My naive 2 minutes solution : (reduce #(assoc %1 %2 (inc (get %1 %2 0))) {} "abcaaabcccaabrrcb") EDIT: While it's shorter and (in

My naive 2 minutes solution : (reduce #(assoc %1 %2 (inc (get %1 %2 0))) {} "abcaaabcccaabrrcb") EDIT: While it's shorter and (in my opinion) a lot clearer than the OP's solution, it's also consistently faster. Functionnal code and composition is cool, but i think the author got trapped into making his code beautifull, or maybe just into the wrong algorithm for the job. It happened to me before in clojure and other functional languages