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[2024 day 24] - Pt2 - Manual Inspection?
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this post was submitted on 24 Dec 2024
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Yeah - dumped out the input into GraphViz, and then inspected it 'by eye' to get the swaps. Nearly finished in the top 100 in the world, too. Feels like a really bad way to get the solution, though.
If you add eg. 1111 and 1111 and expect 11110, then you'll get an output like 11010 if there's a mistake in "bit 2". Can try all the swaps between x2 / y2 / z2 until you get the "right answer", and then continue. There's only about five different ops for each "bit" of the input, so trying all of them won't take too long.
I know - thank you, though, good to know it's not just me. Not the first puzzle that I've solved using GraphViz, either.
Some of them do depend on some unstated property of the input that can only be discerned by inspecting it - I don't feel too bad about that kind of 'cheat', as long as it goes from "the input" -> "your code" -> "the output".
Some of them - and I'm thinking another that ludicrous "line up the hailstones" one from day 24 from last year - are the kind where you parse the input so you can output it in the right format for Wolfram Alpha. Most unsatisfying as a coding puzzle.
Im halfway through doing that, definitely feels wrong though. I'm curious to see if there is a good programatic way of doing it.
Dont you need a pair of broken bits? For mine, bit 6 is broken, because its just x6^y6. So I need to find where the carry bit got swapped to. Or are you suggesting that I swap my bit 6 operation with every other operation until it resolves?
Mine definitely had outputs swapped between adders,
z06was justx06 XOR y06. The circuit was completely broken there.Every "pair" of bits has the same pattern of AND and XOR, and the previous "carry bit" is passed into the same OR / (XOR + AND) combo to produce an output bit and the next "carry bit". The "whole chain" is nearly right - otherwise your 44 bit inputs wouldn't give a 45 bit output - it's just a few are swapped over. (In my case, anyway - haven't seen any others.) All my swaps were either in the "same column" of GraphViz output, or the next column.
So, yeah. Either "random swaps" with "nearby" outputs, because it's nearly right and you don't need to check further away; or use the fact that this is the well-known pattern for adding two numbers in a CPU's ALU to generate the "correct" sequence, identify which ones are wrong, and output them in alphabetical order.. The answer you need doesn't even require you to pair them up.