chrisvest-4clojure-solution63.clj

;; chrisvest's solution to http://4clojure.com/problem/63

(fn [f coll]
  (reduce (fn [m x]
            (let [k (f x)]
              (update-in m [k] #(conj (or % []) x))))
          {} coll))

#(F f, Seq coll) (
  reduce(
    #(Map m, Object x) {
      Object k = f.call(x);
      updateIn(m, list(k), #(y) (conj(coalesce(y, list()), x)));
      return m;
    },
    new HashMap(),
    coll)
)

#(F f, Seq coll) {
  reduce(
    #(Map m, Object x) {
      Object k = f.call(x);
      updateIn(m, list(k), #(y) {conj(coalesce(y, list()), x)});
      return m;
    },
    new HashMap(),
    coll)
}

{F f, Seq coll ->
  reduce(
    {Map m, Object x ->
      Object k = f.call(x);
      updateIn(m, list(k), {y -> conj(coalesce(y, list()), x)});
      return m;
    },
    new HashMap(),
    coll)
}

#{F f, Seq coll ->
  reduce(
    #{Map m, Object x ->
      Object k = f.call(x);
      updateIn(m, list(k), #{y -> conj(coalesce(y, list()), x)});
      return m;
    },
    new HashMap(),
    coll)
}

(F f, Seq coll) -> {
  reduce(
    (Map m, Object x) -> {
      Object k = f.call(x);
      updateIn(m, list(k), (y) -> {conj(coalesce(y, list()), x)});
      return m;
    },
    new HashMap(),
    coll)
}