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))))