C

Bit of typing and testing again but part 2 was fun and not hard, although I did make a few logic mistakes.

Didn't optimize anything in particular but as in day 8 (iirc) I avoid dealing with strings for references to things, so I keep a string table and only use indices.

Abridged excerpt of data structures and eval():

struct expr { int type, var, imm, next; };
struct partrange { int min[4], max[4]; };

static struct expr flows[600][5];
static int accept_id, reject_id, in_id;

static int64_t eval(int id, struct partrange p)
{
	struct partrange q;
	struct expr *e;
	int64_t acc=0;
	int i;

	if (id == reject_id ||
	    p.min[0] > p.max[0] || p.min[1] > p.max[1] ||
	    p.min[2] > p.max[2] || p.min[3] > p.max[3])
	    	return 0;

	if (id == accept_id)
		return (int64_t)
		    (p.max[0] -p.min[0] +1) * (p.max[1] -p.min[1] +1) *
		    (p.max[2] -p.min[2] +1) * (p.max[3] -p.min[3] +1);

	for (i=0; i < (int)LEN(*flows); i++)
		switch ((e = &flows[id][i])->type) {
		case EXPR_LT:
			q = p;
			q.max[e->var] = MIN(q.max[e->var], e->imm-1);
			p.min[e->var] = MAX(p.min[e->var], e->imm);
			acc += eval(e->next, q);
			break;
		case EXPR_GT:
			q = p;
			q.min[e->var] = MAX(q.min[e->var], e->imm+1);
			p.max[e->var] = MIN(p.max[e->var], e->imm);
			acc += eval(e->next, q);
			break;
		case EXPR_CALL:
			acc += eval(e->next, p);
			return acc;
		}
	
	assert(!"bad flow");
}

https://github.com/sjmulder/aoc/blob/master/2023/c/day19.c