Implement merge method in ruby

Question

I'm trying to implement merge method in my code.

A = { "a" => 200, "b" => 100 }
B = { "b" => 100, "c" => 300 }

So when I call A.merge_method(B) in my main function, it should return

A.merge_method(B) #=> {"a"=>200, "b"=>200, "c"=>300}

How can I implement without using merge method?

Answer 1

Here is the general idea: collect all the keys of any hashes to be merged, then for each of the keys, collect the values in all the hashes that have that key, and sum them.

module HashWithMergeReduce
  refine Hash do
    def merge_reduce(*others, &op)
      hashes = [self, *others]
      hash_keys = hashes.map(&:keys).inject(Set.new, &:+)
      hash_keys.each do |key|
        hashes_with_key = hashes.select { |hash| hash.has_key?(key) }
        self[key] = hashes_with_key.map { |hash| hash[key] }.reduce(&op)
      end
      self
    end
  end
end

module TestHashWithMergeReduce
  using HashWithMergeReduce
  a = { "a" => 200, "b" => 100 }
  b = { "b" => 100, "c" => 300 }  
  puts a.merge_reduce(b, &:+)
  # => {"a"=>200, "b"=>200, "c"=>300}
end