naive nested ruby hash flattener

flathash.rb

class FlatHash
  def self.flatten_hval(pre, val)
    out = {}
    val.each do |k, v|
      out["#{pre}_#{k}"] = v
    end
    flatten(out)
  end

  def self.flatten_aval(pre, val)
    out = {}
    val.each_with_index do |v, i|
      case v
      when String, Symbol, Numeric
        out["#{pre}_#{i}"] = v.to_s
      else
        raise TypeError, "String, Symbol or Numeric required. Got #{v.class}"
      end
    end
    out
  end

  def self.flatten(hash)
    out = {}
    hash.each do |k, v|
      case v
      when String, Symbol, Numeric
        out[k.to_s] = v.to_s
      when Array
        out.merge!(flatten_aval(k, v))
      when Hash
        out.merge!(flatten_hval(k, v))
      end
    end
    out
  end
end

require 'minitest/autorun'

class TestFlatHash < MiniTest::Test
  def test_it_flattens
    i1 = {
      this: {
        that: {
          other: 'thing',
          bibbity: {
            bobbity: {
              boo: :foo!
            }
          }
        }
      },
      foo: ['bar', 'baz', 'biddy'],
      boom: 'bip',
      tech: 9
    }

    o1 = {
      'this_that_bibbity_bobbity_boo' => 'foo!',
      'this_that_other' => 'thing',
      'foo_0' => 'bar',
      'foo_1' => 'baz',
      'foo_2' => 'biddy',
      'boom' => 'bip',
      'tech' => '9'
    }

    assert_equal o1, FlatHash.flatten(i1)
  end
end