Data Driven

module Raindrops
  DROPS = {3 => "Pling", 5 => "Plang", 7 => "Plong"}
  def self.convert(number : Int)
    DROPS.join { |k, v| v if number.divisible_by?(k) }.presence || number.to_s
  end
end

Using a data-driven approach to create logic to solve the raindrops problem, makes the solution both concise and flexible. If the rules change, it is as easy as to change the rules. The solution starts by creating a Hash with the rules. Then it uses Enumerable#join used on the newly created Hash. The join method will iterate over the Hash and join the values together as a String. The join method takes a block, which feds the key and value of the Hash. Then if the number is divisible by the key, it will return the value, otherwise, it will return nil.

The result of the join method is then fed to String#presence. It will return the String if it is not empty, otherwise, it will return nil. Then the || operator is used to return the number as a String if the result is nil, otherwise it will return the result.