intelo.rb

# frozen_string_literal: true

require 'minitest/autorun'

def intelo(foo, bar)
  head = foo.flat_map.with_index do |n, i|
    [n, bar[i * 2, 2]]
  end
  tail = bar[head.size..-1].to_a

  (head + tail).flatten.compact
end

describe "Intelo's problem" do
  it 'is as desired' do
    assert_equal [], intelo([], [])
    assert_equal [1, 2, 3, 4], intelo([], [1, 2, 3, 4])
    assert_equal [1, 2, 3, 4], intelo([1], [2, 3, 4])
    assert_equal [1, 3, 4, 2], intelo([1, 2], [3, 4])
    assert_equal [1, 4, 2, 3], intelo([1, 2, 3], [4])
    assert_equal [1, 2, 3, 4], intelo([1, 2, 3, 4], [])
  end
end

Running:

.

Finished in 0.000796s, 1256.2814 runs/s, 7537.6884 assertions/s.
1 runs, 6 assertions, 0 failures, 0 errors, 0 skips