/**

* Validate a topic to see if it's valid or not.

* Topics can support Multi-level wildcards (‘#’ U+0023) and single-level wildcards (‘+’ U+002B).

*

* The multi-level wildcard character MUST be specified either on its own or following a topic level separator.

* In either case it MUST be the last character specified in the Topic Filter.

*

* The single-level wildcard character MUST occupy an entire level of the filter.

* For example: "+" is valid, "+/tennis/#" is valid, "sport+" is not valid.

*

* Topics beginning with $: The Server MUST NOT match Topic Filters starting with a wildcard character with Topic Names

*

* Topic names and topic filters must not include the null character (Unicode U+0000)

*

* Topic names and Topic Filters are UTF-8 encoded strings; they must not encode to more than 65,535 bytes.

*

* @param {String} topic - A topic

* @returns {Boolean} If the topic is valid, returns true. Otherwise, returns false.

*/

export function validateTopic(topic: string) {

// Topic must be at least 1 character.

if (topic.length === 0) {

return false;

}

const levels: string[] = topic.split('/');

for (const [i, level] of levels.entries()) {

// If SLWC, MUST occupy entire level.

if (level === '+') {

continue;

}

if (level === '#') {

// Validate MLWC at end of topic filter.

return i === levels.length - 1;

}

// Level must not contain more than one MLWC or SLWC character.

if (level.includes('+') || level.includes('#')) {

return false;

}

}

return true;

}

/**

* Validate an array of topics to see if any of them is valid or not

* @param {Array} topics - Array of topics

* @returns {String} If the topics is valid, returns null. Otherwise, returns the invalid one

*/

export function validateTopics(topics: Array<string>) {

if (topics.length === 0) {

return 'empty_topic_list';

}

for (let i = 0; i < topics.length; i++) {

if (!validateTopic(topics[i] as string)) {

return topics[i];

}

}

return null;

}