Day 13: Claw Contraption

Megathread guidelines

  • Keep top level comments as only solutions, if you want to say something other than a solution put it in a new post. (replies to comments can be whatever)
  • You can send code in code blocks by using three backticks, the code, and then three backticks or use something such as https://topaz.github.io/paste/ if you prefer sending it through a URL

FAQ

  • Sparrow_1029@programming.dev
    2·
    2 days ago

    Thank you so much for your explanation! I kind of found some clues looking up perp dot products & other vector math things, but this breaks it down very nicely.

    I implemented your solution in rust, and part 2 failed by +447,761,194,259 (this was using signed 64-bit integers, i64). When I changed it to use signed 64 bit floating-point f64 and checked that the solution for x produces a whole number it worked.

    • Acters@lemmy.world
      3·
      2 days ago

      Did you run my python code as is? I would hope it works for everyone. though, I am unsure what the edge cases are, then.

      • Sparrow_1029@programming.dev
        1·
        1 day ago

        I did run your code as-is in an ipython REPL to check. These were the results:

        REPL session 
        # With unmodified `main` function & `import string` not shown
In [4]: with open("inputs/day13.txt", "r") as f:
   ...:     input_data = f.read().strip().replace(',', '').split('\n\n')
   ...:

In [5]: part_one, part_two = main(input_data)

In [6]: part_one
Out[6]: 39748

In [7]: part_two
Out[7]: 74926346266863

# Then I modified the function to check if x is fractional
In [8]: def main(input_data):
   ...:     part1_total_cost = 0
   ...:     part2_total_cost = 0
   ...:     for machine in input_data:
   ...:         Ax,Ay,Bx,By,Px,Py = [ int(l[2:]) for l in machine.split() if l[-1] in string.digits ]
   ...:         y,r = divmod((Ay * Px - Ax * Py), (Ay * Bx - Ax * By))
   ...:         if r == 0:
   ...:             x = (Px - Bx * y) / Ax
   ...:             if x % 1 == 0:
   ...:                 part1_total_cost += 3*x + y
   ...:         y,r = divmod((Ay * (Px+10000000000000) - Ax * (Py+10000000000000)), (Ay * Bx - Ax * By))
   ...:         if r == 0:
   ...:             x = ((Px+10000000000000) - Bx * y) / Ax
   ...:             if x % 1 == 0:
   ...:                 part2_total_cost += 3*x + y
   ...:
   ...:     return part1_total_cost,part2_total_cost
   ...:

In [9]: part_one, part_two = main(input_data)

In [10]: part_one
Out[10]: 39748.0

In [11]: part_two
Out[11]: 74478585072604.0  # Correct answer for pt 2 of my input

        If you’re curious to check against my puzzle input, it’s here

        Thank you again for the back & forth, and for sharing your solution!

        • Acters@lemmy.world
          2·
          23 hours ago

          there is exactly ONE “machine” that causes your result to be incorrect. ONLY for part 2.

          Button A: X+67, Y+67
Button B: X+16, Y+73
Prize: X=4877, Y=7214

          I see now what your corner case causes. so when my script solves for y first. it will be exact. BUT when you solve for x, it will be not divisible… makes sense now. I didn’t expect this. This only occurs because of part 2! so dastardly. well, that was interesting. I guess I am forced to add that extra check… rip those microsecond gains.