aoc_2024_6b

aoc_2024_6b_faster.py

dirs = ['^','>','v','<']
dir_map = {'^':0,'>':1,'v':2,'<':3}
delta = [(-1,0),(0,1),(1,0),(0,-1)]

def parse_grid(grid_str):
    grid = [list(line) for line in grid_str.split('\n')]
    rows, cols = len(grid), len(grid[0])
    gpos, gdir = None, None
    for r in range(rows):
        for c in range(cols):
            if grid[r][c] in dir_map:
                gpos = (r,c)
                gdir = dir_map[grid[r][c]]
                grid[r][c] = '.' 
                break
        if gpos is not None:
            break
    return grid, gpos[0], gpos[1], gdir, rows, cols

def can_move(r,c,d,grid,rows,cols):
    nr, nc = r+delta[d][0], c+delta[d][1]
    if not (0<=nr<rows and 0<=nc<cols): return True # allow moving outside
    return grid[nr][nc] != '#'

def rotate(d): return (d+1)%4

def check_timeloop(grid, gr, gc, gd, rows, cols):
    max_iter = rows*cols*4+1
    visited = set()
    visited.add((gr,gc,gd))
    for _ in range(max_iter):
        if can_move(gr,gc,gd,grid,rows,cols): gr, gc = gr+delta[gd][0], gc+delta[gd][1]
        else:                                 gd = rotate(gd)
        if not (0<=gr<rows and 0<=gc<cols): return False
        state = (gr,gc,gd)
        if state in visited: return True
        visited.add(state)
    return False # Should never reach here

def solve_1b(grid_str):
    grid, gr, gc, gd, rows, cols = parse_grid(grid_str)
    empties = [(r,c) for r in range(rows) for c in range(cols) if grid[r][c]=='.' and (r,c)!=(gr,gc)]
    count = 0
    for (er,ec) in empties:
        grid[er][ec] = '#'
        if check_timeloop(grid, gr, gc, gd, rows, cols):
            count += 1
        grid[er][ec] = '.'
    return count
  
from aocd import get_data
inp = get_data(year=2024, day=6)
solve_1b(inp)

This is a faster version of https://gist.github.com/UmerHA/3b0d52fbd345f75ba634529d0e7c150e, which doesn't use dicts and classes so heavily.