city_finder.rb

cities = ('a'..'p').to_a

def generate_lists(cities)
  pos = cities.combination(3).to_a.shuffle; nil
  map = {}
  cities.each do |c|
    taken = []
    while taken.length < 3
      c3 = pos.shift
      if c3.include? c
        pos.push c3
      else
        taken.push c3
      end
    end
    # puts "#{ c }\t#{ taken.inspect }"
    map[c] = taken
  end
  origins = map.values.flatten
  counts = Hash.new(0)
  origins.each { |c| counts[c] += 1 }
  # puts counts.inspect
  # puts counts.values.min
  { map: map, counts: counts, min: counts.values.min }
end

def find_city_list(cities, max_runs, min)
  max_runs.times do
    res = generate_lists(cities)
    if res[:min] >= min
      cities.each do |c|
        o = res[:map][c]
        puts "#{ c }\t#{ o[0].join(' ') }\t#{ o[1].join(' ') }\t#{ o[2].join(' ') }"
      end
      puts res[:counts].inspect
      break
    end
  end
end