counts2.clj

(reduce
   (fn [res counts]
     (merge-with + res counts))
   (pmap (fn [[start end]]
           (count-area-codes 
             (io/read-lines-from-file-segment inp-file start end)))
         (partition 2 1
           (io/byte-partitions-at-line-boundaries
             inp-file
             (* 1024 1024)))))