const fs = require('fs');
const path = require('path');

function addTag(tag, articleId) {
  const dir = path.join('./tags_index', tag);
  fs.mkdirSync(dir, { recursive: true });
  fs.writeFileSync(path.join(dir, articleId), '1');
}

function removeTag(tag, articleId) {
  const filePath = path.join('./tags_index', tag, articleId);
  if (fs.existsSync(filePath)) fs.unlinkSync(filePath);
}

function getArticlesForTag(tag) {
  const dir = path.join('./tags_index', tag);
  return fs.existsSync(dir) ? fs.readdirSync(dir) : [];
}

function getArticlesByTags(tags) {
  return tags.reduce((acc, tag) => {
    const articles = new Set(getArticlesForTag(tag));
    return acc === null ? articles : new Set([...acc].filter(x => articles.has(x)));
  }, null);
}