Round-tripping from LinkedHashMaps to Clojure collections

Question

I'm trying to implement round-tripping from LinkedHashMaps to Clojure collections. The implementation below works for smaller collections, but larger collections raise OutOfMemoryError: GC overhead limit exceeded. Below is a generative test that passes, when the number of tests is small (~10), but exhausts memory for larger numbers of tests (~100).

I would appreciate any tips how to reduce the memory requirements of the implementation as well as general tips on how to improve the code's quality.

(import '[clojure.lang IPersistentMap IPersistentVector]
        '[java.util ArrayList LinkedHashMap])

(defprotocol LinkedHashMappable
  "Convert Clojure data structure to LinkedHashMap."
  (->linked-hash-map [data]))

(extend-protocol LinkedHashMappable
  IPersistentMap
  (->linked-hash-map [m]
    (let [lhm (LinkedHashMap.)]
      (doseq [[k v] m]
        (.put lhm
              (if (keyword? k) (name k) k)
              (->linked-hash-map v)))
      lhm))

  IPersistentVector
  (->linked-hash-map [v]
    (let [alist (ArrayList.)]
      (doseq [i v] (.add alist (->linked-hash-map i)))
      alist))

  Object
  (->linked-hash-map [o] o))

(defprotocol Clojurizable
  "Convert LinkedHashMap to Clojure data structure."
  (->clj [lhm]))

(extend-protocol Clojurizable
  LinkedHashMap
  (->clj [lhm]
    (reduce (fn [m [k v]]
              (assoc m (keyword k) (->clj v)))
            {}
            (iterator-seq (.. lhm entrySet iterator))))

  ArrayList
  (->clj [alist]
    (mapv ->clj alist))

  Object
  (->clj [o] o))

; ----- Test -----

(require '[clojure.test.check :as tc]
         '[clojure.test.check.generators :as gen]
         '[clojure.test.check.properties :as prop])

(def map-generator
  (gen/recursive-gen (fn [inner]
                       (gen/map gen/keyword (gen/one-of [(gen/map gen/keyword inner)
                                                         (gen/vector inner)])))
                     gen/string))

(def linked-hash-map-round-tripping
  (prop/for-all [m map-generator]
                (= (->clj (->linked-hash-map m)) m)))

; The test passes for smaller numbers of tests (~10), but exhausts memory for larger numbers (~100).
#_(tc/quick-check 100 linked-hash-map-round-tripping)

Answer 1

I think automatically creating keywords from string keys and string keys from keywords is not a good idea, for three reasons.

1. It would be surprising to a user to have values change classes when theoretically just pouring the contents of one map into another.
2. In the space of possible key transformations, keyword<-->string is just one point. As a user, I might want to do many other kinds of transformations while converting. For a more general approach, think about how transducers can be supplied to the four-argument form of into.
3. Keywords are interned. This may be a source of your memory trouble. Every new keyword gets stored permanently in the heap. That's how Clojure does fast keyword comparison by just checking object references. That means a property-based test that generates huge numbers of unique keywords is going to consume a lot of memory.