Improve speed by using Set

life_game.rb

N = 30

BOARDS = (N * N).times.map { |n| [n / N, n % N] }
NEIGHBORS = BOARDS.map { |(i, j)| [ [i, j], ((-1..1).to_a.product((-1..1).to_a).map { |(x, y)| [(i + x) % N, (j + y) % N] } - [[i, j]]).to_set ] }.to_h

def print_cells(cells)
  BOARDS
    .each_slice(N)
    .map { |row| row.map { |cell| cells.include?(cell) ? '0' : '.' }.join }
    .join("\n")
    .tap { |s| puts s }
end

def update(cells)
  NEIGHBORS.map { |(cell, neighbors)| [cell, (neighbors & cells).size] }
    .select { |cell, count| count == 3 || (count == 2 && cells.include?(cell)) }
    .map(&:first)
    .to_set
end

GENERATIONS = 500

initial_cells = BOARDS.sample(N * N / 2).to_set

GENERATIONS.times.reduce(initial_cells) do |cells, g|
  puts "Generation #{g + 1}"
  print_cells(cells)
  update(cells)
end