;;; -*- lexical-binding: t -*-
;;;
;;; this file:
;;;   http://user.it.uu.se/~embe8573/emacs-init/my-math.el
;;;   https://dataswamp.org/~incal/emacs-init/my-math.el

(defun mean-value (vs)
  (let*((sum  (apply #'+ vs))
        (mean (/ sum (length vs) 1.0)) )
    mean) )

(defun hypotenuse (c1 c2)
  (sqrt (+ (* c1 c1) (* c2 c2))) )

(defun speed (hour minute second km)
  (let*((s (+ second (* 60 minute) (* 60 60 hour)))
        (m (* 1000 km))
        (mps (/ m s)) )
    (* 3.6 mps) ))
;; (speed 04 44 10  42.195) ;  8.909208211143694
;; (speed 02 01 39  42.195) ; 20.811344019728732
;; (speed 19 04 00 168    ) ;  7.2

(defun tau-ceti (speed)
  (let*((dist       (* 11.8 9.46 (expt 10 12)))
        (time       (/ dist speed) )
        (days       (/ time 24))
        (whole-days (round days)) )
    (format-seconds "%y years"
                    (- (float-time (encode-time 0 0 0 whole-days 0 0))
                       (float-time (encode-time 0 0 0 0 0 0)) ))))
;; (tau-ceti 5000)