C

Chose not to do transposing/flipping or fancy indexing so it’s rather verbose, but it’s also clear and (I think) fast. I also tried to limit the number of search steps by keeping two cursors in the current row/col, rather than shooting a ray every time.

Part 2 immediately reminded me of that Tetris puzzle from day 22 last year so I knew how to find and apply the solution. State hashes are stored in an array and (inefficiently) scanned until a loop is found.

One direction of the shift function:

/*
 * Walk two cursors i and j through each column x. The i cursor
 * looks for the first . where an O can go. The j cursor looks
 * ahead for O's. When j finds a # we start again beyond it.
 */
for (x=0; x<w; x++)
for (i=0, j=1; i<h && j<h; )
	if (j <= i) j = i+1;
	else if (g[j][x] == '#') i = j+1;
	else if (g[j][x] != 'O') j++;
	else if (g[i][x] != '.') i++;
	else { g[i++][x] = 'O'; g[j++][x] = '.'; vis14_emit(); }

The main loop:

for (nleft = 1*1000*1000*1000; nleft; nleft--) {
	shift_all();

	if (!period) {
		assert(nhist < (int)LEN(hist));		
		hist[nhist++] = hash_grid();

		for (i=0; i<nhist-1; i++)
			if (hist[i] == hist[nhist-1]) {
				period = nhist-1 - i;
				nleft = nleft % period;
				break;
			}
	}
}

Full solution: https://github.com/sjmulder/aoc/blob/master/2023/c/day14.c