rippled/include/xrpl/protocol/SeqProxy.h

#ifndef XRPL_PROTOCOL_SEQ_PROXY_H_INCLUDED
#define XRPL_PROTOCOL_SEQ_PROXY_H_INCLUDED

#include <cstdint>
#include <ostream>

namespace ripple {

/** A type that represents either a sequence value or a ticket value.

  We use the value() of a SeqProxy in places where a sequence was used
  before.  An example of this is the sequence of an Offer stored in the
  ledger.  We do the same thing with the in-ledger identifier of a
  Check, Payment Channel, and Escrow.

  Why is this safe?  If we use the SeqProxy::value(), how do we know that
  each ledger entry will be unique?

  There are two components that make this safe:

  1. A "TicketCreate" transaction carefully avoids creating a ticket
     that corresponds with an already used Sequence or Ticket value.
     The transactor does this by referring to the account root's
     sequence number.  Creating the ticket advances the account root's
     sequence number so the same ticket (or sequence) value cannot be
     used again.

  2. When a "TicketCreate" transaction creates a batch of tickets it advances
     the account root sequence to one past the largest created ticket.

     Therefore all tickets in a batch other than the first may never have
     the same value as a sequence on that same account.  And since a ticket
     may only be used once there will never be any duplicates within this
     account.
*/
class SeqProxy
{
public:
    enum Type : std::uint8_t { seq = 0, ticket };

private:
    std::uint32_t value_;
    Type type_;

public:
    constexpr explicit SeqProxy(Type t, std::uint32_t v) : value_{v}, type_{t}
    {
    }

    SeqProxy(SeqProxy const& other) = default;

    SeqProxy&
    operator=(SeqProxy const& other) = default;

    /** Factory function to return a sequence-based SeqProxy */
    static constexpr SeqProxy
    sequence(std::uint32_t v)
    {
        return SeqProxy{Type::seq, v};
    }

    constexpr std::uint32_t
    value() const
    {
        return value_;
    }

    constexpr bool
    isSeq() const
    {
        return type_ == seq;
    }

    constexpr bool
    isTicket() const
    {
        return type_ == ticket;
    }

    // Occasionally it is convenient to be able to increase the value_
    // of a SeqProxy.  But it's unusual.  So, rather than putting in an
    // addition operator, you must invoke the method by name.  That makes
    // if more difficult to invoke accidentally.
    SeqProxy&
    advanceBy(std::uint32_t amount)
    {
        value_ += amount;
        return *this;
    }

    // Comparison
    //
    // The comparison is designed specifically so _all_ Sequence
    // representations sort in front of Ticket representations.  This
    // is true even if the Ticket value() is less that the Sequence
    // value().
    //
    // This somewhat surprising sort order has benefits for transaction
    // processing.  It guarantees that transactions creating Tickets are
    // sorted in from of transactions that consume Tickets.
    friend constexpr bool
    operator==(SeqProxy lhs, SeqProxy rhs)
    {
        if (lhs.type_ != rhs.type_)
            return false;
        return (lhs.value() == rhs.value());
    }

    friend constexpr bool
    operator!=(SeqProxy lhs, SeqProxy rhs)
    {
        return !(lhs == rhs);
    }

    friend constexpr bool
    operator<(SeqProxy lhs, SeqProxy rhs)
    {
        if (lhs.type_ != rhs.type_)
            return lhs.type_ < rhs.type_;
        return lhs.value() < rhs.value();
    }

    friend constexpr bool
    operator>(SeqProxy lhs, SeqProxy rhs)
    {
        return rhs < lhs;
    }

    friend constexpr bool
    operator>=(SeqProxy lhs, SeqProxy rhs)
    {
        return !(lhs < rhs);
    }

    friend constexpr bool
    operator<=(SeqProxy lhs, SeqProxy rhs)
    {
        return !(lhs > rhs);
    }

    friend std::ostream&
    operator<<(std::ostream& os, SeqProxy seqProx)
    {
        os << (seqProx.isSeq() ? "sequence " : "ticket ");
        os << seqProx.value();
        return os;
    }
};
}  // namespace ripple

#endif