advent-of-code/2023/day10.py

import os
import sys
from typing import Literal, cast

VERBOSE = os.getenv("AOC_VERBOSE") == "True"

Symbol = Literal["|", "-", "L", "J", "7", "F", ".", "S"]

lines: list[list[Symbol]] = [
    [cast(Symbol, symbol) for symbol in line] for line in sys.stdin.read().splitlines()
]

# find starting point
si, sj = next(
    (i, j)
    for i in range(len(lines))
    for j in range(len(lines[0]))
    if lines[i][j] == "S"
)

# find one of the two outputs
ni, nj = si, sj
for ni, nj, chars in (
    (si - 1, sj, "|7F"),
    (si + 1, sj, "|LJ"),
    (si, sj - 1, "-LF"),
    (si, sj + 1, "-J7"),
):
    if lines[ni][nj] in chars:
        break

# part 1 - find the loop (re-used in part 2)
loop = [(si, sj), (ni, nj)]
while True:
    pi, pj = loop[-2]
    i, j = loop[-1]

    sym = lines[i][j]

    if sym == "|" and pi > i or sym in "JL" and pi == i:
        i -= 1
    elif sym == "|" and pi < i or sym in "7F" and pi == i:
        i += 1
    elif sym == "-" and pj > j or sym in "J7" and pj == j:
        j -= 1
    elif sym == "-" and pj < j or sym in "LF" and pj == j:
        j += 1

    if (i, j) == (si, sj):
        break

    loop.append((i, j))

answer_1 = len(loop) // 2
print(f"answer 1 is {answer_1}")

# part 2

# replace S by an appropriate character for the loop below
di1, dj1 = loop[1][0] - loop[0][0], loop[1][1] - loop[0][1]
di2, dj2 = loop[0][0] - loop[-1][0], loop[0][1] - loop[-1][1]
mapping: dict[tuple[int, int], dict[tuple[int, int], Symbol]] = {
    (0, 1): {(0, 1): "-", (-1, 0): "F", (1, 0): "L"},
    (0, -1): {(0, -1): "-", (-1, 0): "7", (1, 0): "J"},
    (1, 0): {(1, 0): "|", (0, 1): "7", (0, -1): "F"},
    (-1, 0): {(-1, 0): "|", (0, -1): "L", (0, 1): "J"},
}
lines[si][sj] = mapping[di1, dj1][di2, dj2]

# find the points inside the loop using an adaptation of ray casting for a discrete
# grid (https://en.wikipedia.org/wiki/Ray_casting)
#
# use a set for faster '... in loop' check
#
loop_s = set(loop)
inside: set[tuple[int, int]] = set()
for i in range(len(lines)):
    cnt = 0
    for j in range(len(lines[0])):
        if (i, j) not in loop_s and cnt % 2 == 1:
            inside.add((i, j))

        if (i, j) in loop_s and lines[i][j] in "|LJ":
            cnt += 1

if VERBOSE:
    for i in range(len(lines)):
        for j in range(len(lines[0])):
            if (i, j) == (si, sj):
                print("\033[91mS\033[0m", end="")
            elif (i, j) in loop:
                print(lines[i][j], end="")
            elif (i, j) in inside:
                print("\033[92mI\033[0m", end="")
            else:
                print(".", end="")
        print()

answer_2 = len(inside)
print(f"answer 2 is {answer_2}")