Source: Like a Rogue
Part 1: Starting with a sequence of
.and^, generate additional rows using the rules based on the three characters above the new position.
^^.->^.^^->^^..->^..^->^- Otherwise ->
.
How many safe tiles (
.) are there after 40 generations?
This is an elementary cellular automaton. Specifically, Rule 90. I wrote a solution to these five years ago 1 with a pretty cool web based simulator. Here’s a way to do it in Python with zip
though:
# If the previous row left, center, right looks like this, the next element is a trap
trap_map = {
('^', '^', '.'), # Its left and center tiles are traps, but its right tile is not.
('.', '^', '^'), # Its center and right tiles are traps, but its left tile is not.
('^', '.', '.'), # Only its left tile is a trap.
('.', '.', '^'), # Only its right tile is a trap.
}
def next(row):
return ''.join(
'^' if previous in trap_map else '.'
for previous in zip('.' + row, row, row[1:] + '.')
)
def rows(row, height):
yield row
for i in range(height - 1):
row = next(row)
yield row
safe_count = 0
trap_count = 0
for row in rows(args.initial, args.height):
safe_count += row.count('.')
trap_count += row.count('^')
logging.info(row)
print('{} safe, {} trap'.format(safe_count, trap_count))
Part 2: How many safe tiles are there after 400,000 rows?
The solution doesn’t change. It’s already as memory efficient as it can be, only keeping one row at a time.
I feel old now. ↩︎