calculates the location of a destination point

README.md

@@ -25,6 +25,8 @@ Crystal implementation of the [Haversine formula](https://en.wikipedia.org/wiki/
 require "haversine"
 ```
 
+### Distance
+
 Calling `Haversine.distance` with four latitude/longitude coordinates returns a `Haversine::Distance` object which can provide output in kilometers, meters, miles, feet, or nautical miles.
 
 Each "coordinates" member **must** be a pair of coordinates - `latitude` and `longitude`.
@@ -79,6 +81,16 @@ distance2 = Haversine.distance(london, shanghai)
 distance1 < distance2 # => true
 ```
 
+### Destination
+
+Takes the starting point by `latitude`, `longitude` and calculates the location of a destination point
+given a `distance` factor in degrees, radians, miles, or kilometers; and `bearing` in degrees.
+
+```crystal
+Haversine.destination(39, -75, 5000, 90, :kilometers)
+# => {26.440010707631124, -22.885355549364313}
+```
+
 ## Contributing
 
 1. Fork it (<https://github.com/geocrystal/haversine/fork>)

spec/haversine_spec.cr

@@ -31,4 +31,12 @@ describe Haversine do
       it { dist.to_feet.should eq(18275860.669896744) }
     end
   end
+
+  describe ".destination" do
+    describe "calculates the location of a destination point" do
+      it { Haversine.destination(39, -75, 5000, 90, :kilometers).should eq({26.440010707631124, -22.885355549364313}) }
+      it { Haversine.destination([39, -75], 5000, 90, :kilometers).should eq({26.440010707631124, -22.885355549364313}) }
+      it { Haversine.destination({39, -75}, 5000, 90, :kilometers).should eq({26.440010707631124, -22.885355549364313}) }
+    end
+  end
 end

src/haversine.cr

@@ -1,22 +1,46 @@
-require "./haversine/*"
-
 # The haversine formula determines the great-circle distance between two points on a sphere
 # given their latitudes and longitudes.
 #
 # https://en.wikipedia.org/wiki/Haversine_formula
 module Haversine
   extend self
 
-  alias Number = Int32 | Float32 | Float64
+  EARTH_RADIUS = 6371008.8
 
-  RAD_PER_DEG = Math::PI / 180
+  # Unit of measurement factors using a spherical (non-ellipsoid) earth radius.
+  #
+  # Keys are the name of the unit, values are the number of that unit in a single radians
+  FACTORS = {
+    centimeters:    EARTH_RADIUS * 100,
+    centimetres:    EARTH_RADIUS * 100,
+    degrees:        360 / (2 * Math::PI),
+    feet:           EARTH_RADIUS * 3.28084,
+    inches:         EARTH_RADIUS * 39.37,
+    kilometers:     EARTH_RADIUS / 1000,
+    kilometres:     EARTH_RADIUS / 1000,
+    meters:         EARTH_RADIUS,
+    metres:         EARTH_RADIUS,
+    miles:          EARTH_RADIUS / 1609.344,
+    millimeters:    EARTH_RADIUS * 1000,
+    millimetres:    EARTH_RADIUS * 1000,
+    nautical_miles: EARTH_RADIUS / 1852,
+    radians:        1,
+    yards:          EARTH_RADIUS * 1.0936,
+  }
+
+  alias Number = Int32 | Float32 | Float64
 
   # Calculates the haversine distance between two locations using latitude and longitude.
   def distance(lat1 : Number, lon1 : Number, lat2 : Number, lon2 : Number) : Haversine::Distance
-    dlon = lon2 - lon1
-    dlat = lat2 - lat1
+    dlon = to_radians(lon2 - lon1)
+    dlat = to_radians(lat2 - lat1)
+    lat1 = to_radians(lat1)
+    lat2 = to_radians(lat2)
+
+    a =
+      Math.sin(dlat / 2) ** 2 +
+        (Math.sin(dlon / 2) ** 2) * Math.cos(lat1) * Math.cos(lat2)
 
-    a = calc(dlat, lat1, lat2, dlon)
     c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a))
 
     Haversine::Distance.new(c)
@@ -38,11 +62,55 @@ module Haversine
     distance(lat1, lon1, lat2, lon2)
   end
 
-  private def calc(dlat : Number, lat1 : Number, lat2 : Number, dlon : Number) : Number
-    (Math.sin(rpd(dlat) / 2)) ** 2 + Math.cos(rpd(lat1)) * Math.cos((rpd(lat2))) * (Math.sin(rpd(dlon) / 2)) ** 2
+  # Takes the staring point by `latitude`, `longitude` and calculates the location of a destination point
+  # given a `distance` factor in `Haversine::FACTORS`; and `bearing` in degrees(ranging from -180 to 180).
+  #
+  # https://github.com/Turfjs/turf/blob/master/packages/turf-destination/index.ts
+  def destination(latitude : Number, longitude : Number, distance : Number, bearing : Number, unit : Symbol = :kilometers) : Tuple(Float64, Float64)
+    factor = FACTORS[unit]
+
+    radians = distance / factor
+    bearing_rad = to_radians(bearing)
+
+    latitude1 = to_radians(latitude)
+    longitude1 = to_radians(longitude)
+
+    latitude2 = Math.asin(
+      Math.sin(latitude1) * Math.cos(radians) +
+      Math.cos(latitude1) * Math.sin(radians) * Math.cos(bearing_rad)
+    )
+
+    longitude2 =
+      longitude1 +
+        Math.atan2(
+          Math.sin(bearing_rad) * Math.sin(radians) * Math.cos(latitude1),
+          Math.cos(radians) - Math.sin(latitude1) * Math.sin(latitude2)
+        )
+
+    {to_degrees(latitude2), to_degrees(longitude2)}
   end
 
-  private def rpd(num : Number) : Number
-    num * RAD_PER_DEG
+  # :ditto:
+  def destination(coord : Array(Number), distance : Number, bearing : Number, unit : Symbol = :kilometers) : Tuple(Float64, Float64)
+    latitude, longitude = coord
+
+    destination(latitude, longitude, distance, bearing, unit)
+  end
+
+  # :ditto:
+  def destination(coord : Tuple(Number, Number), distance : Number, bearing : Number, unit : Symbol = :kilometers) : Tuple(Float64, Float64)
+    latitude, longitude = coord
+
+    destination(latitude, longitude, distance, bearing, unit)
+  end
+
+  private def to_radians(degrees : Number) : Number
+    degrees * Math::PI / 180.0
+  end
+
+  private def to_degrees(radians : Number) : Number
+    radians * 180.0 / Math::PI
   end
 end
+
+require "./haversine/*"

src/haversine/distance.cr

@@ -2,37 +2,14 @@ module Haversine
   class Distance
     include Comparable(self)
 
-    EARTH_RADIUS = 6371008.8
-
-    # Unit of measurement factors using a spherical (non-ellipsoid) earth radius.
-    #
-    # Keys are the name of the unit, values are the number of that unit in a single radians
-    FACTORS = {
-      centimeters:    EARTH_RADIUS * 100,
-      centimetres:    EARTH_RADIUS * 100,
-      degrees:        360 / (2 * Math::PI),
-      feet:           EARTH_RADIUS * 3.28084,
-      inches:         EARTH_RADIUS * 39.37,
-      kilometers:     EARTH_RADIUS / 1000,
-      kilometres:     EARTH_RADIUS / 1000,
-      meters:         EARTH_RADIUS,
-      metres:         EARTH_RADIUS,
-      miles:          EARTH_RADIUS / 1609.344,
-      millimeters:    EARTH_RADIUS * 1000,
-      millimetres:    EARTH_RADIUS * 1000,
-      nautical_miles: EARTH_RADIUS / 1852,
-      radians:        1,
-      yards:          EARTH_RADIUS * 1.0936,
-    }
-
     property distance
 
     def initialize(@distance : Number)
     end
 
-    {% for factor in FACTORS.keys %}
+    {% for factor in Haversine::FACTORS.keys %}
       def to_{{factor.id}} : Number
-        @distance * FACTORS[:{{factor.id}}]
+        @distance * Haversine::FACTORS[:{{factor.id}}]
       end
     {% end %}