triangle.rb

def triangle(a, b, c)
  sides = [ a, b, c ]
  unless sides.all? { |s| s > 0 }
    raise TriangleError, 'Sides must be positive'
  end

  s1, s2, s3 = sides.sort
  unless s1 + s2 > s3
    raise TriangleError, 'Sides violate the triangle inequality'
  end

  case sides.uniq.count
    when 1 then :equilateral
    when 2 then :isosceles
    else :scalene
  end
end