/* Below flags apply in the context of BIP 68 */
/* If this flag set, CTxIn::nSequence is NOT interpreted as a
 * relative lock-time. */
static const uint32_t SEQUENCE_LOCKTIME_DISABLE_FLAG = (1 << 31);

/* If CTxIn::nSequence encodes a relative lock-time and this flag
 * is set, the relative lock-time has units of 512 seconds,
 * otherwise it specifies blocks with a granularity of 1. */
static const uint32_t SEQUENCE_LOCKTIME_TYPE_FLAG = (1 << 22);

/* If CTxIn::nSequence encodes a relative lock-time, this mask is
 * applied to extract that lock-time from the sequence field. */
static const uint32_t SEQUENCE_LOCKTIME_MASK = 0x0000ffff;
   
case OP_NOP3:
{
    if (!(flags & SCRIPT_VERIFY_CHECKSEQUENCEVERIFY)) {
        // not enabled; treat as a NOP3
        if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS) {
            return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS);
        }
        break;
    }

    if (stack.size() < 1)
       return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);

    // Note that elsewhere numeric opcodes are limited to
    // operands in the range -2**31+1 to 2**31-1, however it is
    // legal for opcodes to produce results exceeding that
    // range. This limitation is implemented by CScriptNum's
    // default 4-byte limit.
    //
    // Thus as a special case we tell CScriptNum to accept up
    // to 5-byte bignums, which are good until 2**39-1, well
    // beyond the 2**32-1 limit of the nSequence field itself.
    const CScriptNum nSequence(stacktop(-1), fRequireMinimal, 5);

    // In the rare event that the argument may be < 0 due to
    // some arithmetic being done first, you can always use
    // 0 MAX CHECKSEQUENCEVERIFY.
    if (nSequence < 0)
        return set_error(serror, SCRIPT_ERR_NEGATIVE_LOCKTIME);

    // To provide for future soft-fork extensibility, if the
    // operand has the disabled lock-time flag set,
    // CHECKSEQUENCEVERIFY behaves as a NOP.
    if ((nSequence & CTxIn::SEQUENCE_LOCKTIME_DISABLE_FLAG) != 0)
        break;

    // Compare the specified sequence number with the input.
    if (!checker.CheckSequence(nSequence))
        return set_error(serror, SCRIPT_ERR_UNSATISFIED_LOCKTIME);

    break;
}
    
bool TransactionSignatureChecker::CheckSequence(const CScriptNum& nSequence) const
{
    // Relative lock times are supported by comparing the passed
    // in operand to the sequence number of the input.
    const int64_t txToSequence = (int64_t)txTo->vin[nIn].nSequence;

    // Fail if the transaction's version number is not set high
    // enough to trigger BIP 68 rules.
    if (static_cast(txTo->nVersion) < 2)
        return false;

    // Sequence numbers with their most significant bit set are not
    // consensus constrained. Testing that the transaction's sequence
    // number do not have this bit set prevents using this property
    // to get around a CHECKSEQUENCEVERIFY check.
    if (txToSequence & CTxIn::SEQUENCE_LOCKTIME_DISABLE_FLAG)
        return false;

    // Mask off any bits that do not have consensus-enforced meaning
    // before doing the integer comparisons
    const uint32_t nLockTimeMask = CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG | CTxIn::SEQUENCE_LOCKTIME_MASK;
    const int64_t txToSequenceMasked = txToSequence & nLockTimeMask;
    const CScriptNum nSequenceMasked = nSequence & nLockTimeMask;

    // There are two kinds of nSequence: lock-by-blockheight
    // and lock-by-blocktime, distinguished by whether
    // nSequenceMasked < CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG.
    //
    // We want to compare apples to apples, so fail the script
    // unless the type of nSequenceMasked being tested is the same as
    // the nSequenceMasked in the transaction.
    if (!(
        (txToSequenceMasked <  CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG && nSequenceMasked <  CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG) ||
        (txToSequenceMasked >= CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG && nSequenceMasked >= CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG)
    ))
        return false;

    // Now that we know we're comparing apples-to-apples, the
    // comparison is a simple numeric one.
    if (nSequenceMasked > txToSequenceMasked)
        return false;

    return true;
}