gistfile1.clj

(ns encounters.geo
  (:require [geohash.core :as geohash]))

(def ^:dynamic *geohash-precision* 1)

(def ^:dynamic *radius-km* 6371.009)

(defn great-circle-distance
  ([lat lon lat' lon']
     (great-circle-distance lat lon lat' lon' *radius-km*))
  ([lat lon lat' lon' radius]
     (let [[lat-r lon-r lat-r' lon-r']
           (map #(Math/toRadians %) [lat lon lat' lon'])]
       (* radius
          (Math/acos (+ (* (Math/sin lat-r) (Math/sin lat-r'))
                        (* (Math/cos lat-r)
                           (Math/cos lat-r')
                           (Math/cos (- lon-r lon-r')))))))))

(def ^:dynamic *percentage-margin* 0.05)

(defn get-geohashes
  [lat lon]
  (let [gh (geohash/encode lat lon *geohash-precision*)
        {:keys [minlon minlat maxlon maxlat]} (geohash/decode-bbox gh)
        
        percentage-north (/ (great-circle-distance lat lon maxlat lon)
                            (great-circle-distance minlat lon maxlat lon))
        percentage-east (/ (great-circle-distance lat lon lat maxlon)
                           (great-circle-distance lat minlon lat maxlon))
        neighbors #{}
        neighbors (if (<= percentage-north *percentage-margin*)
                    (conj neighbors :north)
                    (if (<= (- 1 percentage-north) *percentage-margin*)
                      (conj neighbors :south)
                      neighbors))
        neighbors (if (<= percentage-east *percentage-margin*)
                    (conj neighbors :east)
                    (if (<= (- 1 percentage-east) *percentage-margin*)
                      (conj neighbors :west)
                      neighbors))]
    neighbors))