Problem B for qualification round of Google Code Jam 2014

problem_b.lisp

#!/usr/bin/sbcl --script

(setf *read-default-float-format* 'double-float)

(defun calculate-time (c f x)
  (labels ((calculate-time-helper (c f x n time-spent)
             (let* ((time-with-current (/ x (+ (* n f) 2)))
                    (time-for-farm (/ c (+ (* n f) 2)))
                    (time-with-farm (+ time-for-farm
                                       (/ x (+ (* (1+ n) f) 2)))))
               (if (> time-with-farm time-with-current)
                   (+ time-spent time-with-current)
                   (calculate-time-helper c f x (1+ n) (+ time-spent
                                                          time-for-farm))))))
    (calculate-time-helper c f x 0 0)))

(with-open-file (in (cadr *posix-argv*) :direction :input)
  (with-open-file (out "output" :direction :output :if-exists :supersede)
    (loop with cases = (read in)
          for case from 1 upto cases
          for c = (read in)
          for f = (read in)
          for x = (read in)
          do (format out "Case #~D: ~7$~%" case (calculate-time c f x)))))