Problem 14 on project Euler is to find the number under 1,000,000 that
is the start of the longest Collatz sequence. My solution is below,
basically it uses a hash map to cache the lengths of collatz
sequences. But when I get to 113383 the collatz function just bounces
between the numbers 1, 2, and 4. Those numbers seem to be in the map
but contains? just keeps returning false.
(ns euler.problem14)
(def lengths (atom {1 0}))
(defn collatz [input]
(loop [current input accum 0]
(when (= input 113383) (println current accum (filter #(< % 10)
(keys @lengths))))
(if (contains? @lengths current)
(do
(swap! lengths #(assoc % input (+ accum (get % current))))
(get @lengths input))
(if (zero? (mod current 2))
(recur (/ current 2) (inc accum))
(recur (inc (* 3 current)) (inc accum))))))
(time (println (last (sort-by #(second %) (for [x (range 1 1000000)]
(do (println x) [x (collatz x)]))))))
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