C

Just tracing the ray. When it splits, recurse one way and continue the other. Didn’t bother with a direction lookup table this time, just a few ifs. The ray ends when it goes out of bounds or a ray in that direction has been previously traced on a given cell (this is tracked with a separate table).

It would’ve been straightforward if I hadn’t gotten the ‘previously visited’ check wrong 😞. I was checking against the direction coming in of the tile but marking the direction going out.

Ray function:

static void
ray(int x, int y, int dir)
{
	int c;

	while (x>=0 && y>=0 && x<w && y<h) {
		if (beams[y][x] & (1 << dir))
			break;

		beams[y][x] |= 1 << dir;
		c = map[y][x];

		if (dir==NN && c=='/')  dir = EE; else
		if (dir==EE && c=='/')  dir = NN; else
		if (dir==SS && c=='/')  dir = WW; else
		if (dir==WW && c=='/')  dir = SS; else
		if (dir==NN && c=='\\') dir = WW; else
		if (dir==EE && c=='\\') dir = SS; else
		if (dir==SS && c=='\\') dir = EE; else
		if (dir==WW && c=='\\') dir = NN; else
		if (dir==NN && c=='-') { ray(x,y,WW); dir = EE; } else
		if (dir==SS && c=='-') { ray(x,y,WW); dir = EE; } else
		if (dir==EE && c=='|') { ray(x,y,NN); dir = SS; } else
		if (dir==WW && c=='|') { ray(x,y,NN); dir = SS; } 

		x += dir==EE ? 1 : dir==WW ? -1 : 0;
		y += dir==SS ? 1 : dir==NN ? -1 : 0;
	}
}

https://github.com/sjmulder/aoc/blob/master/2023/c/day16.c-