this post was submitted on 10 Dec 2025
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Advent Of Code

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Day 10: Factory

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[โ€“] Avicenna@programming.dev 3 points 1 week ago* (last edited 1 week ago)

In this case, I formulated both questions as a linear algebra question. The first one is over the finite field F2, which I solved by using the library galois and some manual row reduction. The second was over positive integers which is not a field, so I solved it over Q using sympy and then looked for positive integer minimal solutions. In some cases these are under determined, and in some cases exactly solvable systems of the form Ax = b for x where A is the vector made from button switch vectors, b is the light switch pattern vector in the first case or jolts in the second. For under determined ones, your solutions are of the form particular + linear combinations of null space (mod 2 in the first case) so you only search for the minimal one there and in the second you have to search both minimal and positive integer one (because your solution will be over Q and not Z+) in the second case. Wonders of vectorization makes a quick work of these last parts (0.2 second in the first problem about 20s in the second). Also nullspace seems to generally have less than or equal to two dimensions so search space is much smaller than using all the button press vectors.

import itertools as it
from pathlib import Path

import numpy as np
import galois
from sympy import Matrix, symbols, linsolve

cwd = Path(__file__).parent
GF2 = galois.GF(2)

def convert_line(line):

  target = line.split('] ')[0][1:]
  vectors = line.split('] ')[1].split(' ')[:-1]
  jolts = line.split('] ')[1].split(' ')[-1].strip()

  ndims = len(target)

  target = np.array([0 if l=='.' else 1 for l in target], dtype=int)
  jolts = np.array(list(map(int,jolts[1:-1].split(','))))

  M = []

  for v in vectors:
    coords = [int(x) for x in v if x.isnumeric()]
    vec = np.zeros(ndims, dtype=int)
    vec[coords] = 1
    M.append(vec)

  return np.array(M).T,target,jolts


def parse_input(file_path):
  with file_path.open("r") as fp:
    manual = list(map(convert_line, fp.readlines()))

  return manual


def find_pivots(R):
    pivots = []
    m, n = R.shape
    row = 0

    for col in range(n):
      if row < m and R[row, col] == 1:
        pivots.append(col)
        row += 1

    return pivots


def solve_GF2(A, x):

  nullspace = A.null_space()

  M = GF2(np.hstack([np.array(A), np.array(x)[:,None]]))
  R = M.row_reduce()

  pivots = find_pivots(R)

  m, n = R.shape
  n -= 1

  particular = GF2.Zeros(n)

  for r, c in enumerate(pivots):
      particular[c] = R[r, n]

  return np.array(particular), nullspace


def solve_Q(M, x):
  b = symbols(" ".join([f"b{i}" for i in range(M.shape[1])]))
  solution = list(linsolve((M, x), b))[0]
  syms = list(solution.free_symbols)
  func = Matrix(solution)

  particular = np.array(func.subs({s:0 for s in syms}).tolist()).flatten().astype(float)
  nullspace = np.array([np.array(x.tolist()).flatten() for x in M.nullspace()]).astype(float)

  return particular, nullspace


def minimize(nullspace, particular, jolt):

  nvecs = nullspace.shape[0]

  if not jolt:
    coef = np.array(list(it.product(np.arange(0, 2), repeat=nvecs)))
    A = np.sum(np.mod(coef@np.array(nullspace) + particular[None,:],2),axis=-1)
    I = np.argmin(A)
    res = np.mod(coef[I]@np.array(nullspace) + particular[None,:],2)

    return np.sum(res)
  else:
    N = 100
    I = []

    while len(I)==0: # look for a positive integer solution, if does not exist increase N

      coef = np.array(list(it.product(np.arange(-N, N), repeat=nvecs)))
      A = coef@np.array(nullspace) + particular[None,:]
      mask = (A >= 0) & np.isclose(A, A.astype(int))
      I = np.where(mask.all(axis=1))[0]
      N += 500

    return np.min(np.sum(A[I,:],axis=-1))


def solve_problem(file_name, jolt=False):

  manual = parse_input(Path(cwd, file_name))
  sum_press = 0

  for ind,(M, light_target, jolt_target) in enumerate(manual):

    if not jolt:
      #part1 solve over GF2, looks for minimal solution of the form particular + null
      M = GF2(M)
      target = GF2(light_target)
      particular, nullspace = solve_GF2(M, target)

    else:
      #part2 solve over Q, look for minimal integer, positive solution of the form particular + null
      target = Matrix(jolt_target.astype(int))
      M = Matrix(M.astype(int))
      particular, nullspace = solve_Q(M, target)

    sum_press += minimize(nullspace, particular, jolt)

  return sum_press


if __name__ == "__main__":

  assert solve_problem("test_input") == 7
  assert solve_problem("input") == 475
  assert solve_problem("test_input", True) == 33
  assert solve_problem("input", True) == 18273