pebbles.rb

# Scholten/Dijkstra pebble game
# https://youtu.be/31KI7fk15QA?t=90

# my implementation (an abomination)
class Pebble 
    attr_accessor :color

    def initialize
        @color = 1 + rand(2) > 1 ? :black : :white
    end 

    def to_s
        @color.to_s
    end
end

class Urn
    attr_accessor :pebbles

    # Starting position: an urn containing one
    # or more pebbles, each pebble being black 
    # or white
    def initialize
        n = 1 + rand(500)
        @pebbles = []
        puts "Fill: #{n}"
        fill(n)
    end 

    def fill(n)
        n.times do 
            @pebbles << Pebble.new
        end
    end

    # A move. Take two pebbles out of the urn;
    # if colors are different, a white pebble is 
    # put into the urn, if equal, a black one.
    def take
        p1 = @pebbles.pop
        p2 = @pebbles.pop
        p3 = Pebble.new

        puts "Got #{p1} and #{p2} pebbles."

        if p1.color != p2.color 
            p3.color = :white 
        else 
            p3.color = :black
        end 

        put(p3)
    end 

    def put(peb)
        @pebbles << peb
    end

    def count
        @pebbles.length
    end
end 

urn = Urn.new 

# Continue making moves as long as possible
# A move is possible if the urn contains two 
# or more pebbles.
i = 0
init_count = urn.count
while urn.count >= 2
    urn.take 
    i += 1
end 

puts "Finished in #{i} turns for #{init_count} pebbles."

p urn.pebbles


# Dijkstra's implementation
b = 10
w = 10

until b + w <= 2 do
  b -= 1

  b -= 1 if b >= 2

  if w >= 2
    w -= 2
    b += 1
  end 
end

puts "Finished, b: #{b} w: #{w}"