Converting a nested hash into a flat hash

Question

This question is the inverse of this question.

Given a nested hash like

{
    :a => {
       :b => {:c => 1, :d => 2},
       :e => 3,
    },
    :f => 4,
}

what is the best way to convert it into a flat hash like

{
    [:a, :b, :c] => 1,
    [:a, :b, :d] => 2,
    [:a, :e] => 3,
    [:f] => 4,
}

Answer 1

A functional approach (see the history for an alternative implementations):

def recursive_flatten(hash)
  hash.flat_map do |key, value|
    if value.is_a?(Hash)
      recursive_flatten(value).map { |ks, v| [[key] + ks, v] } 
    else
      [[[key], value]]
    end
  end.to_h
end

Answer 2

Another way:

def flat_hash(h,f=[],g={})
  return g.update({ f=>h }) unless h.is_a? Hash
  h.each { |k,r| flat_hash(r,f+[k],g) }
  g
end

h = { :a => { :b => { :c => 1,
                      :d => 2 },
              :e => 3 },
      :f => 4 }

flat_hash(h) #=> {[:a, :b, :c]=>1, [:a, :b, :d]=>2, [:a, :e]=>3, [:f]=>4}