macher.rb

require "test/unit"

class CountryMatcherTest < Test::Unit::TestCase
  def test_valid_case
    result = CountryMatcher.call("DE") do |match|
      match.us { "US" }
      match.de { "DE" }
      match.gb { "GB" }
    end

    assert_equal result, "DE"
  end

  def test_valid_case_with_nil_result
    result = CountryMatcher.call("DE") do |match|
      match.us { "US" }
      match.de {}
      match.gb { "GB" }
    end

    assert_equal result, nil
  end

  def test_multiple_definitions
    result = proc do
      CountryMatcher.call("DE") do |match|
        match.us { "US" }
        match.de { "DE" }
        match.de { "DE" }
        match.gb { "GB" }
      end
    end

    assert_raise(CountryMatcher::MultipleDefinitionError) { result.call }
  end

  def test_missing_definition
    result = proc do
      CountryMatcher.call("DE") do |match|
        match.de { "DE" }
        match.gb { "GB" }
      end
    end

    assert_raise(CountryMatcher::MissingDefinitionError) { result.call }
  end

  def test_unknown_country
    result = proc do
      CountryMatcher.call("XX") do |match|
        match.de { "DE" }
        match.us { "US" }
        match.gb { "GB" }
      end
    end

    assert_raise(CountryMatcher::UnknownValueError) { result.call }
  end

  def test_unknown_case
    result = proc do
      CountryMatcher.call("DE") do |match|
        match.xx { "XX" }
        match.de { "DE" }
        match.us { "US" }
        match.gb { "GB" }
      end
    end

    assert_raise(NoMethodError) { result.call }
  end
end

class CountryMatcher
  COUNTRIES = ["DE", "GB", "US"]

  class MultipleDefinitionError < StandardError; end
  class MissingDefinitionError < StandardError; end
  class UnknownValueError < StandardError; end

  class Match
    def initialize(country)
      @country = country
      @calls = []
    end

    COUNTRIES.each do |country|
      define_method country.downcase do |&block|
        if @calls.include?(country)
          raise MultipleDefinitionError, country.inspect
        end

        @calls << country
        @result = block.call if @country == country
      end
    end

    def verify_and_return
      difference.then do |missing|
        if missing.any?
          raise MissingDefinitionError, missing.inspect
        end
      end

      if !COUNTRIES.include?(@country)
        raise UnknownValueError, @country.inspect
      end

      @result
    end

    private

    def difference
      COUNTRIES.difference(@calls) +
        @calls.difference(COUNTRIES)
    end
  end

  def self.call(*args, **kwargs, &block)
    new(*args, **kwargs).call(&block)
  end

  def initialize(value)
    @value = value
  end

  def call
    Match.new(@value).then do |match|
      yield match

      match.verify_and_return
    end
  end
end