Enable equivalence and hash equality to Observer (#57)

An observer can be seen as a value object. If all the characteristics of
an observer are equal to those of another, they can be considered as the
same.

This change implements `#==` (and its alias `#eql?`) and `#hash` to
implement value object behaviours.

lib/astronoby/observer.rb

@@ -10,7 +10,7 @@ class Observer
     MOLAR_MASS_OF_AIR = 0.0289644
     UNIVERSAL_GAS_CONSTANT = 8.31432
 
-    attr_reader :latitude, :longitude, :elevation, :temperature
+    attr_reader :latitude, :longitude, :elevation, :temperature, :pressure
 
     # @param latitude [Angle] geographic latitude of the observer
     # @param longitude [Angle] geographic longitude of the observer
@@ -31,19 +31,38 @@ def initialize(
       @longitude = longitude
       @elevation = elevation
       @temperature = temperature
-      @pressure = pressure
+      @pressure = pressure || compute_pressure
     end
 
-    # Compute an estimation of the atmospheric pressure based on the elevation
-    # and temperature
-    #
-    # @return [Float] the atmospheric pressure in millibars.
-    def pressure
-      @pressure ||= PRESSURE_AT_SEA_LEVEL * pressure_ratio
+    def ==(other)
+      return false unless other.is_a?(self.class)
+
+      @latitude == other.latitude &&
+        @longitude == other.longitude &&
+        @elevation == other.elevation &&
+        @temperature == other.temperature &&
+        @pressure == other.pressure
+    end
+    alias_method :eql?, :==
+
+    def hash
+      [
+        self.class,
+        @latitude,
+        @longitude,
+        @elevation,
+        @temperature,
+        @pressure
+      ].hash
     end
 
     private
 
+    # @return [Float] the atmospheric pressure in millibars.
+    def compute_pressure
+      @pressure ||= PRESSURE_AT_SEA_LEVEL * pressure_ratio
+    end
+
     # Source:
     # Barometric formula
     # https://en.wikipedia.org/wiki/Barometric_formula

spec/astronoby/observer_spec.rb

@@ -1,6 +1,90 @@
 # frozen_string_literal: true
 
+require "set" # rubocop:disable Lint/RedundantRequireStatement
+
 RSpec.describe Astronoby::Observer do
+  describe "equivalence (#==)" do
+    it "returns true when the observers are equivalent" do
+      observer1 = described_class.new(
+        latitude: Astronoby::Angle.from_degrees(45),
+        longitude: Astronoby::Angle.from_degrees(90),
+        elevation: 100,
+        temperature: 280,
+        pressure: 10
+      )
+      observer2 = described_class.new(
+        latitude: Astronoby::Angle.from_degrees(45),
+        longitude: Astronoby::Angle.from_degrees(90),
+        elevation: 100,
+        temperature: 280,
+        pressure: 10
+      )
+
+      expect(observer1).to eq observer2
+    end
+
+    it "returns true when the observers are not equivalent" do
+      observer1 = described_class.new(
+        latitude: Astronoby::Angle.from_degrees(45),
+        longitude: Astronoby::Angle.from_degrees(90),
+        elevation: 100,
+        temperature: 280,
+        pressure: 10
+      )
+      observer2 = described_class.new(
+        latitude: Astronoby::Angle.from_degrees(45),
+        longitude: Astronoby::Angle.from_degrees(90),
+        elevation: 100,
+        temperature: 280,
+        pressure: 15
+      )
+
+      expect(observer1).not_to eq observer2
+    end
+  end
+
+  describe "hash equality" do
+    it "makes an angle foundable as a Hash key" do
+      observer1 = described_class.new(
+        latitude: Astronoby::Angle.from_degrees(45),
+        longitude: Astronoby::Angle.from_degrees(90),
+        elevation: 100,
+        temperature: 280,
+        pressure: 10
+      )
+      observer2 = described_class.new(
+        latitude: Astronoby::Angle.from_degrees(45),
+        longitude: Astronoby::Angle.from_degrees(90),
+        elevation: 100,
+        temperature: 280,
+        pressure: 10
+      )
+      map = {observer1 => :observer}
+
+      expect(map[observer2]).to eq :observer
+    end
+
+    it "makes an angle foundable in a Set" do
+      observer1 = described_class.new(
+        latitude: Astronoby::Angle.from_degrees(45),
+        longitude: Astronoby::Angle.from_degrees(90),
+        elevation: 100,
+        temperature: 280,
+        pressure: 10
+      )
+      observer2 = described_class.new(
+        latitude: Astronoby::Angle.from_degrees(45),
+        longitude: Astronoby::Angle.from_degrees(90),
+        elevation: 100,
+        temperature: 280,
+        pressure: 10
+      )
+      set = Set.new([observer1])
+
+      expect(set.include?(observer2)).to be true
+    end
+  end
+
   describe "#pressure" do
     it "returns the computed pression in millibars" do
       latitude = Astronoby::Angle.from_degrees(0)