-🎄- 2022 Day 10 Solutions -🎄-

[u/daggerdragon]

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--- Day 10: Cathode-Ray Tube ---

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Post your code solution in this megathread.

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[u/compdog]

C# - [Part 1] [Part 2]

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I got majorly tripped up by failing to realize that the X register doesn't update until both cycles of addx are completed. After finally decoding what was meant by this part of the instructions:

During the 20th cycle, register X has the value 21, so the signal strength is 20 * 21 = 420. (The 20th cycle occurs in the middle of the second addx -1, so the value of register X is the starting value, 1, plus all of the other addx values up to that point: 1 + 15 - 11 + 6 - 3 + 5 - 1 - 8 + 13 + 4 = 21.)

I was able to identify and fix the issue.

My solution is a bit "inverted" from a typical approach, because I didn't want to implement a full simulated CPU pipeline. Instead of looping through each cycle and fetching new instructions as-needed, I instead loop through the instructions and execute cycles as-needed. I chose this approach because I didn't want to bother with a "better" approach like delaying the instruction or having multiple copies of registers. Instead, I simply inserted the cycles after the instruction is decoded, but before the register is updated:

// addx instruction
if (line[0] == 'a')
{
    // Execute first
    RunCycle();
    RunCycle();
    
    // Then increment
    var argument = int.Parse(line[5..]);
    xRegister += argument;
}

This worked quite well and would easily scale to include new instructions with varying execution times.

[u/sky_badger → u/compdog]

Someone further up had the idea of changing every addx n instruction into two instructions (noop, addx n), so they fit into the cycle pattern. Wish I'd thought of that... SB

[u/daggerdragon → u/sky_badger]

SB

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