aoc2023_23.txt

Let X be the pre-step state.

Compute horizontal and vertical convolutions (4):

    let H = X← | X | X→;
    let V = X↑ | X | X↓;

Compute active elves (4):

    let A = X & (H↑ | H↓ | V← | V→);

Compute proposals (7):

    let U = A↑ & ¬H;
    let R = (¬U↓ & A)→ & ¬V;
    let D = (¬R← & A)↓ & ¬H
    let L = (¬U↑ & A)← & ¬V;

Resolve conflicts (5):

    let M = (U ^ D) | (R ^ L);
    let Vk = U & D;
    let Hk = R & L;

Put together (6):

    let O = (X & ¬A);
    let S = Hk← | Hk→ | Vk↑ | Vk↓;
    let Result = O | S | M;

Total combining operations per step: 26
Above the 21 I assumed...

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Another thing to consider:
Only performing operations on the parts of the grid that have been reached.
This would only really apply to rows if using SIMD.
Each row could be 256 bits wide though, so maybe the point is moot.