From 1e1e22be250ff2c755d15e67a61b899c9c6654e9 Mon Sep 17 00:00:00 2001
From: Kevin Caffrey <kcaffrey@gmail.com>
Date: Mon, 18 Dec 2023 10:47:02 -0500
Subject: [PATCH] day 18

---
 README.md            |   3 +-
 data/examples/18.txt |  14 +++++
 src/bin/18.rs        | 119 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 135 insertions(+), 1 deletion(-)
 create mode 100644 data/examples/18.txt
 create mode 100644 src/bin/18.rs

diff --git a/README.md b/README.md
index 9794559..466ba7a 100644
--- a/README.md
+++ b/README.md
@@ -50,8 +50,9 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 15](./src/bin/15.rs) | `20.4µs` | `85.9µs` |
 | [Day 16](./src/bin/16.rs) | `58.3µs` | `1.7ms` |
 | [Day 17](./src/bin/17.rs) | `1.6ms` | `3.7ms` |
+| [Day 18](./src/bin/18.rs) | `8.1µs` | `7.7µs` |
 
-**Total: 14.07ms**
+**Total: 14.09ms**
 <!--- benchmarking table --->
 
 ---
diff --git a/data/examples/18.txt b/data/examples/18.txt
new file mode 100644
index 0000000..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 0000000..b17530d
--- /dev/null
+++ b/src/bin/18.rs
@@ -0,0 +1,119 @@
+advent_of_code::solution!(18);
+
+pub fn part_one(input: &str) -> Option<i64> {
+    let mut border_points = 0;
+    let mut area = 0;
+    let mut prev = Point::origin();
+    for line in input
+        .as_bytes()
+        .split(|&ch| ch == b'\n')
+        .filter(|line| !line.is_empty())
+    {
+        let dir: Direction = line[0].try_into().expect("valid direction");
+        let distance = line[2..]
+            .iter()
+            .copied()
+            .take_while(|&ch| ch != b' ')
+            .fold(0, |acc, ch| acc * 10 + i64::from(ch - b'0'));
+        let next = prev.move_in_dir(dir, distance);
+        area += prev.x * next.y - prev.y * next.x;
+        border_points += distance;
+        prev = next;
+    }
+    area = area.abs() / 2;
+    Some(area + border_points / 2 + 1)
+}
+
+pub fn part_two(input: &str) -> Option<i64> {
+    let mut border_points = 0;
+    let mut area = 0;
+    let mut prev = Point::origin();
+    for line in input
+        .as_bytes()
+        .split(|&ch| ch == b'\n')
+        .filter(|line| !line.is_empty())
+    {
+        let hex = &line[line.len() - 7..line.len() - 1];
+        let dir: Direction = hex[5].try_into().expect("valid direction");
+        let distance = hex[..5]
+            .iter()
+            .copied()
+            .take_while(|&ch| ch != b' ')
+            .fold(0, |acc, ch| {
+                acc * 16 + char::from(ch).to_digit(16).unwrap() as i64
+            });
+        let next = prev.move_in_dir(dir, distance);
+        area += prev.x * next.y - prev.y * next.x;
+        border_points += distance;
+        prev = next;
+    }
+    area = area.abs() / 2;
+    Some(area + border_points / 2 + 1)
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+struct Point {
+    x: i64,
+    y: i64,
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+enum Direction {
+    North,
+    South,
+    East,
+    West,
+}
+
+impl Point {
+    pub const fn new(x: i64, y: i64) -> Self {
+        Self { x, y }
+    }
+
+    pub const fn origin() -> Self {
+        Self::new(0, 0)
+    }
+
+    pub const fn move_in_dir(self, dir: Direction, distance: i64) -> Self {
+        match dir {
+            Direction::North => Point::new(self.x, self.y - distance),
+            Direction::South => Point::new(self.x, self.y + distance),
+            Direction::East => Point::new(self.x + distance, self.y),
+            Direction::West => Point::new(self.x - distance, self.y),
+        }
+    }
+}
+
+#[derive(Debug)]
+struct InvalidDirectionError(char);
+
+impl TryFrom<u8> for Direction {
+    type Error = InvalidDirectionError;
+
+    fn try_from(value: u8) -> Result<Self, Self::Error> {
+        Ok(match value {
+            b'U' | b'3' => Self::North,
+            b'D' | b'1' => Self::South,
+            b'L' | b'2' => Self::West,
+            b'R' | b'0' => Self::East,
+            _ => return Err(InvalidDirectionError(char::from(value))),
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(62));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(952408144115));
+    }
+}