Ancestors

Written by Jérémie Chassaing on 2024-12-17 at 12:26

Day 17 part 2 was fun.

I guess all programs were similar

Taking bits from A 3by3

This bits are xor with 2 constants to get an index and a value

Bits from a at this index are xor with value before output

Solver for part 2 is trying numbers in depth first from 0 to 7.

Each value impose further bits, so keep a mask of fixed bits.

At each step validate value that is tried with already set bits, this cuts all invalid branches early and terminates in less than 100 microseconds

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Toot

Written by Jérémie Chassaing on 2024-12-17 at 12:32

The only difference is ptobably the 2 constants used to xor, and maybe which registers are used for comptation

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Descendants

Written by Jérémie Chassaing on 2024-12-17 at 12:35

And the second number of the opcode that takes no argument also.

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Written by Jérémie Chassaing on 2024-12-17 at 17:24

It was also important to notice that all operations are bitwise operations.

X mod 8 is X & 0b111

X div 2^N is X shift right by N

The rest is just xor

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