From 0000034077e6846612dc289c4e667eb93a20c60a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Matej=20Jane=C5=BEi=C4=8D?= <janezic.mj@gmail.com>
Date: Mon, 18 Dec 2023 21:02:50 +0100
Subject: [PATCH] solution: day 18

---
 data/examples/18.txt |  14 +++++
 src/bin/18.rs        | 126 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 140 insertions(+)
 create mode 100644 data/examples/18.txt
 create mode 100644 src/bin/18.rs

diff --git a/data/examples/18.txt b/data/examples/18.txt
new file mode 100644
index 000000000..fc7612e
--- /dev/null
+++ b/data/examples/18.txt
@@ -0,0 +1,14 @@
+R 6 (#70c710)
+D 5 (#0dc571)
+L 2 (#5713f0)
+D 2 (#d2c081)
+R 2 (#59c680)
+D 2 (#411b91)
+L 5 (#8ceee2)
+U 2 (#caa173)
+L 1 (#1b58a2)
+U 2 (#caa171)
+R 2 (#7807d2)
+U 3 (#a77fa3)
+L 2 (#015232)
+U 2 (#7a21e3)
diff --git a/src/bin/18.rs b/src/bin/18.rs
new file mode 100644
index 000000000..2bf1150
--- /dev/null
+++ b/src/bin/18.rs
@@ -0,0 +1,126 @@
+use std::str::FromStr;
+
+#[derive(Debug, Clone, Copy)]
+enum Direction {
+    Up,
+    Down,
+    Left,
+    Right,
+}
+
+#[derive(Debug)]
+struct ParseDirectionError;
+
+impl FromStr for Direction {
+    type Err = ParseDirectionError;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        use Direction::*;
+
+        Ok(match s {
+            "U" | "3" => Up,
+            "D" | "1" => Down,
+            "L" | "2" => Left,
+            "R" | "0" => Right,
+            _ => return Err(ParseDirectionError),
+        })
+    }
+}
+
+impl From<Direction> for (isize, isize) {
+    fn from(value: Direction) -> Self {
+        use Direction::*;
+        match value {
+            Up => (-1, 0),
+            Down => (1, 0),
+            Left => (0, -1),
+            Right => (0, 1),
+        }
+    }
+}
+
+fn get_area(border: &[(isize, isize)], border_length: isize) -> isize {
+    // get area with shoelace formula (trapezoid variant)
+    // https://en.wikipedia.org/wiki/Shoelace_formula
+    let mut shoelace: isize = 0;
+    for n in 0..border.len() {
+        let (y1, x1) = border[n];
+        let (y2, x2) = border[(n + 1) % border.len()];
+        shoelace += (y1 + y2) * (x1 - x2);
+    }
+    let area = shoelace / 2;
+
+    // get interior by inverting pick's theorem formula
+    // https://en.wikipedia.org/wiki/Pick%27s_theorem
+    let interior = area + 1 - border_length / 2;
+    interior + border_length
+}
+
+fn get_border(instructions: &[(Direction, isize)]) -> (Vec<(isize, isize)>, isize) {
+    let mut border = Vec::new();
+    let mut border_length = 0;
+    let (mut sy, mut sx) = (0, 0);
+    border.push((sy, sx));
+
+    for (d, l) in instructions.iter().copied() {
+        let (dy, dx) = d.into();
+        (sy, sx) = (sy + l * dy, sx + l * dx);
+        border.push((sy, sx));
+        border_length += l;
+    }
+
+    border.pop();
+
+    (border, border_length)
+}
+
+pub fn part_one(input: &str) -> Option<isize> {
+    let instructions: Vec<(Direction, isize)> = input
+        .lines()
+        .map(|line| {
+            let [d, l, _] = line.splitn(3, ' ').collect::<Vec<_>>().try_into().unwrap();
+            (d.parse().unwrap(), l.parse::<isize>().unwrap())
+        })
+        .collect();
+
+    let (border, border_length) = get_border(&instructions);
+    Some(get_area(&border, border_length))
+}
+
+fn join_option_tuple<T, U>((a, b): (Option<T>, Option<U>)) -> Option<(T, U)> {
+    Some((a?, b?))
+}
+
+pub fn part_two(input: &str) -> Option<isize> {
+    let instructions: Vec<(Direction, isize)> = input
+        .lines()
+        .filter_map(|line| line.split_once(" (#"))
+        .filter_map(|(_, h)| h.strip_suffix(')'))
+        .map(|h| h.split_at(h.len() - 1))
+        .map(|(hex, dir)| (dir.parse().ok(), isize::from_str_radix(hex, 16).ok()))
+        .filter_map(join_option_tuple)
+        .collect();
+
+    let (border, border_length) = get_border(&instructions);
+    Some(get_area(&border, border_length))
+}
+
+aoc::solution!(18);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_part_one() {
+        assert_eq!(
+            part_one(&aoc::template::read_file("examples", 18)),
+            Some(62)
+        );
+    }
+    #[test]
+    fn test_part_two() {
+        assert_eq!(
+            part_two(&aoc::template::read_file("examples", 18)),
+            Some(952408144115)
+        );
+    }
+}