To calculate the Euclidean distance, let's first create a vector from our dictionary and document. This will allow us to easily compare the term frequencies between documents because they will occupy the same index of the vector.

(defn term-id [dict term]
  (get-in @dict [:terms term]))

(defn term-frequencies [dict terms]
  (->> (map #(term-id dict %) terms)
       (remove nil?)
       (frequencies)))

(defn map->vector [dictionary id-counts]
  (let [zeros (vec (replicate (:count @dictionary) 0))]
    (-> (reduce #(apply assoc! %1 %2) (transient zeros) id-counts)
        (persistent!))))

(defn tf-vector [dict document]
  (map->vector dict (term-frequencies dict document)))

The term-frequencies function creates a map of term ID to frequency count for each term in the document. The map->vector function simply takes this map and associates the frequency count at the index of the vector given by the term ID. Since there may be many terms, and...