'use strict';

// Load modules

const Dns = require('dns');


// Declare internals

const internals = {
    hasOwn: Object.prototype.hasOwnProperty,
    indexOf: Array.prototype.indexOf,
    defaultThreshold: 16,
    maxIPv6Groups: 8,

    categories: {
        valid: 1,
        dnsWarn: 7,
        rfc5321: 15,
        cfws: 31,
        deprecated: 63,
        rfc5322: 127,
        error: 255
    },

    diagnoses: {

        // Address is valid

        valid: 0,

        // Address is valid, but the DNS check failed

        dnsWarnNoMXRecord: 5,
        dnsWarnNoRecord: 6,

        // Address is valid for SMTP but has unusual elements

        rfc5321TLD: 9,
        rfc5321TLDNumeric: 10,
        rfc5321QuotedString: 11,
        rfc5321AddressLiteral: 12,

        // Address is valid for message, but must be modified for envelope

        cfwsComment: 17,
        cfwsFWS: 18,

        // Address contains deprecated elements, but may still be valid in some contexts

        deprecatedLocalPart: 33,
        deprecatedFWS: 34,
        deprecatedQTEXT: 35,
        deprecatedQP: 36,
        deprecatedComment: 37,
        deprecatedCTEXT: 38,
        deprecatedIPv6: 39,
        deprecatedCFWSNearAt: 49,

        // Address is only valid according to broad definition in RFC 5322, but is otherwise invalid

        rfc5322Domain: 65,
        rfc5322TooLong: 66,
        rfc5322LocalTooLong: 67,
        rfc5322DomainTooLong: 68,
        rfc5322LabelTooLong: 69,
        rfc5322DomainLiteral: 70,
        rfc5322DomainLiteralOBSDText: 71,
        rfc5322IPv6GroupCount: 72,
        rfc5322IPv62x2xColon: 73,
        rfc5322IPv6BadCharacter: 74,
        rfc5322IPv6MaxGroups: 75,
        rfc5322IPv6ColonStart: 76,
        rfc5322IPv6ColonEnd: 77,

        // Address is invalid for any purpose

        errExpectingDTEXT: 129,
        errNoLocalPart: 130,
        errNoDomain: 131,
        errConsecutiveDots: 132,
        errATEXTAfterCFWS: 133,
        errATEXTAfterQS: 134,
        errATEXTAfterDomainLiteral: 135,
        errExpectingQPair: 136,
        errExpectingATEXT: 137,
        errExpectingQTEXT: 138,
        errExpectingCTEXT: 139,
        errBackslashEnd: 140,
        errDotStart: 141,
        errDotEnd: 142,
        errDomainHyphenStart: 143,
        errDomainHyphenEnd: 144,
        errUnclosedQuotedString: 145,
        errUnclosedComment: 146,
        errUnclosedDomainLiteral: 147,
        errFWSCRLFx2: 148,
        errFWSCRLFEnd: 149,
        errCRNoLF: 150,
        errUnknownTLD: 160,
        errDomainTooShort: 161
    },

    components: {
        localpart: 0,
        domain: 1,
        literal: 2,
        contextComment: 3,
        contextFWS: 4,
        contextQuotedString: 5,
        contextQuotedPair: 6
    }
};


// $lab:coverage:off$
internals.defer = typeof process !== 'undefined' && process && typeof process.nextTick === 'function' ?
    process.nextTick.bind(process) :
    function (callback) {

        return setTimeout(callback, 0);
    };
// $lab:coverage:on$


internals.specials = function () {

    const specials = '()<>[]:;@\\,."';        // US-ASCII visible characters not valid for atext (http://tools.ietf.org/html/rfc5322#section-3.2.3)
    const lookup = new Array(0x100);
    for (let i = 0xff; i >= 0; --i) {
        lookup[i] = false;
    }

    for (let i = 0; i < specials.length; ++i) {
        lookup[specials.charCodeAt(i)] = true;
    }

    return function (code) {

        return lookup[code];
    };
}();


internals.regex = {
    ipV4: /\b(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?)$/,
    ipV6: /^[a-fA-F\d]{0,4}$/
};


internals.checkIpV6 = function (items) {

    return items.every((value) => internals.regex.ipV6.test(value));
};


internals.validDomain = function (tldAtom, options) {

    if (options.tldBlacklist) {
        if (Array.isArray(options.tldBlacklist)) {
            return internals.indexOf.call(options.tldBlacklist, tldAtom) === -1;
        }

        return !internals.hasOwn.call(options.tldBlacklist, tldAtom);
    }

    if (Array.isArray(options.tldWhitelist)) {
        return internals.indexOf.call(options.tldWhitelist, tldAtom) !== -1;
    }

    return internals.hasOwn.call(options.tldWhitelist, tldAtom);
};


/**
 * Check that an email address conforms to RFCs 5321, 5322 and others
 *
 * We distinguish clearly between a Mailbox as defined by RFC 5321 and an
 * addr-spec as defined by RFC 5322. Depending on the context, either can be
 * regarded as a valid email address. The RFC 5321 Mailbox specification is
 * more restrictive (comments, white space and obsolete forms are not allowed).
 *
 * @param {string} email The email address to check. See README for specifics.
 * @param {Object} options The (optional) options:
 *   {boolean} checkDNS If true then will check DNS for MX records. If
 *     true this call to isEmail _will_ be asynchronous.
 *   {*} errorLevel Determines the boundary between valid and invalid
 *     addresses.
 *   {*} tldBlacklist The set of domains to consider invalid.
 *   {*} tldWhitelist The set of domains to consider valid.
 *   {*} minDomainAtoms The minimum number of domain atoms which must be present
 *     for the address to be valid.
 * @param {function(number|boolean)} callback The (optional) callback handler.
 * @return {*}
 */

exports.validate = internals.validate = function (email, options, callback) {

    options = options || {};

    if (typeof options === 'function') {
        callback = options;
        options = {};
    }

    if (typeof callback !== 'function') {
        if (options.checkDNS) {
            throw new TypeError('expected callback function for checkDNS option');
        }

        callback = null;
    }

    let diagnose;
    let threshold;

    if (typeof options.errorLevel === 'number') {
        diagnose = true;
        threshold = options.errorLevel;
    }
    else {
        diagnose = !!options.errorLevel;
        threshold = internals.diagnoses.valid;
    }

    if (options.tldWhitelist) {
        if (typeof options.tldWhitelist === 'string') {
            options.tldWhitelist = [options.tldWhitelist];
        }
        else if (typeof options.tldWhitelist !== 'object') {
            throw new TypeError('expected array or object tldWhitelist');
        }
    }

    if (options.tldBlacklist) {
        if (typeof options.tldBlacklist === 'string') {
            options.tldBlacklist = [options.tldBlacklist];
        }
        else if (typeof options.tldBlacklist !== 'object') {
            throw new TypeError('expected array or object tldBlacklist');
        }
    }

    if (options.minDomainAtoms && (options.minDomainAtoms !== ((+options.minDomainAtoms) | 0) || options.minDomainAtoms < 0)) {
        throw new TypeError('expected positive integer minDomainAtoms');
    }

    let maxResult = internals.diagnoses.valid;
    const updateResult = (value) => {

        if (value > maxResult) {
            maxResult = value;
        }
    };

    const context = {
        now: internals.components.localpart,
        prev: internals.components.localpart,
        stack: [internals.components.localpart]
    };

    let prevToken = '';

    const parseData = {
        local: '',
        domain: ''
    };
    const atomData = {
        locals: [''],
        domains: ['']
    };

    let elementCount = 0;
    let elementLength = 0;
    let crlfCount = 0;
    let charCode;

    let hyphenFlag = false;
    let assertEnd = false;

    const emailLength = email.length;

    let token;                                      // Token is used outside the loop, must declare similarly
    for (let i = 0; i < emailLength; ++i) {
        token = email[i];

        switch (context.now) {
            // Local-part
            case internals.components.localpart:
                // http://tools.ietf.org/html/rfc5322#section-3.4.1
                //   local-part      =   dot-atom / quoted-string / obs-local-part
                //
                //   dot-atom        =   [CFWS] dot-atom-text [CFWS]
                //
                //   dot-atom-text   =   1*atext *("." 1*atext)
                //
                //   quoted-string   =   [CFWS]
                //                       DQUOTE *([FWS] qcontent) [FWS] DQUOTE
                //                       [CFWS]
                //
                //   obs-local-part  =   word *("." word)
                //
                //   word            =   atom / quoted-string
                //
                //   atom            =   [CFWS] 1*atext [CFWS]
                switch (token) {
                    // Comment
                    case '(':
                        if (elementLength === 0) {
                            // Comments are OK at the beginning of an element
                            updateResult(elementCount === 0 ? internals.diagnoses.cfwsComment : internals.diagnoses.deprecatedComment);
                        }
                        else {
                            updateResult(internals.diagnoses.cfwsComment);
                            // Cannot start a comment in an element, should be end
                            assertEnd = true;
                        }

                        context.stack.push(context.now);
                        context.now = internals.components.contextComment;
                        break;

                        // Next dot-atom element
                    case '.':
                        if (elementLength === 0) {
                            // Another dot, already?
                            updateResult(elementCount === 0 ? internals.diagnoses.errDotStart : internals.diagnoses.errConsecutiveDots);
                        }
                        else {
                            // The entire local-part can be a quoted string for RFC 5321; if one atom is quoted it's an RFC 5322 obsolete form
                            if (assertEnd) {
                                updateResult(internals.diagnoses.deprecatedLocalPart);
                            }

                            // CFWS & quoted strings are OK again now we're at the beginning of an element (although they are obsolete forms)
                            assertEnd = false;
                            elementLength = 0;
                            ++elementCount;
                            parseData.local += token;
                            atomData.locals[elementCount] = '';
                        }

                        break;

                        // Quoted string
                    case '"':
                        if (elementLength === 0) {
                            // The entire local-part can be a quoted string for RFC 5321; if one atom is quoted it's an RFC 5322 obsolete form
                            updateResult(elementCount === 0 ? internals.diagnoses.rfc5321QuotedString : internals.diagnoses.deprecatedLocalPart);

                            parseData.local += token;
                            atomData.locals[elementCount] += token;
                            ++elementLength;

                            // Quoted string must be the entire element
                            assertEnd = true;
                            context.stack.push(context.now);
                            context.now = internals.components.contextQuotedString;
                        }
                        else {
                            updateResult(internals.diagnoses.errExpectingATEXT);
                        }

                        break;

                        // Folding white space
                    case '\r':
                        if (emailLength === ++i || email[i] !== '\n') {
                            // Fatal error
                            updateResult(internals.diagnoses.errCRNoLF);
                            break;
                        }

                        // Fallthrough

                    case ' ':
                    case '\t':
                        if (elementLength === 0) {
                            updateResult(elementCount === 0 ? internals.diagnoses.cfwsFWS : internals.diagnoses.deprecatedFWS);
                        }
                        else {
                            // We can't start FWS in the middle of an element, better be end
                            assertEnd = true;
                        }

                        context.stack.push(context.now);
                        context.now = internals.components.contextFWS;
                        prevToken = token;
                        break;

                    case '@':
                        // At this point we should have a valid local-part
                        // $lab:coverage:off$
                        if (context.stack.length !== 1) {
                            throw new Error('unexpected item on context stack');
                        }
                        // $lab:coverage:on$

                        if (parseData.local.length === 0) {
                            // Fatal error
                            updateResult(internals.diagnoses.errNoLocalPart);
                        }
                        else if (elementLength === 0) {
                            // Fatal error
                            updateResult(internals.diagnoses.errDotEnd);
                        }
                            // http://tools.ietf.org/html/rfc5321#section-4.5.3.1.1 the maximum total length of a user name or other local-part is 64
                            //    octets
                        else if (parseData.local.length > 64) {
                            updateResult(internals.diagnoses.rfc5322LocalTooLong);
                        }
                            // http://tools.ietf.org/html/rfc5322#section-3.4.1 comments and folding white space SHOULD NOT be used around "@" in the
                            //    addr-spec
                            //
                            // http://tools.ietf.org/html/rfc2119
                            // 4. SHOULD NOT this phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular
                            //    circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood
                            //    and the case carefully weighed before implementing any behavior described with this label.
                        else if (context.prev === internals.components.contextComment || context.prev === internals.components.contextFWS) {
                            updateResult(internals.diagnoses.deprecatedCFWSNearAt);
                        }

                        // Clear everything down for the domain parsing
                        context.now = internals.components.domain;
                        context.stack[0] = internals.components.domain;
                        elementCount = 0;
                        elementLength = 0;
                        assertEnd = false; // CFWS can only appear at the end of the element
                        break;

                        // ATEXT
                    default:
                        // http://tools.ietf.org/html/rfc5322#section-3.2.3
                        //    atext = ALPHA / DIGIT / ; Printable US-ASCII
                        //            "!" / "#" /     ;  characters not including
                        //            "$" / "%" /     ;  specials.  Used for atoms.
                        //            "&" / "'" /
                        //            "*" / "+" /
                        //            "-" / "/" /
                        //            "=" / "?" /
                        //            "^" / "_" /
                        //            "`" / "{" /
                        //            "|" / "}" /
                        //            "~"
                        if (assertEnd) {
                            // We have encountered atext where it is no longer valid
                            switch (context.prev) {
                                case internals.components.contextComment:
                                case internals.components.contextFWS:
                                    updateResult(internals.diagnoses.errATEXTAfterCFWS);
                                    break;

                                case internals.components.contextQuotedString:
                                    updateResult(internals.diagnoses.errATEXTAfterQS);
                                    break;

                                    // $lab:coverage:off$
                                default:
                                    throw new Error('more atext found where none is allowed, but unrecognized prev context: ' + context.prev);
                                    // $lab:coverage:on$
                            }
                        }
                        else {
                            context.prev = context.now;
                            charCode = token.charCodeAt(0);

                            // Especially if charCode == 10
                            if (charCode < 33 || charCode > 126 || internals.specials(charCode)) {

                                // Fatal error
                                updateResult(internals.diagnoses.errExpectingATEXT);
                            }

                            parseData.local += token;
                            atomData.locals[elementCount] += token;
                            ++elementLength;
                        }
                }

                break;

            case internals.components.domain:
                // http://tools.ietf.org/html/rfc5322#section-3.4.1
                //   domain          =   dot-atom / domain-literal / obs-domain
                //
                //   dot-atom        =   [CFWS] dot-atom-text [CFWS]
                //
                //   dot-atom-text   =   1*atext *("." 1*atext)
                //
                //   domain-literal  =   [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]
                //
                //   dtext           =   %d33-90 /          ; Printable US-ASCII
                //                       %d94-126 /         ;  characters not including
                //                       obs-dtext          ;  "[", "]", or "\"
                //
                //   obs-domain      =   atom *("." atom)
                //
                //   atom            =   [CFWS] 1*atext [CFWS]

                // http://tools.ietf.org/html/rfc5321#section-4.1.2
                //   Mailbox        = Local-part "@" ( Domain / address-literal )
                //
                //   Domain         = sub-domain *("." sub-domain)
                //
                //   address-literal  = "[" ( IPv4-address-literal /
                //                    IPv6-address-literal /
                //                    General-address-literal ) "]"
                //                    ; See Section 4.1.3

                // http://tools.ietf.org/html/rfc5322#section-3.4.1
                //      Note: A liberal syntax for the domain portion of addr-spec is
                //      given here.  However, the domain portion contains addressing
                //      information specified by and used in other protocols (e.g.,
                //      [RFC1034], [RFC1035], [RFC1123], [RFC5321]).  It is therefore
                //      incumbent upon implementations to conform to the syntax of
                //      addresses for the context in which they are used.
                //
                // is_email() author's note: it's not clear how to interpret this in
                // he context of a general email address validator. The conclusion I
                // have reached is this: "addressing information" must comply with
                // RFC 5321 (and in turn RFC 1035), anything that is "semantically
                // invisible" must comply only with RFC 5322.
                switch (token) {
                    // Comment
                    case '(':
                        if (elementLength === 0) {
                            // Comments at the start of the domain are deprecated in the text, comments at the start of a subdomain are obs-domain
                            // http://tools.ietf.org/html/rfc5322#section-3.4.1
                            updateResult(elementCount === 0 ? internals.diagnoses.deprecatedCFWSNearAt : internals.diagnoses.deprecatedComment);
                        }
                        else {
                            // We can't start a comment mid-element, better be at the end
                            assertEnd = true;
                            updateResult(internals.diagnoses.cfwsComment);
                        }

                        context.stack.push(context.now);
                        context.now = internals.components.contextComment;
                        break;

                        // Next dot-atom element
                    case '.':
                        if (elementLength === 0) {
                            // Another dot, already? Fatal error.
                            updateResult(elementCount === 0 ? internals.diagnoses.errDotStart : internals.diagnoses.errConsecutiveDots);
                        }
                        else if (hyphenFlag) {
                            // Previous subdomain ended in a hyphen. Fatal error.
                            updateResult(internals.diagnoses.errDomainHyphenEnd);
                        }
                        else if (elementLength > 63) {
                            // Nowhere in RFC 5321 does it say explicitly that the domain part of a Mailbox must be a valid domain according to the
                            // DNS standards set out in RFC 1035, but this *is* implied in several places. For instance, wherever the idea of host
                            // routing is discussed the RFC says that the domain must be looked up in the DNS. This would be nonsense unless the
                            // domain was designed to be a valid DNS domain. Hence we must conclude that the RFC 1035 restriction on label length
                            // also applies to RFC 5321 domains.
                            //
                            // http://tools.ietf.org/html/rfc1035#section-2.3.4
                            // labels          63 octets or less

                            updateResult(internals.diagnoses.rfc5322LabelTooLong);
                        }

                        // CFWS is OK again now we're at the beginning of an element (although
                        // it may be obsolete CFWS)
                        assertEnd = false;
                        elementLength = 0;
                        ++elementCount;
                        atomData.domains[elementCount] = '';
                        parseData.domain += token;

                        break;

                        // Domain literal
                    case '[':
                        if (parseData.domain.length === 0) {
                            // Domain literal must be the only component
                            assertEnd = true;
                            ++elementLength;
                            context.stack.push(context.now);
                            context.now = internals.components.literal;
                            parseData.domain += token;
                            atomData.domains[elementCount] += token;
                            parseData.literal = '';
                        }
                        else {
                            // Fatal error
                            updateResult(internals.diagnoses.errExpectingATEXT);
                        }

                        break;

                        // Folding white space
                    case '\r':
                        if (emailLength === ++i || email[i] !== '\n') {
                            // Fatal error
                            updateResult(internals.diagnoses.errCRNoLF);
                            break;
                        }

                        // Fallthrough

                    case ' ':
                    case '\t':
                        if (elementLength === 0) {
                            updateResult(elementCount === 0 ? internals.diagnoses.deprecatedCFWSNearAt : internals.diagnoses.deprecatedFWS);
                        }
                        else {
                            // We can't start FWS in the middle of an element, so this better be the end
                            updateResult(internals.diagnoses.cfwsFWS);
                            assertEnd = true;
                        }

                        context.stack.push(context.now);
                        context.now = internals.components.contextFWS;
                        prevToken = token;
                        break;

                        // This must be ATEXT
                    default:
                        // RFC 5322 allows any atext...
                        // http://tools.ietf.org/html/rfc5322#section-3.2.3
                        //    atext = ALPHA / DIGIT / ; Printable US-ASCII
                        //            "!" / "#" /     ;  characters not including
                        //            "$" / "%" /     ;  specials.  Used for atoms.
                        //            "&" / "'" /
                        //            "*" / "+" /
                        //            "-" / "/" /
                        //            "=" / "?" /
                        //            "^" / "_" /
                        //            "`" / "{" /
                        //            "|" / "}" /
                        //            "~"

                        // But RFC 5321 only allows letter-digit-hyphen to comply with DNS rules
                        //   (RFCs 1034 & 1123)
                        // http://tools.ietf.org/html/rfc5321#section-4.1.2
                        //   sub-domain     = Let-dig [Ldh-str]
                        //
                        //   Let-dig        = ALPHA / DIGIT
                        //
                        //   Ldh-str        = *( ALPHA / DIGIT / "-" ) Let-dig
                        //
                        if (assertEnd) {
                            // We have encountered ATEXT where it is no longer valid
                            switch (context.prev) {
                                case internals.components.contextComment:
                                case internals.components.contextFWS:
                                    updateResult(internals.diagnoses.errATEXTAfterCFWS);
                                    break;

                                case internals.components.literal:
                                    updateResult(internals.diagnoses.errATEXTAfterDomainLiteral);
                                    break;

                                    // $lab:coverage:off$
                                default:
                                    throw new Error('more atext found where none is allowed, but unrecognized prev context: ' + context.prev);
                                    // $lab:coverage:on$
                            }
                        }

                        charCode = token.charCodeAt(0);
                        // Assume this token isn't a hyphen unless we discover it is
                        hyphenFlag = false;

                        if (charCode < 33 || charCode > 126 || internals.specials(charCode)) {
                            // Fatal error
                            updateResult(internals.diagnoses.errExpectingATEXT);
                        }
                        else if (token === '-') {
                            if (elementLength === 0) {
                                // Hyphens cannot be at the beginning of a subdomain, fatal error
                                updateResult(internals.diagnoses.errDomainHyphenStart);
                            }

                            hyphenFlag = true;
                        }
                            // Check if it's a neither a number nor a latin letter
                        else if (charCode < 48 || charCode > 122 || (charCode > 57 && charCode < 65) || (charCode > 90 && charCode < 97)) {
                            // This is not an RFC 5321 subdomain, but still OK by RFC 5322
                            updateResult(internals.diagnoses.rfc5322Domain);
                        }

                        parseData.domain += token;
                        atomData.domains[elementCount] += token;
                        ++elementLength;
                }

                break;

                // Domain literal
            case internals.components.literal:
                // http://tools.ietf.org/html/rfc5322#section-3.4.1
                //   domain-literal  =   [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]
                //
                //   dtext           =   %d33-90 /          ; Printable US-ASCII
                //                       %d94-126 /         ;  characters not including
                //                       obs-dtext          ;  "[", "]", or "\"
                //
                //   obs-dtext       =   obs-NO-WS-CTL / quoted-pair
                switch (token) {
                    // End of domain literal
                    case ']':
                        if (maxResult < internals.categories.deprecated) {
                            // Could be a valid RFC 5321 address literal, so let's check

                            // http://tools.ietf.org/html/rfc5321#section-4.1.2
                            //   address-literal  = "[" ( IPv4-address-literal /
                            //                    IPv6-address-literal /
                            //                    General-address-literal ) "]"
                            //                    ; See Section 4.1.3
                            //
                            // http://tools.ietf.org/html/rfc5321#section-4.1.3
                            //   IPv4-address-literal  = Snum 3("."  Snum)
                            //
                            //   IPv6-address-literal  = "IPv6:" IPv6-addr
                            //
                            //   General-address-literal  = Standardized-tag ":" 1*dcontent
                            //
                            //   Standardized-tag  = Ldh-str
                            //                     ; Standardized-tag MUST be specified in a
                            //                     ; Standards-Track RFC and registered with IANA
                            //
                            //   dcontent      = %d33-90 / ; Printable US-ASCII
                            //                 %d94-126 ; excl. "[", "\", "]"
                            //
                            //   Snum          = 1*3DIGIT
                            //                 ; representing a decimal integer
                            //                 ; value in the range 0 through 255
                            //
                            //   IPv6-addr     = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp
                            //
                            //   IPv6-hex      = 1*4HEXDIG
                            //
                            //   IPv6-full     = IPv6-hex 7(":" IPv6-hex)
                            //
                            //   IPv6-comp     = [IPv6-hex *5(":" IPv6-hex)] "::"
                            //                 [IPv6-hex *5(":" IPv6-hex)]
                            //                 ; The "::" represents at least 2 16-bit groups of
                            //                 ; zeros.  No more than 6 groups in addition to the
                            //                 ; "::" may be present.
                            //
                            //   IPv6v4-full   = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal
                            //
                            //   IPv6v4-comp   = [IPv6-hex *3(":" IPv6-hex)] "::"
                            //                 [IPv6-hex *3(":" IPv6-hex) ":"]
                            //                 IPv4-address-literal
                            //                 ; The "::" represents at least 2 16-bit groups of
                            //                 ; zeros.  No more than 4 groups in addition to the
                            //                 ; "::" and IPv4-address-literal may be present.

                            let index = -1;
                            let addressLiteral = parseData.literal;
                            const matchesIP = internals.regex.ipV4.exec(addressLiteral);

                            // Maybe extract IPv4 part from the end of the address-literal
                            if (matchesIP) {
                                index = matchesIP.index;
                                if (index !== 0) {
                                    // Convert IPv4 part to IPv6 format for futher testing
                                    addressLiteral = addressLiteral.slice(0, index) + '0:0';
                                }
                            }

                            if (index === 0) {
                                // Nothing there except a valid IPv4 address, so...
                                updateResult(internals.diagnoses.rfc5321AddressLiteral);
                            }
                            else if (addressLiteral.slice(0, 5).toLowerCase() !== 'ipv6:') {
                                updateResult(internals.diagnoses.rfc5322DomainLiteral);
                            }
                            else {
                                const match = addressLiteral.slice(5);
                                let maxGroups = internals.maxIPv6Groups;
                                const groups = match.split(':');
                                index = match.indexOf('::');

                                if (!~index) {
                                    // Need exactly the right number of groups
                                    if (groups.length !== maxGroups) {
                                        updateResult(internals.diagnoses.rfc5322IPv6GroupCount);
                                    }
                                }
                                else if (index !== match.lastIndexOf('::')) {
                                    updateResult(internals.diagnoses.rfc5322IPv62x2xColon);
                                }
                                else {
                                    if (index === 0 || index === match.length - 2) {
                                        // RFC 4291 allows :: at the start or end of an address with 7 other groups in addition
                                        ++maxGroups;
                                    }

                                    if (groups.length > maxGroups) {
                                        updateResult(internals.diagnoses.rfc5322IPv6MaxGroups);
                                    }
                                    else if (groups.length === maxGroups) {
                                        // Eliding a single "::"
                                        updateResult(internals.diagnoses.deprecatedIPv6);
                                    }
                                }

                                // IPv6 testing strategy
                                if (match[0] === ':' && match[1] !== ':') {
                                    updateResult(internals.diagnoses.rfc5322IPv6ColonStart);
                                }
                                else if (match[match.length - 1] === ':' && match[match.length - 2] !== ':') {
                                    updateResult(internals.diagnoses.rfc5322IPv6ColonEnd);
                                }
                                else if (internals.checkIpV6(groups)) {
                                    updateResult(internals.diagnoses.rfc5321AddressLiteral);
                                }
                                else {
                                    updateResult(internals.diagnoses.rfc5322IPv6BadCharacter);
                                }
                            }
                        }
                        else {
                            updateResult(internals.diagnoses.rfc5322DomainLiteral);
                        }

                        parseData.domain += token;
                        atomData.domains[elementCount] += token;
                        ++elementLength;
                        context.prev = context.now;
                        context.now = context.stack.pop();
                        break;

                    case '\\':
                        updateResult(internals.diagnoses.rfc5322DomainLiteralOBSDText);
                        context.stack.push(context.now);
                        context.now = internals.components.contextQuotedPair;
                        break;

                        // Folding white space
                    case '\r':
                        if (emailLength === ++i || email[i] !== '\n') {
                            updateResult(internals.diagnoses.errCRNoLF);
                            break;
                        }

                        // Fallthrough

                    case ' ':
                    case '\t':
                        updateResult(internals.diagnoses.cfwsFWS);

                        context.stack.push(context.now);
                        context.now = internals.components.contextFWS;
                        prevToken = token;
                        break;

                        // DTEXT
                    default:
                        // http://tools.ietf.org/html/rfc5322#section-3.4.1
                        //   dtext         =   %d33-90 /  ; Printable US-ASCII
                        //                     %d94-126 / ;  characters not including
                        //                     obs-dtext  ;  "[", "]", or "\"
                        //
                        //   obs-dtext     =   obs-NO-WS-CTL / quoted-pair
                        //
                        //   obs-NO-WS-CTL =   %d1-8 /    ; US-ASCII control
                        //                     %d11 /     ;  characters that do not
                        //                     %d12 /     ;  include the carriage
                        //                     %d14-31 /  ;  return, line feed, and
                        //                     %d127      ;  white space characters
                        charCode = token.charCodeAt(0);

                        // '\r', '\n', ' ', and '\t' have already been parsed above
                        if (charCode > 127 || charCode === 0 || token === '[') {
                            // Fatal error
                            updateResult(internals.diagnoses.errExpectingDTEXT);
                            break;
                        }
                        else if (charCode < 33 || charCode === 127) {
                            updateResult(internals.diagnoses.rfc5322DomainLiteralOBSDText);
                        }

                        parseData.literal += token;
                        parseData.domain += token;
                        atomData.domains[elementCount] += token;
                        ++elementLength;
                }

                break;

                // Quoted string
            case internals.components.contextQuotedString:
                // http://tools.ietf.org/html/rfc5322#section-3.2.4
                //   quoted-string = [CFWS]
                //                   DQUOTE *([FWS] qcontent) [FWS] DQUOTE
                //                   [CFWS]
                //
                //   qcontent      = qtext / quoted-pair
                switch (token) {
                    // Quoted pair
                    case '\\':
                        context.stack.push(context.now);
                        context.now = internals.components.contextQuotedPair;
                        break;

                        // Folding white space. Spaces are allowed as regular characters inside a quoted string - it's only FWS if we include '\t' or '\r\n'
                    case '\r':
                        if (emailLength === ++i || email[i] !== '\n') {
                            // Fatal error
                            updateResult(internals.diagnoses.errCRNoLF);
                            break;
                        }

                        // Fallthrough

                    case '\t':
                        // http://tools.ietf.org/html/rfc5322#section-3.2.2
                        //   Runs of FWS, comment, or CFWS that occur between lexical tokens in
                        //   a structured header field are semantically interpreted as a single
                        //   space character.

                        // http://tools.ietf.org/html/rfc5322#section-3.2.4
                        //   the CRLF in any FWS/CFWS that appears within the quoted-string [is]
                        //   semantically "invisible" and therefore not part of the
                        //   quoted-string

                        parseData.local += ' ';
                        atomData.locals[elementCount] += ' ';
                        ++elementLength;

                        updateResult(internals.diagnoses.cfwsFWS);
                        context.stack.push(context.now);
                        context.now = internals.components.contextFWS;
                        prevToken = token;
                        break;

                        // End of quoted string
                    case '"':
                        parseData.local += token;
                        atomData.locals[elementCount] += token;
                        ++elementLength;
                        context.prev = context.now;
                        context.now = context.stack.pop();
                        break;

                        // QTEXT
                    default:
                        // http://tools.ietf.org/html/rfc5322#section-3.2.4
                        //   qtext          =   %d33 /             ; Printable US-ASCII
                        //                      %d35-91 /          ;  characters not including
                        //                      %d93-126 /         ;  "\" or the quote character
                        //                      obs-qtext
                        //
                        //   obs-qtext      =   obs-NO-WS-CTL
                        //
                        //   obs-NO-WS-CTL  =   %d1-8 /            ; US-ASCII control
                        //                      %d11 /             ;  characters that do not
                        //                      %d12 /             ;  include the carriage
                        //                      %d14-31 /          ;  return, line feed, and
                        //                      %d127              ;  white space characters
                        charCode = token.charCodeAt(0);

                        if (charCode > 127 || charCode === 0 || charCode === 10) {
                            updateResult(internals.diagnoses.errExpectingQTEXT);
                        }
                        else if (charCode < 32 || charCode === 127) {
                            updateResult(internals.diagnoses.deprecatedQTEXT);
                        }

                        parseData.local += token;
                        atomData.locals[elementCount] += token;
                        ++elementLength;
                }

                // http://tools.ietf.org/html/rfc5322#section-3.4.1
                //   If the string can be represented as a dot-atom (that is, it contains
                //   no characters other than atext characters or "." surrounded by atext
                //   characters), then the dot-atom form SHOULD be used and the quoted-
                //   string form SHOULD NOT be used.

                break;
                // Quoted pair
            case internals.components.contextQuotedPair:
                // http://tools.ietf.org/html/rfc5322#section-3.2.1
                //   quoted-pair     =   ("\" (VCHAR / WSP)) / obs-qp
                //
                //   VCHAR           =  %d33-126   ; visible (printing) characters
                //   WSP             =  SP / HTAB  ; white space
                //
                //   obs-qp          =   "\" (%d0 / obs-NO-WS-CTL / LF / CR)
                //
                //   obs-NO-WS-CTL   =   %d1-8 /   ; US-ASCII control
                //                       %d11 /    ;  characters that do not
                //                       %d12 /    ;  include the carriage
                //                       %d14-31 / ;  return, line feed, and
                //                       %d127     ;  white space characters
                //
                // i.e. obs-qp       =  "\" (%d0-8, %d10-31 / %d127)
                charCode = token.charCodeAt(0);

                if (charCode > 127) {
                    // Fatal error
                    updateResult(internals.diagnoses.errExpectingQPair);
                }
                else if ((charCode < 31 && charCode !== 9) || charCode === 127) {
                    // ' ' and '\t' are allowed
                    updateResult(internals.diagnoses.deprecatedQP);
                }

                // At this point we know where this qpair occurred so we could check to see if the character actually needed to be quoted at all.
                // http://tools.ietf.org/html/rfc5321#section-4.1.2
                //   the sending system SHOULD transmit the form that uses the minimum quoting possible.

                context.prev = context.now;
                // End of qpair
                context.now = context.stack.pop();
                token = '\\' + token;

                switch (context.now) {
                    case internals.components.contextComment:
                        break;

                    case internals.components.contextQuotedString:
                        parseData.local += token;
                        atomData.locals[elementCount] += token;

                        // The maximum sizes specified by RFC 5321 are octet counts, so we must include the backslash
                        elementLength += 2;
                        break;

                    case internals.components.literal:
                        parseData.domain += token;
                        atomData.domains[elementCount] += token;

                        // The maximum sizes specified by RFC 5321 are octet counts, so we must include the backslash
                        elementLength += 2;
                        break;

                        // $lab:coverage:off$
                    default:
                        throw new Error('quoted pair logic invoked in an invalid context: ' + context.now);
                        // $lab:coverage:on$
                }
                break;

                // Comment
            case internals.components.contextComment:
                // http://tools.ietf.org/html/rfc5322#section-3.2.2
                //   comment  = "(" *([FWS] ccontent) [FWS] ")"
                //
                //   ccontent = ctext / quoted-pair / comment
                switch (token) {
                    // Nested comment
                    case '(':
                        // Nested comments are ok
                        context.stack.push(context.now);
                        context.now = internals.components.contextComment;
                        break;

                        // End of comment
                    case ')':
                        context.prev = context.now;
                        context.now = context.stack.pop();
                        break;

                        // Quoted pair
                    case '\\':
                        context.stack.push(context.now);
                        context.now = internals.components.contextQuotedPair;
                        break;

                        // Folding white space
                    case '\r':
                        if (emailLength === ++i || email[i] !== '\n') {
                            // Fatal error
                            updateResult(internals.diagnoses.errCRNoLF);
                            break;
                        }

                        // Fallthrough

                    case ' ':
                    case '\t':
                        updateResult(internals.diagnoses.cfwsFWS);

                        context.stack.push(context.now);
                        context.now = internals.components.contextFWS;
                        prevToken = token;
                        break;

                        // CTEXT
                    default:
                        // http://tools.ietf.org/html/rfc5322#section-3.2.3
                        //   ctext         = %d33-39 /  ; Printable US-ASCII
                        //                   %d42-91 /  ;  characters not including
                        //                   %d93-126 / ;  "(", ")", or "\"
                        //                   obs-ctext
                        //
                        //   obs-ctext     = obs-NO-WS-CTL
                        //
                        //   obs-NO-WS-CTL = %d1-8 /    ; US-ASCII control
                        //                   %d11 /     ;  characters that do not
                        //                   %d12 /     ;  include the carriage
                        //                   %d14-31 /  ;  return, line feed, and
                        //                   %d127      ;  white space characters
                        charCode = token.charCodeAt(0);

                        if (charCode > 127 || charCode === 0 || charCode === 10) {
                            // Fatal error
                            updateResult(internals.diagnoses.errExpectingCTEXT);
                            break;
                        }
                        else if (charCode < 32 || charCode === 127) {
                            updateResult(internals.diagnoses.deprecatedCTEXT);
                        }
                }

                break;

                // Folding white space
            case internals.components.contextFWS:
                // http://tools.ietf.org/html/rfc5322#section-3.2.2
                //   FWS     =   ([*WSP CRLF] 1*WSP) /  obs-FWS
                //                                   ; Folding white space

                // But note the erratum:
                // http://www.rfc-editor.org/errata_search.php?rfc=5322&eid=1908:
                //   In the obsolete syntax, any amount of folding white space MAY be
                //   inserted where the obs-FWS rule is allowed.  This creates the
                //   possibility of having two consecutive "folds" in a line, and
                //   therefore the possibility that a line which makes up a folded header
                //   field could be composed entirely of white space.
                //
                //   obs-FWS =   1*([CRLF] WSP)

                if (prevToken === '\r') {
                    if (token === '\r') {
                        // Fatal error
                        updateResult(internals.diagnoses.errFWSCRLFx2);
                        break;
                    }

                    if (++crlfCount > 1) {
                        // Multiple folds => obsolete FWS
                        updateResult(internals.diagnoses.deprecatedFWS);
                    }
                    else {
                        crlfCount = 1;
                    }
                }

                switch (token) {
                    case '\r':
                        if (emailLength === ++i || email[i] !== '\n') {
                            // Fatal error
                            updateResult(internals.diagnoses.errCRNoLF);
                        }

                        break;

                    case ' ':
                    case '\t':
                        break;

                    default:
                        if (prevToken === '\r') {
                            // Fatal error
                            updateResult(internals.diagnoses.errFWSCRLFEnd);
                        }

                        crlfCount = 0;

                        // End of FWS
                        context.prev = context.now;
                        context.now = context.stack.pop();

                        // Look at this token again in the parent context
                        --i;
                }

                prevToken = token;
                break;

                // Unexpected context
                // $lab:coverage:off$
            default:
                throw new Error('unknown context: ' + context.now);
                // $lab:coverage:on$
        } // Primary state machine

        if (maxResult > internals.categories.rfc5322) {
            // Fatal error, no point continuing
            break;
        }
    } // Token loop

    // Check for errors
    if (maxResult < internals.categories.rfc5322) {
        // Fatal errors
        if (context.now === internals.components.contextQuotedString) {
            updateResult(internals.diagnoses.errUnclosedQuotedString);
        }
        else if (context.now === internals.components.contextQuotedPair) {
            updateResult(internals.diagnoses.errBackslashEnd);
        }
        else if (context.now === internals.components.contextComment) {
            updateResult(internals.diagnoses.errUnclosedComment);
        }
        else if (context.now === internals.components.literal) {
            updateResult(internals.diagnoses.errUnclosedDomainLiteral);
        }
        else if (token === '\r') {
            updateResult(internals.diagnoses.errFWSCRLFEnd);
        }
        else if (parseData.domain.length === 0) {
            updateResult(internals.diagnoses.errNoDomain);
        }
        else if (elementLength === 0) {
            updateResult(internals.diagnoses.errDotEnd);
        }
        else if (hyphenFlag) {
            updateResult(internals.diagnoses.errDomainHyphenEnd);
        }

            // Other errors
        else if (parseData.domain.length > 255) {
            // http://tools.ietf.org/html/rfc5321#section-4.5.3.1.2
            //   The maximum total length of a domain name or number is 255 octets.
            updateResult(internals.diagnoses.rfc5322DomainTooLong);
        }
        else if (parseData.local.length + parseData.domain.length + /* '@' */ 1 > 254) {
            // http://tools.ietf.org/html/rfc5321#section-4.1.2
            //   Forward-path   = Path
            //
            //   Path           = "<" [ A-d-l ":" ] Mailbox ">"
            //
            // http://tools.ietf.org/html/rfc5321#section-4.5.3.1.3
            //   The maximum total length of a reverse-path or forward-path is 256 octets (including the punctuation and element separators).
            //
            // Thus, even without (obsolete) routing information, the Mailbox can only be 254 characters long. This is confirmed by this verified
            // erratum to RFC 3696:
            //
            // http://www.rfc-editor.org/errata_search.php?rfc=3696&eid=1690
            //   However, there is a restriction in RFC 2821 on the length of an address in MAIL and RCPT commands of 254 characters.  Since
            //   addresses that do not fit in those fields are not normally useful, the upper limit on address lengths should normally be considered
            //   to be 254.
            updateResult(internals.diagnoses.rfc5322TooLong);
        }
        else if (elementLength > 63) {
            // http://tools.ietf.org/html/rfc1035#section-2.3.4
            // labels   63 octets or less
            updateResult(internals.diagnoses.rfc5322LabelTooLong);
        }
        else if (options.minDomainAtoms && atomData.domains.length < options.minDomainAtoms) {
            updateResult(internals.diagnoses.errDomainTooShort);
        }
        else if (options.tldWhitelist || options.tldBlacklist) {
            const tldAtom = atomData.domains[elementCount];

            if (!internals.validDomain(tldAtom, options)) {
                updateResult(internals.diagnoses.errUnknownTLD);
            }
        }
    } // Check for errors

    let dnsPositive = false;
    let finishImmediately = false;

    const finish = () => {

        if (!dnsPositive && maxResult < internals.categories.dnsWarn) {
            // Per RFC 5321, domain atoms are limited to letter-digit-hyphen, so we only need to check code <= 57 to check for a digit
            const code = atomData.domains[elementCount].charCodeAt(0);
            if (code <= 57) {
                updateResult(internals.diagnoses.rfc5321TLDNumeric);
            }
            else if (elementCount === 0) {
                updateResult(internals.diagnoses.rfc5321TLD);
            }
        }

        if (maxResult < threshold) {
            maxResult = internals.diagnoses.valid;
        }

        const finishResult = diagnose ? maxResult : maxResult < internals.defaultThreshold;

        if (callback) {
            if (finishImmediately) {
                callback(finishResult);
            }
            else {
                internals.defer(callback.bind(null, finishResult));
            }
        }

        return finishResult;
    }; // Finish

    if (options.checkDNS && maxResult < internals.categories.dnsWarn) {
        // http://tools.ietf.org/html/rfc5321#section-2.3.5
        //   Names that can be resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed in Section 5) are permitted, as are CNAME RRs whose
        //   targets can be resolved, in turn, to MX or address RRs.
        //
        // http://tools.ietf.org/html/rfc5321#section-5.1
        //   The lookup first attempts to locate an MX record associated with the name.  If a CNAME record is found, the resulting name is processed
        //   as if it were the initial name. ... If an empty list of MXs is returned, the address is treated as if it was associated with an implicit
        //   MX RR, with a preference of 0, pointing to that host.
        //
        // isEmail() author's note: We will regard the existence of a CNAME to be sufficient evidence of the domain's existence. For performance
        // reasons we will not repeat the DNS lookup for the CNAME's target, but we will raise a warning because we didn't immediately find an MX
        // record.
        if (elementCount === 0) {
            // Checking TLD DNS only works if you explicitly check from the root
            parseData.domain += '.';
        }

        const dnsDomain = parseData.domain;
        Dns.resolveMx(dnsDomain, (err, mxRecords) => {

            // If we have a fatal error, then we must assume that there are no records
            if (err && err.code !== Dns.NODATA) {
                updateResult(internals.diagnoses.dnsWarnNoRecord);
                return finish();
            }

            if (mxRecords && mxRecords.length) {
                dnsPositive = true;
                return finish();
            }

            let count = 3;
            let done = false;
            updateResult(internals.diagnoses.dnsWarnNoMXRecord);

            const handleRecords = (ignoreError, records) => {

                if (done) {
                    return;
                }

                --count;

                if (records && records.length) {
                    done = true;
                    return finish();
                }

                if (count === 0) {
                    // No usable records for the domain can be found
                    updateResult(internals.diagnoses.dnsWarnNoRecord);
                    done = true;
                    finish();
                }
            };

            Dns.resolveCname(dnsDomain, handleRecords);
            Dns.resolve4(dnsDomain, handleRecords);
            Dns.resolve6(dnsDomain, handleRecords);
        });

        finishImmediately = true;
    }
    else {
        const result = finish();
        finishImmediately = true;
        return result;
    } // CheckDNS
};


exports.diagnoses = internals.validate.diagnoses = (function () {

    const diag = {};
    const keys = Object.keys(internals.diagnoses);
    for (let i = 0; i < keys.length; ++i) {
        const key = keys[i];
        diag[key] = internals.diagnoses[key];
    }

    return diag;
})();