {

            SpanReader v{signet_solution};

            v >> tx_spending.vin[0].scriptSig;

            v >> tx_spending.vin[0].scriptWitness.stack;

            if (!v.empty()) return std::nullopt; // extraneous data encountered

        } catch (const std::exception&) {

            return std::nullopt; // parsing error

        }

    }

    uint256 signet_merkle = ComputeModifiedMerkleRoot(modified_cb, block);

    std::vector<uint8_t> block_data;

    VectorWriter writer{block_data, 0};

    writer << block.nVersion;

    writer << block.hashPrevBlock;

    writer << signet_merkle;

    writer << block.nTime;

    tx_to_spend.vin[0].scriptSig << block_data;

    tx_spending.vin[0].prevout = COutPoint(tx_to_spend.GetHash(), 0);

    return SignetTxs{tx_to_spend, tx_spending};

}

// Signet block solution checker

bool CheckSignetBlockSolution(const CBlock& block, const Consensus::Params& consensusParams)

{

    if (block.GetHash() == consensusParams.hashGenesisBlock) {

        // genesis block solution is always valid

        return true;

    }

    const CScript challenge(consensusParams.signet_challenge.begin(), consensusParams.signet_challenge.end());

    const std::optional<SignetTxs> signet_txs = SignetTxs::Create(block, challenge);

    if (!signet_txs) {

        LogDebug(BCLog::VALIDATION, "CheckSignetBlockSolution: Errors in block (block solution parse failure)\n");

        return false;

    }

    const CScript& scriptSig = signet_txs->m_to_sign.vin[0].scriptSig;

    const CScriptWitness& witness = signet_txs->m_to_sign.vin[0].scriptWitness;

    PrecomputedTransactionData txdata;

    txdata.Init(signet_txs->m_to_sign, {signet_txs->m_to_spend.vout[0]});

    TransactionSignatureChecker sigcheck(&signet_txs->m_to_sign, /* nInIn= */ 0, /* amountIn= */ signet_txs->m_to_spend.vout[0].nValue, txdata, MissingDataBehavior::ASSERT_FAIL);

    if (!VerifyScript(scriptSig, signet_txs->m_to_spend.vout[0].scriptPubKey, &witness, BLOCK_SCRIPT_VERIFY_FLAGS, sigcheck)) {

        LogDebug(BCLog::VALIDATION, "CheckSignetBlockSolution: Errors in block (block solution invalid)\n");

        return false;

    }

    return true;

}