fun_with_hash.rb

input = {
  'key1a.key2a.key3a' => 'value1',
  'key1a.key2b.key3b' => 'value2',
  'key1a.key2c.key3c' => 'value3',
  'key1a.key2c.key3d' => 'value4'
}

def flat_keys_to_nested(hash)
  hash.each_with_object({}) do |(key,value), all|
    key_parts = key.split('.')
    leaf = key_parts[0...-1].inject(all) { |h, k| h[k] ||= {} }
    leaf[key_parts.last] = value
  end
end

puts flat_keys_to_nested(input)
# {"key1a"=>{"key2a"=>{"key3a"=>"value1"}, "key2b"=>{"key3b"=>"value2"}, "key2c"=>{"key3c"=>"value3", "key3d"=>"value4"}}}