an example of beautiful ruby code used to validate a triangle as part of ruby koans exercise

triangle-koan-ruby.rb

# exercise is from http://rubykoans.com/ - a great way to learn ruby!

#! my not so succinct but works solution
def triangle(a, b, c)
  validate_triangle(a, b, c)
  if a == b && b == c
    :equilateral
  elsif a != b && b != c && c != a
    :scalene
  else
    :isosceles
  end
end

def validate_triangle(a, b, c)
  raise TriangleError, "No such thing as negative or 0-length sides in a triangle" if a <= 0 || b <= 0 || c <= 0
  raise TriangleError, "Incomplete triangle because the sides cannot touch and close up" if a + b <= c || a + c <= b || b + c <= a
end

# ===
# Josh's beautiful solution taken from https://stackoverflow.com/a/14445542/6463816
# ===
def triangle(a, b, c)
  [a, b, c].permutation do |sides|
    raise TriangleError unless sides[0] + sides[1] > sides[2]
  end
  case [a,b,c].uniq.size
    when 3; :scalene
    when 2; :isosceles
    when 1; :equilateral
  end
end

class TriangleError < StandardError
end

[].permutation is so easy to understand as a human! line 24

[].uniq.size and when N express the thing I'm trying to do in the form closest to how I think about it in my human mind. line 27

Rather than sides[0] + sides[1] > sides[2], what about sides[0..1].sum > sides[2]?