From 79b809c3b7bf91d0ffc20999be999ba4ee3a4fbf Mon Sep 17 00:00:00 2001
From: Lars Martens <lars@martens.red>
Date: Tue, 10 Dec 2024 07:37:53 +0100
Subject: [PATCH] Solve 10

---
 src/bin/10.rs | 111 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 111 insertions(+)
 create mode 100644 src/bin/10.rs

diff --git a/src/bin/10.rs b/src/bin/10.rs
new file mode 100644
index 0000000..a0f6fa4
--- /dev/null
+++ b/src/bin/10.rs
@@ -0,0 +1,111 @@
+use aoc::*;
+use glam::IVec2;
+use std::collections::{HashMap, HashSet};
+use std::iter;
+
+const INPUT: &str = include_str!("../../input/10");
+
+fn main() {
+    assert_example!(part1, "10-test", 36);
+    println!("Part 1: {}", part1(INPUT));
+    assert_example!(part2, "10-test", 81);
+    println!("Part 2: {}", part2(INPUT));
+}
+
+fn part1(input: &str) -> usize {
+    let map = Map::parse(input);
+    map.trailheads().map(|t| map.reachable(t).len()).sum()
+}
+
+fn part2(input: &str) -> usize {
+    let map = Map::parse(input);
+    map.trailheads().map(|t| map.distinct_trails(t)).sum()
+}
+
+fn directions() -> [IVec2; 4] {
+    [
+        IVec2::new(1, 0),
+        IVec2::new(-1, 0),
+        IVec2::new(0, 1),
+        IVec2::new(0, -1),
+    ]
+}
+
+#[derive(Debug)]
+struct Map {
+    height: HashMap<IVec2, i64>,
+}
+
+impl Map {
+    fn reachable(&self, from: IVec2) -> HashSet<IVec2> {
+        let from_height = match self.get_height(from) {
+            Some(9) => return [from].into(),
+            None => return [].into(),
+            Some(h) => h,
+        };
+
+        self.climb_up(from, from_height)
+            .into_iter()
+            .map(|pos| self.reachable(pos))
+            .fold(HashSet::new(), |mut acc, other| {
+                acc.extend(other);
+                acc
+            })
+    }
+
+    fn distinct_trails(&self, from: IVec2) -> usize {
+        let from_height = match self.get_height(from) {
+            Some(9) => return 1,
+            None => return 0,
+            Some(h) => h,
+        };
+
+        self.climb_up(from, from_height)
+            .into_iter()
+            .map(|pos| self.distinct_trails(pos))
+            .sum()
+    }
+
+    fn climb_up(&self, from: IVec2, from_height: i64) -> Vec<IVec2> {
+        directions()
+            .into_iter()
+            .filter_map(|dir| {
+                let pos = from + dir;
+                self.get_height(pos).map(|height| (pos, height))
+            })
+            .filter_map(|(pos, h)| {
+                if h == from_height + 1 {
+                    Some(pos)
+                } else {
+                    None
+                }
+            })
+            .collect()
+    }
+
+    fn get_height(&self, at: IVec2) -> Option<i64> {
+        self.height.get(&at).copied()
+    }
+
+    fn trailheads(&self) -> impl Iterator<Item = IVec2> + use<'_> {
+        self.height.iter().filter_map(|(&pos, &h)| match h {
+            0 => Some(pos),
+            _ => None,
+        })
+    }
+
+    fn parse(input: &str) -> Self {
+        let height = input
+            .lines()
+            .enumerate()
+            .flat_map(|(y, line)| line.chars().enumerate().zip(iter::repeat(y)))
+            .map(|((x, c), y)| {
+                let pos = IVec2::new(x as i32, y as i32);
+                let height = c.to_digit(10).unwrap() as i64;
+                (pos, height)
+            })
+            .collect();
+
+        Self { height }
+    }
+}