(ns advent-2020-15)

(def seed [1,2,16,19,18,0])

(defn next-num [[data [i n]]]
  [(assoc data n i)
   [(inc i)
    (- i (data n i))]])

(defn nth-num [seed n]
  (->> [(into {} (map vector (butlast seed) (range)))
        [(dec (count seed)) (last seed)]]
       (iterate next-num)
       (map second)
       (drop-while #(< (first %) (dec n)))
       first))

;; part 1
(nth-num seed 2020)

;; part 2
(nth-num seed 30000000)