hash merge with block

hash_merge.rb

ids = [%w(d b f a), %w(d c a)]
hash = {}
a.map do |list|
  list.each_with_index do |v,idx|
    h = {}
    h[idx] = v
    hash.merge!(h) do |_,left,right|
      Array(left) + Array(right)
    end
    hash
  end
end