/** The focus keywords of a post are stored comma-separated, the first one first. */

export const KEYWORDS_MAX = 5;

export const parseKeywords = (value: string): string[] =>
  value
    .split(',')
    .map(keyword => keyword.trim())
    .filter(keyword => keyword !== '');

/**
 * The stored list with a new keyword leading it: the old first one goes,
 * the secondary ones stay (minus the newcomer, if it was among them), and
 * the list keeps to its five.
 */
export const leadKeyword = (stored: string, keyword: string): string => {
  const lead = keyword.trim();
  if (lead === '') return stored;
  const rest = parseKeywords(stored)
    .slice(1)
    .filter(entry => entry.toLowerCase() !== lead.toLowerCase());
  return [lead, ...rest].slice(0, KEYWORDS_MAX).join(', ');
};

/**
 * The stored list with one of its keywords moved to the front: the others
 * keep their order behind it.
 */
export const promoteKeyword = (stored: string, index: number): string => {
  const list = parseKeywords(stored);
  if (index <= 0 || index >= list.length) return list.join(', ');
  return [list[index], ...list.filter((_, at) => at !== index)].join(', ');
};