combination.rb

class Hoge
  def combination(array, num)
    return [] if num == 0
    return array.map {|elem| Array(elem) } if num == 1

    each_with_rest(array).with_object([]) {|(elem, rest), result|
      result.push(*_combination(rest, [elem], num))
    }
  end

  def _combination(array, candidates, num, result = [])
    each_with_rest(array).with_object(result) {|(elem, rest), result|
      _candidates = candidates.dup << elem

      if _candidates.count == num
        result << _candidates
      else
        _combination(rest, _candidates, num, result)
      end
    }
  end

  def each_with_rest(array)
    if block_given?
      array.each_with_index {|elem, i| yield elem, array[i+1..-1] }
    else
      array.map.with_index {|elem, i| [elem, array[i+1..-1]] }.each
    end
  end
end

array = %w(a b c d e f g)
p Hoge.new.combination(array, 4) == array.combination(4).to_a