Day 3: Gear Ratios

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    6 months ago

    Language: Python

    Classic AoC grid problem… Tedious as usual, but very doable. Took my time and I’m pretty happy with the result. :]

    Part 1

    For the first part, I decided to break the problem into: 1. Reading the schematic, 2. Finding the numbers, 3. Finding the parts. This was useful for Part 2 as I could re-use my read_schematic and find_numbers functions.

    Two things I typically do for grid problems:

    1. Pad the grid so you can avoid annoying boundary checks.
    2. I have a DIRECTIONS list I loop through so I can check easily check the neighbors.
    Schematic  = list[str]
    Number     = tuple[int, int, int]
        (-1, -1),
        (-1,  0),
        (-1,  1),
        ( 0, -1),
        ( 0,  1),
        ( 1, -1),
        ( 1,  0),
        ( 1,  1),
    def read_schematic(stream=sys.stdin) -> Schematic:
        schematic = [line.strip() for line in stream]
        columns   = len(schematic[0]) + 2
        return [
            *['.' + line + '.' for line in schematic],
    def is_symbol(s: str) -> bool:
        return not (s.isdigit() or s == '.')
    def find_numbers(schematic: Schematic) -> Iterator[Number]:
        rows    = len(schematic)
        columns = len(schematic[0])
        for r in range(1, rows):
            for number in re.finditer(r'[0-9]+', schematic[r]):
                yield (r, *number.span())
    def find_parts(schematic: Schematic, numbers: Iterator[Number]) -> Iterator[int]:
        for r, c_head, c_tail in numbers:
            part = int(schematic[r][c_head:c_tail])
            for c in range(c_head, c_tail):
                neighbors = (schematic[r + dr][c + dc] for dr, dc in DIRECTIONS)
                if any(is_symbol(neighbor) for neighbor in neighbors):
                    yield part
    def main(stream=sys.stdin) -> None:
        schematic = read_schematic(stream)
        numbers   = find_numbers(schematic)
        parts     = find_parts(schematic, numbers)
    Part 2

    For the second part, I just found the stars, and then I found the gears by checking if the stars are next to two numbers (which I had found previously).

    def find_stars(schematic: Schematic) -> Iterator[Star]:
        rows    = len(schematic)
        columns = len(schematic[0])
        for r in range(1, rows):
            for c in range(1, columns):
                token = schematic[r][c]
                if token == '*':
                    yield (r, c)
    def find_gears(schematic: Schematic, stars: Iterator[Star], numbers: list[Number]) -> Iterator[int]:
        for star_r, star_c in stars:
            gears = [                                                                                                                      
                for number_r, number_c_head, number_c_tail in numbers
                if any(star_r + dr == number_r and number_c_head <= (star_c + dc) < number_c_tail for dr, dc in DIRECTIONS)
            if len(gears) == 2:
                yield gears[0] * gears[1]
    def main(stream=sys.stdin) -> None:
        schematic = read_schematic(stream)
        numbers   = find_numbers(schematic)
        stars     = find_stars(schematic)
        gears     = find_gears(schematic, stars, list(numbers))

    GitHub Repo