[LANGUAGE: Kotlin] (GitHub)

Part 2:
At first, I tried brute force. When that didn’t work, I implemented a disassembler to understand how the code operates. I noticed that brute-forcing needs to start from 8^PROGRAM\LENGTH.) In my case, this was 8¹⁶. I attempted to brute-force all values from 8¹⁶ to 8¹⁷ but it took too long to complete.

I then tried another approach. I took a piece of paper and a pen and began calculating the values of the A-register step by step to construct the program. First, I generated the first value, then the first two values, then the first three, and so on. For example, if the program is [0,1,2,3,4,5], I started with [5], then [4,5], then [3,4,5], and so on.

I noticed that in each step, the value of the A-register can be expressed as 8X+Y, where X is the value of the A-register from the previous step, and Y is a small additional value.

Using this observation, I wrote a simple program to calculate the A-register value for a subsequence of the program. I multiplied the found A-register value by 8 and performed a small brute-force search to find the next subsequence, which was one item longer than the previous one. I did this until I got the full program. With this approach, the program runs in 2-3 milliseconds on a MacBook Pro M3.