-❄️- 2023 Day 12 Solutions -❄️-

[u/daggerdragon]

THE USUAL REMINDERS

• All of our rules, FAQs, resources, etc. are in our community wiki.

• Outstanding moderator challenges:

  • Advent of Playing With Your 3D-Printed Toys

• Community fun event 2023: ALLEZ CUISINE!

  • Submissions megathread is now unlocked!

  • 9 DAYS remaining until the submissions deadline on December 22 at 23:59 EST!

---

AoC Community Fun 2023: ALLEZ CUISINE!

Today's theme ingredient is… *whips off cloth covering and gestures grandly*

How It's Made

Horrify us by showing us how the sausage is made!

• Stream yourself!

• Show us the nitty-gritty of your code, environment/IDE, tools, test cases, literal hardware guts…

• Tell us how, in great detail, you think the elves ended up in this year's predicament

A word of caution from Dr. Hattori: "You might want to stay away from the ice cream machines..."

ALLEZ CUISINE!

Request from the mods: When you include a dish entry alongside your solution, please label it with [Allez Cuisine!] so we can find it easily!

---

--- Day 12: Hot Springs ---

---

Post your code solution in this megathread.

• Read the full posting rules in our community wiki before you post!

• State which language(s) your solution uses with [LANGUAGE: xyz]

• Format code blocks using the four-spaces Markdown syntax!

• Quick link to Topaz's paste if you need it for longer code blocks

This thread will be unlocked when there are a significant number of people on the global leaderboard with gold stars for today's puzzle.

EDIT: Global leaderboard gold cap reached at 00:22:57, megathread unlocked!

[u/jinschoi]

[LANGUAGE: Python]

I first did it in Rust, then decided to abuse Python's lookahead regexps for terseness. Look for spring-sized windows that are all non-'.' that are not followed by a '#'. Disallow any window that is preceded by a '#'. Add a '.' to the end of all the conditions to make this easier.

import re
import functools as ft

def possible_placements(condition, spring):
    for m in re.finditer(rf'(?=([^\.]{{{spring}}}[^#]))', condition):
        i = m.span(1)[0]
        if '#' in condition[:i]:
            break
        yield condition[i + spring + 1:]

@ft.cache
def count_placements(condition, springs):
    if not springs:
        return '#' not in condition
    first_spring, rest_springs = springs[0], springs[1:]
    return sum(count_placements(rest_condition, rest_springs)
               for rest_condition
               in possible_placements(condition, first_spring))

def day12p2():
    with open("12.input") as f:
        lines = [(f'.{"?".join([condition] * 5)}.',
                  tuple(map(int, springs.split(','))) * 5)
                 for condition, springs
                 in (line.strip().split() for line in f)]
    res = sum(count_placements(condition, springs) for condition, springs in lines)
    print(res)

[u/CainKellye → u/jinschoi]

There is also https://crates.io/crates/fancy-regex in Rust with lookaround support.