Solution: Coloring Cells by Shortest Distance

require 'distances'
class Cell
  attr_reader :row, :column
  attr_accessor :north, :south, :east, :west
  def initialize(row, column)
    @row, @column = row, column
    @links = {}
  end
  def link(cell, bidi=true)
    @links[cell] = true
    cell.link(self, false) if bidi
    self
  end
  def unlink(cell, bidi=true)
    @links.delete(cell)
    cell.unlink(self, false) if bidi
    self
  end
  def links
    @links.keys
  end
  def linked?(cell)
    @links.key?(cell)
  end
  def neighbors
    list = []
    list << north if north
    list << south if south
    list << east  if east
    list << west  if west
    list
  end
  def distances(from: [self])
    # work your magic here!
    distances = Distances.new(from.first) 
    from.each { |cell| distances[cell] = 0 }
    frontier = [ *from ]
    while frontier.any? 
      new_frontier = [] 
      frontier.each do |cell| 
        cell.links.each do |linked| 
          next if distances[linked] 
          distances[linked] = distances[cell] + 1 
          new_frontier << linked 
        end
      end
      frontier = new_frontier 
    end
    distances
  end
end