Natural Number Additions in Ruby

natural.rb

class NaturalNumberAdditions
   
    def self.for num
        NaturalNumberAdditions.new.calc(num)
    end

    def calc num
        return [] if num == 1
        pairs_for(num) | previous_additions_plus_one(num)
    end

    def pairs_for num
        pairs = []
        (1..num).each do |lhs|
            (1..num).each do |rhs|
                next if lhs < rhs
                pairs << "#{lhs} + #{rhs}" if lhs + rhs == num
            end
        end

        pairs
    end

    def previous_additions_plus_one num
        NaturalNumberAdditions.for(num - 1).map { |previous| "#{previous} + 1" }
    end
end

(1..10).each do |num|
    puts "#{num} is #{NaturalNumberAdditions.for(num).join(', ')}"
end

# produces
# 1 is
# 2 is 1 + 1
# 3 is 2 + 1, 1 + 1 + 1
# 4 is 2 + 2, 3 + 1, 2 + 1 + 1, 1 + 1 + 1 + 1
# 5 is 3 + 2, 4 + 1, 2 + 2 + 1, 3 + 1 + 1, 2 + 1 + 1 + 1, 1 + 1 + 1 + 1 + 1
# 6 is 3 + 3, 4 + 2, 5 + 1, 3 + 2 + 1, 4 + 1 + 1, 2 + 2 + 1 + 1, 3 + 1 + 1 + 1, 2 + 1 + 1 + 1 + 1, 1 + 1 + 1 + 1 + 1 + 1
# ....