Project Euler #14 and memoization in Clojure
- by dbyrne
As a neophyte clojurian, it was recommended to me that I go through the Project Euler problems as a way to learn the language. Its definitely a great way to improve your skills and gain confidence. I just finished up my answer to problem #14. It works fine, but to get it running efficiently I had to implement some memoization. I couldn't use the prepackaged memoize function because of the way my code was structured, and I think it was a good experience to roll my own anyways. My question is if there is a good way to encapsulate my cache within the function itself, or if I have to define an external cache like I have done. Also, any tips to make my code more idiomatic would be appreciated.
(use 'clojure.test)
(def mem (atom {}))
(with-test
(defn chain-length
([x] (chain-length x x 0))
([start-val x c]
(if-let [e (last(find @mem x))]
(let [ret (+ c e)]
(swap! mem assoc start-val ret)
ret)
(if (<= x 1)
(let [ret (+ c 1)]
(swap! mem assoc start-val ret)
ret)
(if (even? x)
(recur start-val (/ x 2) (+ c 1))
(recur start-val (+ 1 (* x 3)) (+ c 1)))))))
(is (= 10 (chain-length 13))))
(with-test
(defn longest-chain
([] (longest-chain 2 0 0))
([c max start-num]
(if (>= c 1000000)
start-num
(let [l (chain-length c)]
(if (> l max)
(recur (+ 1 c) l c)
(recur (+ 1 c) max start-num))))))
(is (= 837799 (longest-chain))))
