Advent of code 2020 day 24 notes

if 0:  # Notice the repeated 18-line blocks; find the block differences:
  lines = puzzle.input.strip('\n').split('\n')
  for m in range(18):
    print(m)
    set_ = set(line for i, line in enumerate(lines) if i % 18 == m)
    print('\n'.join(set_))


def test():  # See the 3 differing lines in each block.
  lines = puzzle.input.strip('\n').split('\n')
  params = {4: 'div z ', 5: 'add x ', 15: 'add y '}
  a, b, c = [[int(lines[i].split()[-1]) for i in range(m, 253, 18)]
             for m in params]
  print(f'a = {a}')
  print(list(zip(range(14), a, b, c)))

test()


# Block of 18 lines repeated 14 times:
# inp w     w = input
# mul x 0
# add x z
# mod x 26  x = z % 26
# div z 1   z /= a   where a = 1 or 26
# add x 13  x += b   where b = -14, -13, -9, ..., 11, 12, 13, 15
# eql x w
# eql x 0   x = x != w
# mul y 0
# add y 25
# mul y x
# add y 1   y = x * 25 + 1
# mul z y   z *= y
# mul y 0
# add y w   y = w
# add y 14  y += c  where c = 3, 4, 5, 6, 7, 8, 10, 13, 13, 13, 14, 16
# mul y x   y *= x
# add z y   z += y

# Progressively simplify the code:

# w = input  (1..9)
# x = w != ((z % 26) + b)
# z = int(z / a) * (x * 25 + 1)  +  (w + c) * x

# where a = 1 or 26    (%18 == 4)
# where b = -14, -13, -9, ..., 11, 12, 13, 15  (%18==5)
# where c = 3, 4, 5, 6, 7, 8, 10, 13, 13, 13, 14, 16  (%18==15)


# w = input  (1..9)
# if w != ((z % 26) + b):
#   z = int(z / a) * 26 + w + c
# else:
#   z = int(z / a)


# w = input  (1..9)
# if w != digits[0] + b
#   if a == 26:
#     digits[0] = w + c
#   else:  # a == 1
#     digits.push(w + c)
# else:
#   if a == 26:
#     digits.pop()


# w = input  (1..9)
# match = w == digits[-1] + b
# if a == 26:
#   if match:
#     digits.pop()
#   else:
#     digits[-1] = w + c
# else:  # a == 1
#   if not match:
#     digits.append(w + c)

# valid = all(digit == 0 for digit in digits)
# valid = not digits  # Actually, at least for this input.


# match = w0 == (0) + 13  # False
# append(w0 + 14)
# [w0 + 14]

# match = w1 == (w0 + 14) + 12  # False
# append(w1 + 8)
# [w0 + 14, w1 + 8]

# match = w2 == (w1 + 8) + 11  # False
# append(w2 + 5)
# [w0 + 14, w1 + 8, w2 + 5]

# match = w3 == (w2 + 5) + 0  # True   if w3 == w2 + 5
# pop()    # (if not match, replace w1 + 8 by w3 + 4)
# [w0 + 14, w1 + 8]

# match = w4 == (w1 + 8) + 15  # False
# append(w4 + 10)
# [w0 + 14, w1 + 8, w4 + 10]

# match = w5 == (w4 + 10) - 13  # True if w5 == w4 - 3
# pop()
# [w0 + 14, w1 + 8]

# match = w6 == (w1 + 8) + 10  # False
# append(w6 + 16)
# [w0 + 14, w1 + 8, w6 + 16]

# match = w7 == (w6 + 16) - 9  # True if w7 == w6 + 7
# pop()
# [w0 + 14, w1 + 8]

# match = w8 == (w1 + 8) + 11  # False
# append(w8 + 6)
# [w0 + 14, w1 + 8, w8 + 6]

# match = w9 == (w8 + 6) + 13  # False
# append(w9 + 13)
# [w0 + 14, w1 + 8, w8 + 6, w9 + 13]

# match = w10 == (w9 + 13) - 14  # True if w10 == w9 - 1
# pop()
# [w0 + 14, w1 + 8, w8 + 6]

# match = w11 == (w8 + 6) - 3  # True if w11 == w8 + 3
# pop()
# [w0 + 14, w1 + 8]

# match = w12 == (w1 + 8) - 2  # True if w12 == w1 + 6
# pop()
# [w0 + 14]

# match = w13 == (w0 + 14) - 14  # True if w13 == w0
# pop()
# []

# pos: 01234567890123
# max: 93499629698999
# min: 11164118121471


# Generalized solution:
def process1(s, part2=False):
  lines = s.strip('\n').split('\n')
  a, b, c = [[int(lines[i].split()[-1]) for i in range(m, 253, 18)]
             for m in (4, 5, 15)]
  solution = [1 if part2 else 9] * 14
  stack = []
  for i in range(14):
    if a[i] == 1:
      stack.append((i, c[i]))
    else:
      j, old_c = stack.pop()
      diff = old_c + b[i]
      # The constraint is that solution[i] - solution[j] == diff.
      if part2:
        solution[i] = 1 + diff if diff >= 0 else 1
        solution[j] = 1 if diff >= 0 else 1 - diff
      else:
        solution[i] = 9 if diff >= 0 else 9 + diff
        solution[j] = 9 - diff if diff >= 0 else 9

  return ''.join(map(str, solution))