triangle.rb

class TriangleError < StandardError
end

class Triangle

  attr_accessor :a, :b, :c

  def initialize(a, b, c)
    @a = a
    @b = b
    @c = c
  end

  def kind
    sides = [a,b,c]
    ineq = [a+b > c, a+c > b, b+c > a]

    if sides.min <= 0 || ineq.include?(false)
      raise TriangleError
    end

    if a == b && b == c
      :equilateral
    elsif a != b && a != c && b != c
      :scalene
    elsif (a == b && b != c) || (b == c && c != a) || (a == c && c != b)
      :isosceles
    end
  end
  
end

triangle_spec.rb

require_relative './triangle.rb'

RSpec.configure do |config|
  # Use color in STDOUT
  config.color_enabled = true

  # Use color not only in STDOUT but also in pagers and files
  config.tty = true

  # Use the specified formatter
  config.formatter = :progress # :progress, :html, :textmate
end

describe 'Triangle' do

  it 'knows that equilateral triangles have equal sides' do
    Triangle.new(2, 2, 2).kind.should eq(:equilateral)
  end

  it 'knows that larger equilateral triangles also have equal sides' do
    Triangle.new(10, 10, 10).kind.should eq(:equilateral)
  end

  it 'knows that isosceles triangles have last two sides equal' do
    Triangle.new(3, 4, 4).kind.should eq(:isosceles)
  end

  it 'knows that isosceles triangles have first and last sides equal' do
    Triangle.new(4, 3, 4).kind.should eq(:isosceles)
  end

  it 'knows that isosceles triangles have two first sides equal' do
    Triangle.new(4, 4, 3).kind.should eq(:isosceles)
  end

  it 'knows that isosceles triangles have in fact exactly two sides equal' do
    Triangle.new(10, 10, 2).kind.should eq(:isosceles)
  end

  it 'knows that scalene triangles have no equal sides' do
    Triangle.new(3, 4, 5).kind.should eq(:scalene)
  end

  it 'knows that scalene triangles have no equal sides at a larger scale too' do
    Triangle.new(10, 11, 12).kind.should eq(:scalene)
  end

  it 'knows that scalene triangles have no equal sides in descending order either' do
    Triangle.new(5, 4, 2).kind.should eq(:scalene)
  end

  it 'knows that very small triangles are legal' do
    Triangle.new(0.4, 0.6, 0.3).kind.should eq(:scalene)
  end

  # Bonus Tests
  it 'knows that triangles with no size are illegal' do
    expect{Triangle.new(0, 0, 0).kind}.to raise_error(TriangleError)
  end

  it 'knows that triangles with negative sides are illegal' do
    expect{Triangle.new(3, 4, -5).kind}.to raise_error(TriangleError)
  end

  it 'knows that triangles violating triangle inequality are illegal' do
    expect{Triangle.new(1, 1, 3).kind}.to raise_error(TriangleError)
  end

  it 'knows that triangles violating triangle inequality are illegal 2' do
    expect{Triangle.new(2, 4, 2).kind}.to raise_error(TriangleError)
  end

  it 'knows that triangles violating triangle inequality are illegal 3' do
    expect{Triangle.new(7, 3, 2).kind}.to raise_error(TriangleError)
  end

end