(defn sample
" Returns a sample of the given size from the given collection. If replacement
  is set to false it returns a set, otherwise it returns a list.

  Arguments:
    x -- collection to be sampled from

  Options:
    :size -- (default (count x) sample size
    :replacement (default true) -- sample with replacement


  Examples:
    (sample (range 10)) ; permutation of numbers zero through ten
    (sample [:red :green :blue] :size 10) ; choose 10 items that are either :red, :green, or :blue.
    (sample (seq \"abcdefghijklmnopqrstuvwxyz\")  :size 4 :replacement false) ; choose 4 random letters.

"
  ([x & options]
    (let [opts (when options (apply assoc {} options))
          size (or (:size opts) (count x))
          replacement? (if (false? (:replacement opts)) false true)
          max-idx (dec (count x))]
      (if (= size 1)
        (nth x (rand-int (inc max-idx)))
        (if replacement?
          (map #(nth x %) (sample-uniform size :min 0 :max max-idx :integers true))
          (if (> size (count x))
            (throw (Exception. "'size' can't be larger than (count x) without replacement!"))
            (map #(nth x %)
                 (loop [samp-indices [] indices-set #{}]
                   (if (= (count samp-indices) size)
                     samp-indices
                     (let [i (sample-uniform 1 :min 0 :max max-idx :integers true)]
                       (if (contains? indices-set i)
                         (recur samp-indices indices-set)
                         (recur (conj samp-indices i) (conj indices-set i)))))))))))))