Implement 2023 day 18 part 2

This was not at all the twist I expected, but I like it.

2023/src/day12.rs

@@ -33,7 +33,7 @@ fn number_ways(line: &[u8], groups: &[u8]) -> u64 {
 
                 // Either defective or maybe defective
                 if c != b'.' && cur_group < usize::from(group) {
-                    next[group_pos * group_stride + cur_group as usize + 1] += ways;
+                    next[group_pos * group_stride + cur_group + 1] += ways;
                 }
 
                 if c != b'#' {

2023/src/day18.rs

@@ -1,7 +1,147 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use anyhow::Context;
+use nom::bytes::complete::tag;
+use nom::bytes::complete::take;
+use nom::bytes::complete::take_until;
+use nom::combinator::map;
+use nom::combinator::map_res;
+use nom::multi::many1;
+use nom::sequence::delimited;
+use nom::sequence::pair;
+use nom::sequence::terminated;
+use nom::IResult;
+
+use crate::common::parse_input;
+use crate::common::Direction;
+
+struct Dig {
+    dir: Direction,
+    amount: u64,
 }
 
-pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+fn parse_instructions(i: &[u8]) -> IResult<&[u8], Vec<Dig>> {
+    many1(terminated(
+        map(
+            pair(
+                terminated(
+                    map_res(take(1usize), |c: &[u8]| {
+                        Ok(match c[0] {
+                            b'L' => Direction::Left,
+                            b'U' => Direction::Up,
+                            b'R' => Direction::Right,
+                            b'D' => Direction::Down,
+                            e => return Err(format!("Invalid digging direction {e}")),
+                        })
+                    }),
+                    tag(" "),
+                ),
+                terminated(nom::character::complete::u64, tag(" ")),
+            ),
+            |(dir, amount)| Dig { dir, amount },
+        ),
+        take(10usize),
+    ))(i)
+}
+
+fn compute_points(instructions: &[Dig]) -> Vec<(i64, i64)> {
+    let mut result = Vec::with_capacity(instructions.len() + 1);
+    result.push((0, 0));
+
+    let mut x = 0;
+    let mut y = 0;
+
+    for &Dig { dir, amount } in instructions {
+        match dir {
+            Direction::Up => y -= amount as i64,
+            Direction::Left => x -= amount as i64,
+            Direction::Down => y += amount as i64,
+            Direction::Right => x += amount as i64,
+        }
+
+        result.push((x, y));
+    }
+
+    debug_assert_eq!(result.first(), result.last());
+
+    result
+}
+
+fn shoelace(points: &[(i64, i64)]) -> i64 {
+    points
+        .windows(2)
+        .map(|w| w[0].0 * w[1].1 - w[0].1 * w[1].0)
+        .sum::<i64>()
+        / 2
+}
+
+fn solve(digs: &[Dig]) -> anyhow::Result<String> {
+    let points = compute_points(digs);
+
+    let area = shoelace(&points);
+    // Assumption: we don't cross over ourselves
+    let perimeter = digs.iter().map(|dig| dig.amount as i64).sum::<i64>();
+
+    let total = area + perimeter / 2 + 1;
+
+    Ok(total.to_string())
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let digs = parse_input(input, parse_instructions)?;
+
+    solve(&digs)
+}
+
+fn parse_colors(i: &[u8]) -> IResult<&[u8], Vec<Dig>> {
+    fn parse_color(i: &[u8]) -> anyhow::Result<u64> {
+        // stdlib offers hex parsing but requires going through string, incurring utf-8 validation.
+        // better do it ourselves.
+        let mut num = 0;
+
+        for &c in &i[1..] {
+            num *= 16;
+            num += (c as char).to_digit(16).context("Invalid hex")?;
+        }
+
+        Ok(num.into())
+    }
+    many1(delimited(
+        take_until("#"),
+        map_res(take(7usize), |color: &[u8]| -> anyhow::Result<Dig> {
+            let num = parse_color(color)?;
+            let amount = num >> 4;
+
+            let dir = match num & 0xF {
+                0 => Direction::Right,
+                1 => Direction::Down,
+                2 => Direction::Left,
+                3 => Direction::Up,
+                other => anyhow::bail!("Unknown direction: {other}"),
+            };
+
+            Ok(Dig { dir, amount })
+        }),
+        tag(")\n"),
+    ))(i)
+}
+
+pub fn part2(input: &[u8]) -> anyhow::Result<String> {
+    let digs = parse_input(input, parse_colors)?;
+    solve(&digs)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/18.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!("62", part1(SAMPLE).unwrap());
+    }
+
+    #[test]
+    fn sample_part2() {
+        assert_eq!("952408144115", part2(SAMPLE).unwrap());
+    }
 }

2023/src/samples/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)