(defn $rollup
"Returns a dataset that uses the given summary function (or function identifier keyword)
 to rollup the given column based on a set of group-by columns. The summary function
 should accept a single sequence of values and return a single summary value. Alternatively,
 you can provide a keyword identifer of a set of built-in functions including:
   
   :max -- the maximum value of the data in each group
   :min -- the minimum value of the data in each group
   :sum -- the sum of the data in each group
   :count -- the number of elements in each group
   :mean -- the mean of the data in each group
   

  Like the other '$' dataset functions, $rollup will use the dataset bound to $data
  (see the with-data macro) if a dataset is not provided as an argument.

  Examples:

    (use '(incanter core datasets))

    (def iris (get-dataset :iris))
    ($rollup :mean :Sepal.Length :Species iris)
    ($rollup :count :Sepal.Length :Species iris)
    ($rollup :max :Sepal.Length :Species iris)
    ($rollup :min :Sepal.Length :Species iris)
    
    ;; The following is an example using a custom function, but since all the 
    ;; iris measurements are positive, the built-in mean function could have 
    ;; been used instead.

    (use 'incanter.stats)
    ($rollup #(mean (abs %)) :Sepal.Width :Species iris)

    ($rollup sd :Sepal.Length :Species iris)
    ($rollup variance :Sepal.Length :Species iris)
    ($rollup median :Sepal.Length :Species iris)

    (def hair-eye-color (get-dataset :hair-eye-color))
    ($rollup :mean :count [:hair :eye] hair-eye-color)

    (use 'incanter.charts)
    (with-data ($rollup :mean :Sepal.Length :Species iris)
      (view (bar-chart :Species :Sepal.Length)))

     ;; the following exaples use the built-in data set called hair-eye-color.

     (with-data ($rollup :mean :count [:hair :eye] hair-eye-color)
       (view (bar-chart :hair :count :group-by :eye :legend true)))

     (with-data (->>  (get-dataset :hair-eye-color)
                      ($where {:hair {:in #{\"brown\" \"blond\"}}})
                      ($rollup :sum :count [:hair :eye])
                      ($order :count :desc))
       (view $data)
       (view (bar-chart :hair :count :group-by :eye :legend true)))


"
  ([summary-fun col-name group-by]
     ($rollup summary-fun col-name group-by $data))
  ([summary-fun col-name group-by data]
     (let [key-fn (if (coll? col-name) 
		    (fn [row] 
		      (into [] (map #(map-get row %) col-name)))
		    (fn [row] 
		      (map-get row col-name)))
	   rows (:rows data)
	   rollup-fns {:max (fn [col-data] (apply max col-data))
		       :min (fn [col-data] (apply min col-data))
		       :sum (fn [col-data] (apply + col-data))
		       :count count
		       :mean (fn [col-data] (/ (apply + col-data) (count col-data)))}
	   rollup-fn (if (keyword? summary-fun)
		       (rollup-fns summary-fun)
		       summary-fun)]
       (loop [cur rows reduced-rows {}]
	 (if (empty? cur)
	   (let [group-cols (to-dataset (keys reduced-rows))
		 res (conj-cols group-cols (map rollup-fn (vals reduced-rows)))]
	     (col-names res (concat (col-names group-cols)
				    (if (coll? col-name) col-name [col-name]))))
	   (recur (next cur)
		  (let [row (first cur)
			k (submap row group-by)
			a (reduced-rows k)
			b (key-fn row)]
		    (assoc reduced-rows k (if a (conj a b) [b])))))))))