Solution: Linking Worlds

require 'sphere_grid'
require 'prims'
# reopen and add a new possible neighbor
class HemisphereCell
  attr_accessor :alien_grid, :alien
  def neighbors
    super.tap do |list|
      list << alien if alien
    end
  end
end
class MultiSphereGrid
  attr_reader :subgrids
  def initialize(subgrids, connections)
    @subgrids = subgrids
    @subgrids.each do |subgrid|
      others = @subgrids - [ subgrid ]
      connections.times do
        other = others.sample
        local, alien = subgrid.random_cell, other.random_cell
        # skip cells that are already linked to another subgrid
        next if local.alien || alien.alien
        local.alien_grid = other
        local.alien = alien
        alien.alien_grid = subgrid
        alien.alien = local
      end
    end
  end
  def random_cell
    @subgrids.sample.random_cell
  end
  def each_cell
    @subgrids.each do |subgrid|
      subgrid.each_cell { |cell| yield cell }
    end
  end
  def braid(p=1.0)
    @subgrids.each { |subgrid| subgrid.braid(p) }
  end
end
world = SphereGrid.new(20)
moon1 = SphereGrid.new(10)
moon2 = SphereGrid.new(8)
moon3 = SphereGrid.new(6)
grid = MultiSphereGrid.new([ world, moon1, moon2, moon3 ], 3)
TruePrims.on(grid)
grid.braid(0.5)