From 8f6937ae4299747e7368c60885e91682012c62d0 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Fri, 8 Dec 2023 07:14:36 +0100
Subject: [PATCH] Implement 2023 day 8 part 1

---
 2023/src/day08.rs         | 94 ++++++++++++++++++++++++++++++++++++++-
 2023/src/samples/08.1.txt |  9 ++++
 2023/src/samples/08.2.txt |  5 +++
 3 files changed, 106 insertions(+), 2 deletions(-)
 create mode 100644 2023/src/samples/08.1.txt
 create mode 100644 2023/src/samples/08.2.txt

diff --git a/2023/src/day08.rs b/2023/src/day08.rs
index 7c1760f..1c60e39 100644
--- a/2023/src/day08.rs
+++ b/2023/src/day08.rs
@@ -1,7 +1,97 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use nom::bytes::complete::tag;
+use nom::bytes::complete::take;
+use nom::bytes::complete::take_until;
+use nom::combinator::map;
+use nom::multi::fold_many1;
+use nom::sequence::separated_pair;
+use nom::sequence::terminated;
+use nom::sequence::tuple;
+use nom::IResult;
+
+use crate::common::parse_input;
+
+const NUM_PLACES: usize = 26 * 26 * 26;
+
+struct Map<'a> {
+    instructions: &'a [u8],
+    transitions: Box<[(u16, u16); NUM_PLACES]>,
+}
+
+impl<'a> Map<'a> {
+    #[inline]
+    fn transition(&self, pos: u16, step: u8) -> u16 {
+        if step == b'L' {
+            self.transitions[pos as usize].0
+        } else {
+            self.transitions[pos as usize].1
+        }
+    }
+}
+
+fn place_to_index(place: &[u8]) -> u16 {
+    place
+        .iter()
+        .fold(0u16, |index, &c| index * 26 + u16::from(c - b'A'))
+}
+
+fn parse_map(i: &[u8]) -> IResult<&[u8], Map<'_>> {
+    map(
+        separated_pair(
+            take_until("\n"),
+            tag("\n\n"),
+            fold_many1(
+                tuple((
+                    terminated(take(3usize), tag(" = (")),
+                    terminated(take(3usize), tag(", ")),
+                    terminated(take(3usize), tag(")\n")),
+                )),
+                || Box::new([(0, 0); NUM_PLACES]),
+                |mut transitions, (pos, left, right)| {
+                    let pos = place_to_index(pos);
+                    let left = place_to_index(left);
+                    let right = place_to_index(right);
+                    transitions[pos as usize] = (left, right);
+                    transitions
+                },
+            ),
+        ),
+        |(instructions, transitions)| Map {
+            instructions,
+            transitions,
+        },
+    )(i)
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let map = parse_input(input, parse_map)?;
+    let end = place_to_index(b"ZZZ");
+    let mut pos = 0;
+
+    for (count, &step) in map.instructions.iter().cycle().enumerate() {
+        if pos == end {
+            return Ok(count.to_string());
+        }
+
+        pos = map.transition(pos, step);
+    }
+
+    anyhow::bail!("Unreachable, loop is infinite");
 }
 
 pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
     anyhow::bail!("Not implemented")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/08.1.txt");
+    const SAMPLE2: &[u8] = include_bytes!("samples/08.2.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!("2", part1(SAMPLE).unwrap());
+        assert_eq!("6", part1(SAMPLE2).unwrap());
+    }
+}
diff --git a/2023/src/samples/08.1.txt b/2023/src/samples/08.1.txt
new file mode 100644
index 0000000..9029a1b
--- /dev/null
+++ b/2023/src/samples/08.1.txt
@@ -0,0 +1,9 @@
+RL
+
+AAA = (BBB, CCC)
+BBB = (DDD, EEE)
+CCC = (ZZZ, GGG)
+DDD = (DDD, DDD)
+EEE = (EEE, EEE)
+GGG = (GGG, GGG)
+ZZZ = (ZZZ, ZZZ)
diff --git a/2023/src/samples/08.2.txt b/2023/src/samples/08.2.txt
new file mode 100644
index 0000000..7d1b58d
--- /dev/null
+++ b/2023/src/samples/08.2.txt
@@ -0,0 +1,5 @@
+LLR
+
+AAA = (BBB, BBB)
+BBB = (AAA, ZZZ)
+ZZZ = (ZZZ, ZZZ)