Add conversion methods for country codes

README.md

@@ -264,6 +264,14 @@ ISO3166::Country.collect_likely_countries_by_subdivision_name("San José",:commo
  => ["Costa Rica", "Uruguay"]
 ```
 
+### Conversions
+
+```ruby
+ISO3166::Country.from_alpha3_to_alpha2('USA') # => "US"
+ISO3166::Country.from_alpha2_to_alpha3('US') # => "USA"
+
+ISO3166::Country.from_alpha2_to_alpha3('--') # => nil
+```
 
 ## Currencies
 

lib/countries/country.rb

@@ -3,6 +3,7 @@
 module ISO3166
   class Country
     extend CountryClassMethods
+    extend ConversionMethods
     extend CountryFinderMethods
     include Emoji
     include CountrySubdivisionMethods

lib/countries/country/conversion_methods.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+module ISO3166
+  module ConversionMethods
+    # @param alpha2 [String] ISO 3166 alpha-2 country code.
+    # @return [String] ISO 3166 alpha-3 country code.
+    def from_alpha2_to_alpha3(alpha2)
+      find_country_by_alpha2(alpha2)&.alpha3
+    end
+
+    # @param alpha3 [String] ISO 3166 alpha-3 country code.
+    # @return [String] ISO 3166 alpha-2 country code.
+    def from_alpha3_to_alpha2(alpha3)
+      find_country_by_alpha3(alpha3)&.alpha2
+    end
+  end
+end

lib/countries/iso3166.rb

@@ -11,6 +11,7 @@
 require 'countries/translations'
 require 'countries/country/country_subdivision_methods'
 require 'countries/country/class_methods'
+require 'countries/country/conversion_methods'
 require 'countries/country/finder_methods'
 require 'countries/country/emoji'
 require 'countries/country'

spec/country_spec.rb

@@ -664,6 +664,40 @@
     end
   end
 
+  describe 'conversion methods' do
+    describe 'from_alpha2_to_alpha3' do
+      subject { ISO3166::Country.from_alpha2_to_alpha3(alpha2) }
+
+      context 'when country exists' do
+        let(:alpha2) { 'US' }
+
+        it { is_expected.to eq('USA') }
+      end
+
+      context 'when country does not exist' do
+        let(:alpha2) { '..' }
+
+        it { is_expected.to be_nil }
+      end
+    end
+
+    describe 'from_alpha3_to_alpha2' do
+      subject { ISO3166::Country.from_alpha3_to_alpha2(alpha3) }
+
+      context 'when country exists' do
+        let(:alpha3) { 'USA' }
+
+        it { is_expected.to eq('US') }
+      end
+
+      context 'when country does not exist' do
+        let(:alpha3) { '...' }
+
+        it { is_expected.to be_nil }
+      end
+    end
+  end
+
   describe 'hash finder methods' do
     context "when search name in 'iso_short_name'" do
       subject { ISO3166::Country.find_by_iso_short_name('Poland') }