advent_6.py

def simulate(lanternfish, days):
  new_lanternfish = [0] * 9
  for i in range(days):
    new_hatched = lanternfish[8]
    for i in range(1, 9):
      new_lanternfish[i] = lanternfish[i - 1]
    new_lanternfish[0] = new_hatched
    new_lanternfish[2] += new_hatched
    tmp = lanternfish
    lanternfish = new_lanternfish
    new_lanternfish = tmp
  return sum(lanternfish)

if __name__ == '__main__':
  with open('input.txt') as data:
    initial_input = data.readline().strip()
    lanternfish = [0] * 9
    for datum in initial_input.split(','):
      age = int(datum)
      lanternfish[8 - age] += 1
    print(simulate(lanternfish, 256))