function isDmOnEnvelope(envelope) {
    return nbMembers(envelope) === 2;
}

function nbMembers(envelope) {
    let nbMembers =
        (envelope.bcc?.length ?? 0) +
        (envelope.cc?.length ?? 0) +
        (envelope.to?.length ?? 0) +
        (envelope.from?.length ?? 0);
    if (
        envelope.sender?.length > 0 &&
        !(envelope.sender[0].mailbox == envelope.from[0].mailbox && envelope.sender[0].host == envelope.from[0].host)
    ) {
        nbMembers += envelope.sender?.length ?? 0;
    }
    return nbMembers;
}

module.exports = {
    isDmOnEnvelope,
    nbMembers,
};