validate.cpp

#include <bitcoin/validate.hpp>

#include <set>

#include <boost/date_time/posix_time/posix_time.hpp>

#include <bitcoin/blockchain/blockchain.hpp>
#include <bitcoin/block.hpp>
#include <bitcoin/constants.hpp>
#include <bitcoin/transaction.hpp>
#include <bitcoin/satoshi_serialize.hpp>
#include <bitcoin/error.hpp>
#include <bitcoin/utility/assert.hpp>
#include <bitcoin/utility/logger.hpp>

namespace libbitcoin {

using std::placeholders::_1;
using std::placeholders::_2;
using std::placeholders::_3;
using std::placeholders::_4;

namespace posix_time = boost::posix_time;

constexpr size_t max_block_size = 1000000;
constexpr size_t max_block_script_sig_operations = max_block_size / 50;

validate_transaction::validate_transaction(
    blockchain& chain, const message::transaction& tx,
    const pool_buffer& pool, io_service::strand& async_strand)
  : strand_(async_strand), chain_(chain),
    tx_(tx), tx_hash_(hash_transaction(tx)), pool_(pool)
{
}

void validate_transaction::start(validate_handler handle_validate)
{
    handle_validate_ = handle_validate;
    std::error_code ec = basic_checks();
    if (ec)
    {
        handle_validate_(ec, index_list());
        return;
    }

    // Check for duplicates in the blockchain
    chain_.fetch_transaction(tx_hash_,
        strand_.wrap(std::bind(
            &validate_transaction::handle_duplicate_check,
                shared_from_this(), _1)));
}

std::error_code validate_transaction::basic_checks() const
{
    std::error_code ec;
    ec = check_transaction(tx_);
    if (ec)
        return ec;

    if (is_coinbase(tx_))
        return error::coinbase_transaction;

    // Ummm...
    //if ((int64)nLockTime > INT_MAX)

    if (!is_standard())
        return error::is_not_standard;

    // Check for conflicts
    if (fetch(tx_hash_))
        return error::duplicate;
    // Check for blockchain duplicates done next in start() after
    // this function exits.

    return std::error_code();
}

bool validate_transaction::is_standard() const
{
    return true;
}

const message::transaction* validate_transaction::fetch(
    const hash_digest& tx_hash) const
{
    for (const transaction_entry_info& entry: pool_)
        if (entry.hash == tx_hash)
            return &entry.tx;
    return nullptr;
}

void validate_transaction::handle_duplicate_check(const std::error_code& ec)
{
    if (ec != error::not_found)
    {
        handle_validate_(error::duplicate, index_list());
        return;
    }
    // Check for conflicts with memory txs
    for (size_t input_index = 0; input_index < tx_.inputs.size(); 
        ++input_index)
    {
        const message::output_point& previous_output =
            tx_.inputs[input_index].previous_output;
        if (is_spent(previous_output))
        {
            handle_validate_(error::double_spend, index_list());
            return;
        }
    }
    // Check inputs

    // We already know it is not a coinbase tx

    chain_.fetch_last_depth(strand_.wrap(std::bind(
        &validate_transaction::set_last_depth, shared_from_this(), _1, _2)));
}
 
bool validate_transaction::is_spent(const message::output_point outpoint) const
{
    for (const transaction_entry_info& entry: pool_)
        for (const message::transaction_input current_input: entry.tx.inputs)
            if (current_input.previous_output == outpoint)
                return true;
    return false;
}

void validate_transaction::set_last_depth(
    const std::error_code& ec, size_t last_depth)
{
    if (ec)
    {
        handle_validate_(ec, index_list());
        return;
    }
    // Used for checking coinbase maturity
    last_block_depth_ = last_depth;
    value_in_ = 0;
    BITCOIN_ASSERT(tx_.inputs.size() > 0);
    current_input_ = 0;
    // Begin looping through the inputs, fetching the previous tx
    next_previous_transaction();
}

void validate_transaction::next_previous_transaction()
{
    BITCOIN_ASSERT(current_input_ < tx_.inputs.size());
    // First we fetch the parent block depth for a transaction.
    // Needed for checking the coinbase maturity.
    chain_.fetch_transaction_index(
        tx_.inputs[current_input_].previous_output.hash,
        strand_.wrap(std::bind(
            &validate_transaction::previous_tx_index,
                shared_from_this(), _1, _2)));
}

void validate_transaction::previous_tx_index(
    const std::error_code& ec, size_t parent_depth)
{
    if (ec)
    {
        search_pool_previous_tx();
    }
    else
    {
        // Now fetch actual transaction body
        BITCOIN_ASSERT(current_input_ < tx_.inputs.size());
        chain_.fetch_transaction(
            tx_.inputs[current_input_].previous_output.hash,
            strand_.wrap(std::bind(
                &validate_transaction::handle_previous_tx,
                    shared_from_this(), _1, _2, parent_depth)));
    }
}

void validate_transaction::search_pool_previous_tx()
{
    const hash_digest& previous_tx_hash =
        tx_.inputs[current_input_].previous_output.hash;
    const message::transaction* previous_tx = fetch(previous_tx_hash);
    if (!previous_tx)
    {
        handle_validate_(error::input_not_found,
            index_list{current_input_});
        return;
    }
    BITCOIN_ASSERT(!is_coinbase(*previous_tx));
    // parent_depth ignored here as memory pool transactions can
    // never be a coinbase transaction.
    handle_previous_tx(std::error_code(), *previous_tx, 0);
    unconfirmed_.push_back(current_input_);
}

void validate_transaction::handle_previous_tx(const std::error_code& ec,
    const message::transaction& previous_tx, size_t parent_depth)
{
    if (ec)
    {
        handle_validate_(error::input_not_found,
            index_list{current_input_});
        return;
    }
    // Should check for are inputs standard here...
    if (!connect_input(tx_, current_input_, previous_tx,
        parent_depth, last_block_depth_, value_in_))
    {
        handle_validate_(error::validate_inputs_failed, index_list());
        return;
    }
    // Search for double spends...
    chain_.fetch_spend(tx_.inputs[current_input_].previous_output,
        strand_.wrap(std::bind(
            &validate_transaction::check_double_spend,
                shared_from_this(), _1)));
}

bool validate_transaction::connect_input(
    const message::transaction& tx, size_t current_input,
    const message::transaction& previous_tx, size_t parent_depth,
    size_t last_block_depth, uint64_t& value_in)
{
    const message::transaction_input& input = tx.inputs[current_input];
    const message::output_point& previous_outpoint =
        tx.inputs[current_input].previous_output;
    if (previous_outpoint.index >= previous_tx.outputs.size())
        return false;
    const message::transaction_output& previous_output =
        previous_tx.outputs[previous_outpoint.index];
    uint64_t output_value = previous_output.value;
    if (output_value > max_money())
        return false;
    if (is_coinbase(previous_tx))
    {
        size_t depth_difference = last_block_depth - parent_depth;
        if (depth_difference < coinbase_maturity)
            return false;
    }
    script output_script = previous_output.output_script;
    if (!output_script.run(input.input_script, tx, current_input))
        return false;
    value_in += output_value;
    if (value_in > max_money())
        return false;
    return true;
}

void validate_transaction::check_double_spend(const std::error_code& ec)
{
    if (ec == error::unspent_output)
    {
        // End of connect_input checks
        ++current_input_;
        if (current_input_ == tx_.inputs.size())
            check_fees();
        else
        {
            BITCOIN_ASSERT(current_input_ < tx_.inputs.size());
            // Keep looping
            next_previous_transaction();
        }
    }
    else
    {
        BITCOIN_ASSERT(!ec || ec != error::unspent_output);
        handle_validate_(error::double_spend, index_list());
    }
}

bool validate_transaction::tally_fees(const message::transaction& tx,
    uint64_t value_in, uint64_t& total_fees)
{
    uint64_t value_out = total_output_value(tx);
    if (value_in < value_out)
        return false;
    uint64_t fee = value_in - value_out;
    total_fees += fee;
    if (total_fees > max_money())
        return false;
    return true;
}

void validate_transaction::check_fees()
{
    uint64_t fee = 0;
    tally_fees(tx_, value_in_, fee);
    // Who cares?
    // Fuck the police
    // Every tx equal!
    handle_validate_(std::error_code(), unconfirmed_);
}

std::error_code validate_transaction::check_transaction(
    const message::transaction& tx)
{
    if (tx.inputs.empty() || tx.outputs.empty())
        return error::empty_transaction;

    // Maybe not needed since we try to serialise block in CheckBlock()
    //if (exporter_->save(tx, false).size() > max_block_size)
    //    return false;

    // Check for negative or overflow output values
    uint64_t total_output_value = 0;
    for (message::transaction_output output: tx.outputs)
    {
        if (output.value > max_money())
            return error::output_value_overflow;
        total_output_value += output.value;
        if (total_output_value > max_money())
            return error::output_value_overflow;
    }

    if (is_coinbase(tx))
    {
        const script& coinbase_script = tx.inputs[0].input_script;
        size_t coinbase_script_size = save_script(coinbase_script).size();
        if (coinbase_script_size < 2 || coinbase_script_size > 100)
            return error::invalid_coinbase_script_size;
    }
    else
    {
        for (message::transaction_input input: tx.inputs)
            if (previous_output_is_null(input.previous_output))
                return error::previous_output_null;
    }

    return std::error_code();
}

validate_block::validate_block(
    size_t depth, const message::block& current_block)
  : depth_(depth), current_block_(current_block)
{
}

std::error_code validate_block::start()
{
    std::error_code ec;
    ec = check_block();
    if (ec)
        return ec;
    ec = accept_block();
    if (ec)
        return ec;
    ec = connect_block();
    if (ec)
        return ec;
    return std::error_code();
}

std::error_code validate_block::check_block()
{
    // CheckBlock()
    // These are checks that are independent of context
    // that can be validated before saving an orphan block
    // ...

    // Size limits
    if (current_block_.transactions.empty() || 
        current_block_.transactions.size() > max_block_size ||
        satoshi_raw_size(current_block_) > max_block_size)
    {
        return error::size_limits;
    }

    const hash_digest current_block_hash = hash_block_header(current_block_);
    if (!check_proof_of_work(current_block_hash, current_block_.bits))
        return error::proof_of_work;

    const posix_time::ptime block_time =
        posix_time::from_time_t(current_block_.timestamp);
    const posix_time::ptime two_hour_future =
        posix_time::second_clock::universal_time() + posix_time::hours(2);
    if (block_time > two_hour_future)
        return error::futuristic_timestamp;

    if (!is_coinbase(current_block_.transactions[0]))
        return error::first_not_coinbase;
    for (size_t i = 1; i < current_block_.transactions.size(); ++i)
    {
        const message::transaction& tx = current_block_.transactions[i];
        if (is_coinbase(tx))
            return error::extra_coinbases;
    }

    std::set<hash_digest> unique_txs;
    for (message::transaction tx: current_block_.transactions)
    {
        std::error_code ec = validate_transaction::check_transaction(tx);
        if (ec)
            return ec;
        unique_txs.insert(hash_transaction(tx));
    }
    if (unique_txs.size() != current_block_.transactions.size())
        return error::duplicate;

    // Check that it's not full of nonstandard transactions
    if (legacy_sigops_count() > max_block_script_sig_operations)
        return error::too_many_sigs;

    if (current_block_.merkle != 
            generate_merkle_root(current_block_.transactions))
    {
        return error::merkle_mismatch;
    }

    return std::error_code();
}

bool validate_block::check_proof_of_work(hash_digest block_hash, uint32_t bits)
{
    big_number target;
    target.set_compact(bits);

    if (target <= 0 || target > max_target())
        return false;
    
    big_number our_value;
    our_value.set_hash(block_hash);
    if (our_value > target)
        return false;

    return true;
}

inline size_t count_script_sigops(
    const operation_stack& operations, bool accurate)
{
    size_t total_sigs = 0, last_number = 0;
    for (const operation& op: operations)
    {
        if (op.code == opcode::checksig ||
            op.code == opcode::checksigverify)
        {
            total_sigs++;
        }
        else if (op.code == opcode::checkmultisig ||
            op.code == opcode::checkmultisigverify)
        {
            if (accurate && last_number != 0)
                total_sigs += last_number;
            else
                total_sigs += 20;
        }
        byte raw_code = static_cast<byte>(op.code);
        if (static_cast<byte>(opcode::op_1) <= raw_code &&
            static_cast<byte>(opcode::op_16) >= raw_code)
        {
            last_number = raw_code;
        }
    }
    return total_sigs;
}
size_t tx_legacy_sigops_count(const message::transaction& tx)
{
    size_t total_sigs = 0;
    for (message::transaction_input input: tx.inputs)
    {
        const operation_stack& operations = input.input_script.operations();
        total_sigs += count_script_sigops(operations, false);
    }
    for (message::transaction_output output: tx.outputs)
    {
        const operation_stack& operations = output.output_script.operations();
        total_sigs += count_script_sigops(operations, false);
    }
    return total_sigs;
}
size_t validate_block::legacy_sigops_count()
{
    size_t total_sigs = 0;
    for (message::transaction tx: current_block_.transactions)
        total_sigs += tx_legacy_sigops_count(tx);
    return total_sigs;
}

std::error_code validate_block::accept_block()
{
    if (current_block_.bits != work_required())
        return error::incorrect_proof_of_work;
    if (current_block_.timestamp <= median_time_past())
        return error::timestamp_too_early;
    // Txs should be final when included in a block
    for (const message::transaction& tx: current_block_.transactions)
        if (!is_final(tx, depth_, current_block_.timestamp))
            return error::non_final_transaction;
    if (!passes_checkpoints())
        return error::checkpoints_failed;
    return std::error_code();
}

template<typename Value>
inline Value range_constraint(Value value, Value minimum, Value maximum)
{
    if (value < minimum)
        return minimum;
    else if (value > maximum)
        return maximum;
    return value;
}

uint32_t validate_block::work_required()
{
    if (depth_ == 0)
        return max_bits;
    else if (depth_ % readjustment_interval != 0)
        return previous_block_bits();

    uint64_t actual = actual_timespan(readjustment_interval);
    actual = range_constraint(actual, target_timespan / 4, target_timespan * 4);

    big_number retarget;
    retarget.set_compact(previous_block_bits());
    retarget *= actual;
    retarget /= target_timespan;

    if (retarget > max_target())
        retarget = max_target();

    return retarget.compact();
}

bool validate_block::passes_checkpoints()
{
    const hash_digest block_hash = hash_block_header(current_block_);

    if (depth_ == 11111 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x69, 0xe2, 0x44, 0xf7, 
                        0x3d, 0x78, 0xe8, 0xfd, 0x29, 0xba, 0x2f, 0xd2, 
                        0xed, 0x61, 0x8b, 0xd6, 0xfa, 0x2e, 0xe9, 0x25, 
                        0x59, 0xf5, 0x42, 0xfd, 0xb2, 0x6e, 0x7c, 0x1d})
        return false;

    if (depth_ ==  33333 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x2d, 0xd5, 0x58, 0x8a, 
                        0x74, 0x78, 0x4e, 0xaa, 0x7a, 0xb0, 0x50, 0x7a, 
                        0x18, 0xad, 0x16, 0xa2, 0x36, 0xe7, 0xb1, 0xce, 
                        0x69, 0xf0, 0x0d, 0x7d, 0xdf, 0xb5, 0xd0, 0xa6})
        return false;

    if (depth_ ==  68555 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x00, 0x1e, 0x1b, 0x49, 
                        0x03, 0x55, 0x0a, 0x0b, 0x96, 0xe9, 0xa9, 0x40, 
                        0x5c, 0x8a, 0x95, 0xf3, 0x87, 0x16, 0x2e, 0x49, 
                        0x44, 0xe8, 0xd9, 0xfb, 0xe5, 0x01, 0xcd, 0x6a})
        return false;

    if (depth_ ==  70567 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x00, 0x6a, 0x49, 0xb1, 
                        0x4b, 0xcf, 0x27, 0x46, 0x20, 0x68, 0xf1, 0x26, 
                        0x4c, 0x96, 0x1f, 0x11, 0xfa, 0x2e, 0x0e, 0xdd, 
                        0xd2, 0xbe, 0x07, 0x91, 0xe1, 0xd4, 0x12, 0x4a})
        return false;

    if (depth_ ==  74000 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x00, 0x57, 0x39, 0x93, 
                        0xa3, 0xc9, 0xe4, 0x1c, 0xe3, 0x44, 0x71, 0xc0, 
                        0x79, 0xdc, 0xf5, 0xf5, 0x2a, 0x0e, 0x82, 0x4a, 
                        0x81, 0xe7, 0xf9, 0x53, 0xb8, 0x66, 0x1a, 0x20})
        return false;

    if (depth_ == 105000 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0x91, 0xce, 
                        0x28, 0x02, 0x7f, 0xae, 0xa3, 0x20, 0xc8, 0xd2, 
                        0xb0, 0x54, 0xb2, 0xe0, 0xfe, 0x44, 0xa7, 0x73, 
                        0xf3, 0xee, 0xfb, 0x15, 0x1d, 0x6b, 0xdc, 0x97})
        return false;

    if (depth_ == 118000 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x77, 0x4a, 
                        0x7f, 0x8a, 0x7a, 0x12, 0xdc, 0x90, 0x6d, 0xdb, 
                        0x9e, 0x17, 0xe7, 0x5d, 0x68, 0x4f, 0x15, 0xe0, 
                        0x0f, 0x87, 0x67, 0xf9, 0xe8, 0xf3, 0x65, 0x53})
        return false;

    if (depth_ == 134444 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x05, 0xb1, 
                        0x2f, 0xfd, 0x4c, 0xd3, 0x15, 0xcd, 0x34, 0xff, 
                        0xd4, 0xa5, 0x94, 0xf4, 0x30, 0xac, 0x81, 0x4c, 
                        0x91, 0x18, 0x4a, 0x0d, 0x42, 0xd2, 0xb0, 0xfe})
        return false;

    if (depth_ == 140700 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03, 0x3b,
                        0x51, 0x20, 0x28, 0xab, 0xb9, 0x0e, 0x16, 0x26,
                        0xd8, 0xb3, 0x46, 0xfd, 0x0e, 0xd5, 0x98, 0xac, 
                        0x0a, 0x3c, 0x37, 0x11, 0x38, 0xdc, 0xe2, 0xbd})
        return false;

    if (depth_ == 168000 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x09, 0x9e,
                        0x61, 0xea, 0x72, 0x01, 0x5e, 0x79, 0x63, 0x2f,
                        0x21, 0x6f, 0xe6, 0xcb, 0x33, 0xd7, 0x89, 0x9a,
                        0xcb, 0x35, 0xb7, 0x5c, 0x83, 0x03, 0xb7, 0x63})
        return false;

    if (depth_ == 193000 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x05, 0x9f,
                        0x45, 0x2a, 0x5f, 0x73, 0x40, 0xde, 0x66, 0x82,
                        0xa9, 0x77, 0x38, 0x7c, 0x17, 0x01, 0x0f, 0xf6,
                        0xe6, 0xc3, 0xbd, 0x83, 0xca, 0x8b, 0x13, 0x17})
        return false;

    if (depth_ == 210000 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x8b,
                        0x95, 0x34, 0x7e, 0x83, 0x19, 0x2f, 0x69, 0xcf,
                        0x03, 0x66, 0x07, 0x63, 0x36, 0xc6, 0x39, 0xf9,
                        0xb7, 0x22, 0x8e, 0x9b, 0xa1, 0x71, 0x34, 0x2e})
        return false;

    if (depth_ == 216116 && block_hash !=
            hash_digest{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0xb4,
                        0xf4, 0xb4, 0x33, 0xe8, 0x1e, 0xe4, 0x64, 0x94,
                        0xaf, 0x94, 0x5c, 0xf9, 0x60, 0x14, 0x81, 0x6a,
                        0x4e, 0x23, 0x70, 0xf1, 0x1b, 0x23, 0xdf, 0x4e})
        return false;

    return true;
}

std::error_code validate_block::connect_block()
{
    // BIP 30 security fix
    if (depth_ != 91842 && depth_ != 91880)
        for (const auto& current_tx: current_block_.transactions)
            if (!not_duplicate_or_spent(current_tx))
                return error::duplicate_or_spent;

    uint64_t fees = 0;
    size_t total_sigops = 0;
    for (size_t tx_index = 1; tx_index < current_block_.transactions.size();
            ++tx_index)
    {
        uint64_t value_in = 0;
        const message::transaction& tx = current_block_.transactions[tx_index];
        total_sigops += tx_legacy_sigops_count(tx);
        if (total_sigops > max_block_script_sig_operations)
            return error::too_many_sigs;
        if (!validate_inputs(tx, tx_index, value_in, total_sigops))
            return error::validate_inputs_failed;
        if (!validate_transaction::tally_fees(tx, value_in, fees))
            return error::fees_out_of_range;
    }
    uint64_t coinbase_value = 
        total_output_value(current_block_.transactions[0]);
    if (coinbase_value  > block_value(depth_) + fees)
        return error::coinbase_too_large;
    return std::error_code();
}

bool validate_block::not_duplicate_or_spent(const message::transaction& tx)
{
    const hash_digest& tx_hash = hash_transaction(tx);
    // Is there a matching previous tx?
    if (!transaction_exists(tx_hash))
        return true;
    // Then for a duplicate transaction to exist, all its outputs
    // must have been spent.
    for (uint32_t output_index = 0; output_index < tx.outputs.size();
        ++output_index)
    {
        if (!is_output_spent({tx_hash, output_index}))
            return false;
    }
    return true;
}

bool validate_block::validate_inputs(const message::transaction& tx, 
    size_t index_in_parent, uint64_t& value_in, size_t& total_sigops)
{
    BITCOIN_ASSERT(!is_coinbase(tx));
    for (size_t input_index = 0; input_index < tx.inputs.size(); ++input_index)
    {
        if (!connect_input(index_in_parent, tx, input_index,
                value_in, total_sigops))
            return false;
    }
    return true;
}

size_t validate_block::script_hash_signature_operations_count(
    const script& output_script, const script& input_script)
{
    if (output_script.type() != payment_type::script_hash)
        return count_script_sigops(output_script.operations(), true);
    if (input_script.operations().empty())
        return 0;
    script eval_script = parse_script(input_script.operations().back().data);
    return count_script_sigops(eval_script.operations(), true);
}

bool validate_block::connect_input(size_t index_in_parent,
    const message::transaction& current_tx,
    size_t input_index, uint64_t& value_in, size_t& total_sigops)
{
    // Lookup previous output
    BITCOIN_ASSERT(input_index < current_tx.inputs.size());
    const message::transaction_input& input = current_tx.inputs[input_index];
    const message::output_point& previous_output = input.previous_output;
    message::transaction previous_tx;
    size_t previous_depth;
    if (!fetch_transaction(previous_tx, previous_depth, previous_output.hash))
        return false;
    const message::transaction_output& previous_tx_out =
        previous_tx.outputs[previous_output.index];
    // Signature operations count
    total_sigops +=
        script_hash_signature_operations_count(
            previous_tx_out.output_script, input.input_script);
    if (total_sigops > max_block_script_sig_operations)
        return false;
    // Get output amount
    uint64_t output_value = previous_tx_out.value;
    if (output_value > max_money())
        return false;
    // Check coinbase maturity has been reached
    if (is_coinbase(previous_tx))
    {
        BITCOIN_ASSERT(previous_depth <= depth_);
        uint32_t depth_difference = depth_ - previous_depth;
        if (depth_difference < coinbase_maturity)
            return false;
    }
    // Pay to script hash BIP 16 scripts
    // block 170060 contains an invalid BIP 16 transaction
    // before the switchover date.
    bool bip16_enabled =
        current_block_.timestamp >= bip16_switchover_timestamp;
    BITCOIN_ASSERT(!bip16_enabled || depth_ >= bip16_switchover_depth);
    // Validate script
    script output_script = previous_tx_out.output_script;
    if (!output_script.run(input.input_script,
            current_tx, input_index, bip16_enabled))
        return false;
    // Search for double spends
    if (is_output_spent(previous_output, index_in_parent, input_index))
        return false;
    // Increase value_in by this output's value
    value_in += output_value;
    if (value_in > max_money())
        return false;
    return true;
}

} // namespace libbitcoin