101 lines
2.6 KiB
Python
101 lines
2.6 KiB
Python
import os
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import sys
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from typing import Literal, cast
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VERBOSE = os.getenv("AOC_VERBOSE") == "True"
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Symbol = Literal["|", "-", "L", "J", "7", "F", ".", "S"]
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lines: list[list[Symbol]] = [
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[cast(Symbol, symbol) for symbol in line] for line in sys.stdin.read().splitlines()
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]
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# find starting point
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si, sj = next(
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(i, j)
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for i in range(len(lines))
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for j in range(len(lines[0]))
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if lines[i][j] == "S"
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)
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# find one of the two outputs
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ni, nj = si, sj
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for ni, nj, chars in (
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(si - 1, sj, "|7F"),
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(si + 1, sj, "|LJ"),
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(si, sj - 1, "-LF"),
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(si, sj + 1, "-J7"),
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):
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if lines[ni][nj] in chars:
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break
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# part 1 - find the loop (re-used in part 2)
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loop = [(si, sj), (ni, nj)]
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while True:
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pi, pj = loop[-2]
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i, j = loop[-1]
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sym = lines[i][j]
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if sym == "|" and pi > i or sym in "JL" and pi == i:
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i -= 1
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elif sym == "|" and pi < i or sym in "7F" and pi == i:
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i += 1
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elif sym == "-" and pj > j or sym in "J7" and pj == j:
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j -= 1
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elif sym == "-" and pj < j or sym in "LF" and pj == j:
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j += 1
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if (i, j) == (si, sj):
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break
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loop.append((i, j))
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answer_1 = len(loop) // 2
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print(f"answer 1 is {answer_1}")
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# part 2
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# replace S by an appropriate character for the loop below
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di1, dj1 = loop[1][0] - loop[0][0], loop[1][1] - loop[0][1]
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di2, dj2 = loop[0][0] - loop[-1][0], loop[0][1] - loop[-1][1]
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mapping: dict[tuple[int, int], dict[tuple[int, int], Symbol]] = {
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(0, 1): {(0, 1): "-", (-1, 0): "F", (1, 0): "L"},
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(0, -1): {(0, -1): "-", (-1, 0): "7", (1, 0): "J"},
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(1, 0): {(1, 0): "|", (0, 1): "7", (0, -1): "F"},
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(-1, 0): {(-1, 0): "|", (0, -1): "L", (0, 1): "J"},
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}
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lines[si][sj] = mapping[di1, dj1][di2, dj2]
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# find the points inside the loop using an adaptation of ray casting for a discrete
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# grid (https://stackoverflow.com/a/218081/2666289)
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#
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# use a set for faster '... in loop' check
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#
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loop_s = set(loop)
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inside: set[tuple[int, int]] = set()
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for i in range(len(lines)):
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cnt = 0
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for j in range(len(lines[0])):
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if (i, j) not in loop_s and cnt % 2 == 1:
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inside.add((i, j))
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if (i, j) in loop_s and lines[i][j] in "|LJ":
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cnt += 1
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if VERBOSE:
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for i in range(len(lines)):
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for j in range(len(lines[0])):
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if (i, j) == (si, sj):
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print("\033[91mS\033[0m", end="")
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elif (i, j) in loop:
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print(lines[i][j], end="")
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elif (i, j) in inside:
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print("\033[92mI\033[0m", end="")
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else:
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print(".", end="")
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print()
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answer_2 = len(inside)
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print(f"answer 2 is {answer_2}")
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