diff --git a/2023/src/common.rs b/2023/src/common.rs
index a43a168..075e5b2 100644
--- a/2023/src/common.rs
+++ b/2023/src/common.rs
@@ -327,8 +327,16 @@ impl<T: AsRef<[u8]>> Index<(usize, usize)> for Grid<T> {
     type Output = u8;
 
     fn index(&self, (y, x): (usize, usize)) -> &Self::Output {
-        debug_assert!(y < self.height());
-        debug_assert!(x < self.width());
+        debug_assert!(
+            y < self.height(),
+            "Y {y} should be lower than {}",
+            self.height()
+        );
+        debug_assert!(
+            x < self.width(),
+            "X {x} should be lower than {}",
+            self.width()
+        );
 
         let offset = self.width * y + x;
         &self.data.as_ref()[offset]
diff --git a/2023/src/day23.rs b/2023/src/day23.rs
index 7c1760f..58f95c1 100644
--- a/2023/src/day23.rs
+++ b/2023/src/day23.rs
@@ -1,7 +1,164 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use std::collections::HashMap;
+use std::collections::HashSet;
+
+use crate::common::Grid;
+use crate::common::IndexSet;
+
+#[derive(Clone, Copy, Debug)]
+enum Slope {
+    Level,
+    Up,
+    Down,
+}
+
+impl Slope {
+    fn combine(self, other: Self) -> Self {
+        match (self, other) {
+            (Slope::Level, next) | (next, Slope::Level) => next,
+            (Slope::Up, Slope::Up) => Slope::Up,
+            (Slope::Up, Slope::Down) | (Slope::Down, Slope::Up) => {
+                debug_assert!(false, "Inverted slope somehow");
+                Slope::Level
+            }
+            (Slope::Down, Slope::Down) => Slope::Down,
+        }
+    }
+}
+
+fn simplify_graph(input: &[u8]) -> anyhow::Result<Vec<Vec<(usize, u32)>>> {
+    let map = Grid::new(input)?;
+    let width = map.width();
+    let height = map.height();
+
+    let is_junction = |x, y| {
+        let mut entrance_count = 0;
+
+        if map[(y, x + 1)] != b'#' {
+            entrance_count += 1;
+        }
+        if map[(y, x - 1)] != b'#' {
+            entrance_count += 1;
+        }
+        if map[(y - 1, x)] != b'#' {
+            entrance_count += 1;
+        }
+        if map[(y + 1, x)] != b'#' {
+            entrance_count += 1;
+        }
+
+        entrance_count >= 3
+    };
+
+    let mut visited = IndexSet::with_capacity(width * height);
+    let mut graph = vec![vec![]; 3];
+    visited.insert(1); // x = 1, y = 0;
+    visited.insert(1 + width); // x = 1, y = 1
+    visited.insert(width * height - 2);
+
+    let mut nodes = HashMap::new();
+    nodes.insert((1, 0), 0usize);
+    nodes.insert((width - 2, height - 1), 1);
+    nodes.insert((1, 1), 2);
+
+    let mut todo_junctions = vec![(2, 1, 1)];
+
+    let mut todo_positions = Vec::new();
+
+    while let Some((id, start_x, start_y)) = todo_junctions.pop() {
+        todo_positions.push((0, Slope::Level, start_x, start_y));
+
+        while let Some((dist, slope, x, y)) = todo_positions.pop() {
+            let mut enqueue = |x: usize, y: usize, up, down| {
+                if map[(y, x)] == b'#' {
+                    return;
+                } else if let Some(&other) = nodes.get(&(x, y)) {
+                    if other == id {
+                        return;
+                    }
+
+                    if matches!(slope, Slope::Down | Slope::Level) {
+                        graph[id].push((other, dist + 1));
+                    }
+                    if matches!(slope, Slope::Up | Slope::Level) {
+                        graph[other].push((id, dist + 1));
+                    }
+                } else if visited.insert(x + y * width) {
+                    let tile_slope = if map[(y, x)] == up {
+                        Slope::Up
+                    } else if map[(y, x)] == down {
+                        Slope::Down
+                    } else {
+                        Slope::Level
+                    };
+                    let resulting_slope = slope.combine(tile_slope);
+
+                    if is_junction(x, y) {
+                        let new_id = graph.len();
+                        nodes.insert((x, y), new_id);
+                        graph.push(Vec::new());
+                        if matches!(slope, Slope::Down | Slope::Level) {
+                            graph[id].push((new_id, dist + 1));
+                        }
+                        if matches!(slope, Slope::Up | Slope::Level) {
+                            graph[new_id].push((id, dist + 1));
+                        }
+
+                        todo_junctions.push((new_id, x, y));
+                    } else {
+                        todo_positions.push((dist + 1, resulting_slope, x, y));
+                    }
+                }
+            };
+            enqueue(x - 1, y, b'>', b'<');
+            enqueue(x + 1, y, b'<', b'>');
+            enqueue(x, y - 1, b'v', b'^');
+            enqueue(x, y + 1, b'^', b'v');
+        }
+    }
+
+    Ok(graph)
+}
+
+fn longest_path(
+    pos: usize,
+    travelled: u32,
+    graph: &[Vec<(usize, u32)>],
+    visited: &mut HashSet<usize>,
+) -> u32 {
+    if pos == 1 {
+        return travelled;
+    }
+
+    let mut best = 0;
+
+    for &(other, dist) in &graph[pos] {
+        if visited.insert(other) {
+            best = Ord::max(best, longest_path(other, travelled + dist, graph, visited));
+            visited.remove(&other);
+        }
+    }
+
+    best
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let graph = simplify_graph(input)?;
+
+    Ok(longest_path(0, 0, &graph, &mut HashSet::new()).to_string())
 }
 
 pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
     anyhow::bail!("Not implemented")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/23.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!("94", part1(SAMPLE).unwrap());
+    }
+}
diff --git a/2023/src/samples/23.txt b/2023/src/samples/23.txt
new file mode 100644
index 0000000..ea945a4
--- /dev/null
+++ b/2023/src/samples/23.txt
@@ -0,0 +1,23 @@
+#.#####################
+#.......#########...###
+#######.#########.#.###
+###.....#.>.>.###.#.###
+###v#####.#v#.###.#.###
+###.>...#.#.#.....#...#
+###v###.#.#.#########.#
+###...#.#.#.......#...#
+#####.#.#.#######.#.###
+#.....#.#.#.......#...#
+#.#####.#.#.#########v#
+#.#...#...#...###...>.#
+#.#.#v#######v###.###v#
+#...#.>.#...>.>.#.###.#
+#####v#.#.###v#.#.###.#
+#.....#...#...#.#.#...#
+#.#########.###.#.#.###
+#...###...#...#...#.###
+###.###.#.###v#####v###
+#...#...#.#.>.>.#.>.###
+#.###.###.#.###.#.#v###
+#.....###...###...#...#
+#####################.#