Wherein I waste time on a repeat

Problem
Code

I spent so much time today trying to count the points inside the lagoon by going along the edge and trying to smartly count the points inside per-row... when it is simply the exact same problem as in day 10—you need to calculate the area of a polygon. Though, unlike day 10, this time including the entire outline.

Once I had the realization, it was simply a matter of writing the whole shoelace/Pick's thing again. I haven't described it in my day 10 post yet, because it was a solution I found out about later; but the idea is this:

• calculate the "real" area of the shape using the Shoelace formula;
• using the inverse of Pick's theorem, go from the "real" area to the number of integer points inside the shape.

fn lagoon_size(input: &str, decode: bool) -> i64 {
    let mut points = 1;
    let mut pos = (0, 0);
    let mut area = 0;

    for (dir, count) in parse(input, decode) {
        points += count;
        let next_pos = match dir {
            b'L' => (pos.0 - count, pos.1),
            b'U' => (pos.0, pos.1 - count),
            b'R' => (pos.0 + count, pos.1),
            b'D' => (pos.0, pos.1 + count),
            _ => unreachable!(),
        };
        area += pos.0 * next_pos.1 - pos.1 * next_pos.0;
        pos = next_pos;
    }

    area / 2 + points / 2 + 1
}

It's very little code, and it's very efficient. I go through the instructions once, counting how many points the edge has, and doing the shoelace formula, at the same time. Then, at the end I turn area into the number of points inside (area / 2 - points / 2 + 1) plus the number of points on the shape, thus giving me the final result.

I can't believe it took me so long to do this, but sometimes you just forget things you should know.