Advent of Code 2023 Day 24 Solution

day24.py

# Parse input

def parse_input(s):
  out = []
  for line in s.strip().split('\n'):
    p,v = line.split(' @ ')
    pp = [int(i) for i in p.split(',')]
    vv = [int(i) for i in v.split(',')]
    out.append((pp,vv))
  return out

# Part 1 Nothing special

def solve_part_1(input):
  if len(input) < 10:
    min_coord = 7
    max_coord = 27
  else:
    min_coord = 200000000000000
    max_coord = 400000000000000

  def sign(a):
    if a < 0:
      return -1
    elif a == 0:
      return 0
    else:
      return 1

  def line_intersection(line1, line2):
    """https://stackoverflow.com/questions/20677795/how-do-i-compute-the-intersection-point-of-two-lines"""
    xdiff = (line1[0][0] - line1[1][0], line2[0][0] - line2[1][0])
    ydiff = (line1[0][1] - line1[1][1], line2[0][1] - line2[1][1])

    def det(a, b):
        return a[0] * b[1] - a[1] * b[0]

    div = det(xdiff, ydiff)
    if div == 0:
       return None

    d = (det(*line1), det(*line2))
    x = det(d, xdiff) / div
    y = det(d, ydiff) / div
    return x, y

  def hail_to_line(h):
    p,v = h
    p1 = (p[0], p[1])
    p2 = (p[0] + v[0], p[1] + v[1])
    pt = [p1, p2]
    return pt

  def hail_intersection(a,b):
    sect = line_intersection(hail_to_line(a), hail_to_line(b))
    if sect != None:
      def crossed_future(sect, a):
        sx,sy = sect
        return sign(sx - a[0][0]) == sign(a[1][0]) and sign(sy - a[0][1]) == sign(a[1][1])
      if crossed_future(sect, a) and crossed_future(sect, b):
        return sect
      return None
  
  cross_count = 0
  for i, a in enumerate(input):
    for j, b in enumerate(input):
      if i == j:
        break
      sect =hail_intersection(a,b)
      print(f'{a} {b} = {sect}')
      if sect != None and min_coord <= sect[0] <= max_coord and  min_coord <= sect[1] <= max_coord:
        print("Crossed")
        cross_count += 1
  print(cross_count)
  return cross_count

def solve_part_2(input):
  """Print out a set of instructions to the Open-Source "SageMath" program to solve the linear equations."""
  eqn = []
  fn = []
  t = []
  eqn.append('var(\'xg yg zg dxg dyg dzg\')')
  for i,line in enumerate(input[:10]):
    eqn.append(f'var(\'t{i}\')')
    t.append(f't{i}')
    for d in range(3):
      c = "xyz"[d]
      eqn.append(f'f{i}{d} = {c}g+d{c}g*t{i} == {line[0][d]} + {line[1][d]}*t{i}')
      fn.append(f'f{i}{d}')
  fns = ','.join(fn)
  ts = ','.join(t)
  eqn.append(f'solve([{fns}], [xg,yg,zg,dxg,dyg,dzg,{ts}])')
  print('\n'.join(eqn))
  return None