clouncil.clj

(ns the-clouncil)

(def posts
  (sort-by :date (read-string (slurp "posts.edn"))))

(->> posts
     (mapcat (fn [{:keys [categories] :as post}]
               (map (fn [category]
                      [category post])
                    categories)))
     (reduce (fn [acc [category post]]
               (update acc category #(conj % post)))
             {}))

(reduce
 (fn [acc {:keys [categories] :as post}]
   (reduce
    (fn [acc category]
      (update acc category conj post))
    acc
    categories))
 {}
 posts)

(update-vals
 (group-by
  first
  (for [post posts
        category (:categories post)]
    [category post]))
 #(map second %))