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Based on this, I wrote the following function that I've just shared.

  def self.get_length(vertices_a, vertices_b)
    length = 0.0

    vertices_a.each_with_index do |vertex_a, index_a|
      prev_a = vertices_a[index_a-1]
      vertices_b.each_with_index do |vertex_b, index_b|
        prev_b = vertices_b[index_b-1]

        p1 = prev_a
        p2 = vertex_a
        p3 = prev_b
        p4 = vertex_b

        p12 = p2-p1
        p34 = p4-p3

        next unless p12.cross(p34).length < 1e-6

        p13 = p3-p1

        next unless p12.cross(p13).length < 1e-6

        p14 = p4-p1

        k2 = p12.dot(p12)
        k3 = p12.dot(p13)
        k4 = p12.dot(p14)

        table = [
          [0.0, p14.length, p12.length],
          [p13.length, p34.length, (p3-p2).length],
          [p12.length, (p4-p2).length, 0.0]
        ]

        row = if k3 < 1e-6 then
          0
        elsif k3 < k2 then
          1
        else
          2
        end

        col = if k4 < 1e-6 then
          0
        elsif k4 < k2 then
          1
        else
          2
        end

        length += table[row][col]
      end
    end
    length /= 2 if vertices_a.length.eql?(2)
    length /= 2 if vertices_b.length.eql?(2)

    return length
  end