A guess at what old_relik was looking for in #ruby IRC

same_generation_and_age.rb

def recurse_ages(hash, depth: 0, history: [])
  hash.flat_map do |key, value|
    if value.instance_of?(Hash)
      history[depth] = key
      method(__method__).call value, depth: depth.succ, history: history
    else
      [history[0..depth.pred], value]
    end
  end
end

def folk_by_age(hash)
  recurse_ages(hash).each_slice(2).to_h.transform_keys do |pairs|
    pairs.each_slice(2).map(&:last)
  end
end

def same_generation_and_age(hash)
  hash.group_by do |ancestry, age|
    [ancestry.size, age]
  end.map(&:last).select do |grouped|
    grouped.size > 1
  end.to_h do |grouped|
    [grouped.map(&:first), grouped.first.last]
  end
end

lineage = {grandparents:
  {grandpa_1:
    {age: 58,
    parents:
      {parent_1: {age: 34, children: {child_1: {age: 12}}},
       parent_2: {age: 32, children: {child_2: {age: 11}}}}},
  grandpa_2:
    {age: 59,
    parents:
      {parent_3: {age: 31, children: {child_4: {age: 12}}},
       parent_4: {age: 32, children: {child_5: {age: 11}}}}}}}

same_generation_and_age folk_by_age lineage
#=> {[[:grandpa_1, :parent_1, :child_1], [:grandpa_2, :parent_3, :child_4]]=>12,
# [[:grandpa_1, :parent_2], [:grandpa_2, :parent_4]]=>32,
# [[:grandpa_1, :parent_2, :child_2], [:grandpa_2, :parent_4, :child_5]]=>11}