import type { Nullable } from '../utils';
/**
 * Transforms a domain name into a canonical domain name. The canonical domain name is a domain name
 * that has been trimmed, lowercased, stripped of leading dot, and optionally punycode-encoded
 * ({@link https://www.rfc-editor.org/rfc/rfc6265.html#section-5.1.2 | Section 5.1.2 of RFC 6265}). For
 * the most part, this function is idempotent (calling the function with the output from a previous call
 * returns the same output).
 *
 * @remarks
 * A canonicalized host name is the string generated by the following
 * algorithm:
 *
 * 1.  Convert the host name to a sequence of individual domain name
 *     labels.
 *
 * 2.  Convert each label that is not a Non-Reserved LDH (NR-LDH) label,
 *     to an A-label (see Section 2.3.2.1 of [RFC5890] for the former
 *     and latter), or to a "punycode label" (a label resulting from the
 *     "ToASCII" conversion in Section 4 of [RFC3490]), as appropriate
 *     (see Section 6.3 of this specification).
 *
 * 3.  Concatenate the resulting labels, separated by a %x2E (".")
 *     character.
 *
 * @example
 * ```
 * canonicalDomain('.EXAMPLE.com') === 'example.com'
 * ```
 *
 * @param domainName - the domain name to generate the canonical domain from
 * @public
 */
export declare function canonicalDomain(domainName: Nullable<string>): string | undefined;