Solver for the game of Set

set_game.rb

#!/usr/bin/env ruby

#
# Solver for the game of Set.
#

class Card
	# Constants deriving from the rules of the game 
	COLORS = ["red", "blue", "green"]
	SHAPES = ["circle", "square", "triangle"]
	NUMBERS = [1, 2, 3]

	attr_accessor :color
	attr_accessor :shape
	attr_accessor :number

	def initialize(color: nil, shape: nil, number: nil)
		@color = color
		@shape = shape
		@number = number
	end

	# Check if a card is a valid card in Set.
	def valid?
		COLORS.include?(@color)
		SHAPES.include?(@shape)
		NUMBERS.include?(@number)
	end
end

# set? takes 3 Cards and checks to see if they make a set.
# Returns true or false.
def set?(card1, card2, card3)
	cards = [card1, card2, card3]

	# Check to make sure all the cards are valid.
	if cards.any? { |c| !c.valid? }
		return "Cards are invalid."
	end

	# Checks each attribute to see if it would contribute to a valid set.
	# For each attribute, 1 or 3 are valid numbers after the array of attributes is uniqued.
	cards.collect(&:color).uniq.length != 2 &&
		cards.collect(&:shape).uniq.length != 2 &&
		cards.collect(&:number).uniq.length != 2
end

# generate takes 2 cards and generates the 3rd that would complete the set.
# Returns a Card object.
def generate(card1, card2)
	# Checks if both Cards are valid.
	if [card1, card2].any? { |c| !c.valid? }
		return "Cards are invalid."
	end

	card3 = Card.new(color: nil, shape: nil, number: nil)

	# Assigns color.
	if card1.color == card2.color
		card3.color = card1.color
	else
		card3.color = (Card::COLORS - [card1.color, card2.color]).first
	end

	# Assigns shape.
	if card1.shape == card2.shape
		card3.shape = card1.shape
	else
		card3.shape = (Card::SHAPES - [card1.shape, card2.shape]).first
	end

	# Assigns number.
	if card1.number == card2.number
		card3.number = card1.number
	else
		card3.number = (Card::NUMBERS - [card1.number, card2.number]).first
	end

	card3
end

# find_set takes a list of Cards and finds the first available set.
# Returns an array of 3 Cards that make a set, if one is found.
def find_set(list)
	# Check number and validity of Cards.
	if list.length < 3
		return "Not enough cards."
	elsif list.any? { |c| !c.valid? }
		return "Invalid list."
	end

	# Brute force way to find first valid set.
	# Worst case: steps increase as Cards cubed.
	(0..(list.length - 3)).each do |i|
		(1..(list.length - 2)).each do |j|
			(2..(list.length - 1)).each do |k|
				if set?(list[i], list[j], list[k])
					return [list[i], list[j], list[k]]
				end
			end
		end
	end

	"No set found."
end