haversine.clj

(ns haversine
  (:require [math.numeric-tower :refer [expt sqrt]]))

(def RADIUS_OF_EARTH 6371.0088)

(defn radians [degrees]
  (/ (* degrees Math/PI) 180))

(defn haversine
  [lat1 lon1 lat2 lon2]
  (let [[lon1 lat1 lon2 lat2] (map radians [lon1 lat1 lon2 lat2])
        lng (- lon2 lon1)
        lat (- lat2 lat1)
        a (+ (expt (Math/sin (* lat 1/2)) 2)
             (* (Math/cos lat1)
                (Math/cos lat2)
                (expt (Math/sin (/ lng 2)) 2)))
        c (* 2 (Math/asin (sqrt a)))]
    (* 1000 (* c RADIUS_OF_EARTH))))                        ; meters