/** A variable the site knows, as the "Insert variable" menu lists it. */
interface KnownVariable {
  name: string;
}

/** Variables that differ from one page to the next. */
const PER_PAGE =
  /%(title|excerpt|focuskw|category|primary_category|date|author|term|term_description|search_query|customfield\([^)]*\)|customterm\([^)]*\))%/i;

/**
 * Templates that print one page each, and the variables that tell those
 * pages apart; without one, every page of the kind prints the same text.
 */
const NEEDS: { field: RegExp; kind: string; variables: string[] }[] = [
  {
    field: /^pt_.+(?<!_archive)_title$/,
    kind: 'page of this type',
    variables: ['%title%'],
  },
  { field: /^tax_.+_title$/, kind: 'term', variables: ['%term%', '%title%'] },
  { field: /^author_title$/, kind: 'author', variables: ['%author%'] },
  {
    field: /^date_archive_title$/,
    kind: 'date archive',
    variables: ['%date%'],
  },
  { field: /^search_title$/, kind: 'search', variables: ['%search_query%'] },
];

/** Descriptions of one page each, which read the page in some way. */
const DESCRIPTIONS = /^(pt_.+(?<!_archive)_description|tax_.+_description)$/;

/**
 * What is wrong with a title or description template before it is saved:
 * a variable the site does not know prints nothing, and a template with
 * nothing of the page in it gives every page the same title, which search
 * engines treat as duplicates.
 *
 * @param {string}          field     - The option key, e.g. "tax_category_title".
 * @param {string}          template  - What is typed.
 * @param {KnownVariable[]} variables - The variables the site knows.
 * @returns {string[]} The warnings, in the order to read them.
 */
export const templateWarnings = (
  field: string,
  template: string,
  variables: KnownVariable[]
): string[] => {
  const warnings: string[] = [];
  const typed = template.trim();
  if (typed === '') return warnings;

  const known = (token: string): boolean => {
    const base = token.replace(/\(.*\)%$/, '');
    return variables.some(
      variable =>
        variable.name === token ||
        variable.name.startsWith(`${base}(`) ||
        variable.name === `${base}%`
    );
  };
  const unknown = Array.from(
    new Set(Array.from(typed.matchAll(/%[a-z_]+(?:\([^)]*\))?%/gi), m => m[0]))
  ).filter(token => !known(token));
  if (unknown.length > 0) {
    warnings.push(
      `${unknown.join(', ')} ${unknown.length === 1 ? 'is not a variable' : 'are not variables'} this site knows, so ${unknown.length === 1 ? 'it prints' : 'they print'} nothing.`
    );
  }

  const need = NEEDS.find(rule => rule.field.test(field));
  if (
    need &&
    !need.variables.some(variable => typed.toLowerCase().includes(variable))
  ) {
    warnings.push(
      `Every ${need.kind} gets this same title: add ${need.variables.join(' or ')} so each one reads as its own page.`
    );
  } else if (DESCRIPTIONS.test(field) && !PER_PAGE.test(typed)) {
    warnings.push(
      'Every page gets this same description: add %excerpt%, %title% or another variable of the page.'
    );
  }
  return warnings;
};