From 7647cd8487d4a20bc4d1e88ecccff10a018ad96c Mon Sep 17 00:00:00 2001
From: Bauke <me@bauke.xyz>
Date: Wed, 15 Dec 2021 18:11:19 +0100
Subject: [PATCH] Solve day 15!

---
 Cargo.toml           |   1 +
 source/day_15/mod.rs | 105 +++++++++++++++++++++++++++++++++++++++++++
 source/main.rs       |   2 +
 3 files changed, 108 insertions(+)
 create mode 100644 source/day_15/mod.rs

diff --git a/Cargo.toml b/Cargo.toml
index c295b2b..390479f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -13,3 +13,4 @@ path = "source/main.rs"
 [dependencies]
 color-eyre = "0.5.11"
 itertools = "0.10.1"
+pathfinding = "3.0.5"
diff --git a/source/day_15/mod.rs b/source/day_15/mod.rs
new file mode 100644
index 0000000..c2dbfdc
--- /dev/null
+++ b/source/day_15/mod.rs
@@ -0,0 +1,105 @@
+use std::collections::HashMap;
+
+use color_eyre::{eyre::eyre, Result};
+use pathfinding::prelude::{absdiff, astar};
+
+pub fn solve() -> Result<()> {
+  let input_data = include_str!("../../data/day_15.txt").trim();
+  println!("Day 15 Part 1: {}", part_1(input_data)?);
+  println!("Day 15 Part 2: {}", part_2(input_data)?);
+  Ok(())
+}
+
+type Grid = HashMap<Coordinate, isize>;
+
+#[derive(Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+struct Coordinate(isize, isize);
+
+impl Coordinate {
+  fn distance(&self, target: &Coordinate) -> isize {
+    absdiff(self.0, target.0) + absdiff(self.1, target.1)
+  }
+
+  fn successors(&self, grid: &Grid) -> Vec<(Coordinate, isize)> {
+    let &Coordinate(x, y) = self;
+    let mut successors = vec![];
+
+    for coordinate in [
+      Coordinate(x, y - 1), // Up
+      Coordinate(x - 1, y), // Left
+      Coordinate(x, y + 1), // Down
+      Coordinate(x + 1, y), // Right
+    ] {
+      if let Some(value) = grid.get(&coordinate) {
+        successors.push((coordinate, *value));
+      }
+    }
+
+    successors
+  }
+}
+
+fn parse(input: &str) -> Result<(Grid, Coordinate)> {
+  let mut grid = Grid::new();
+  let mut end = Coordinate(0, 0);
+
+  for (y, line) in input.lines().enumerate() {
+    let y = (y + 1).try_into()?;
+
+    for (x, character) in line.char_indices() {
+      let x = (x + 1).try_into()?;
+      grid.insert(Coordinate(x, y), character.to_string().parse()?);
+
+      if end.0 < x && end.1 < y {
+        end = Coordinate(x, y);
+      }
+    }
+  }
+
+  Ok((grid, end))
+}
+
+fn enlarge_grid(grid: Grid, end: Coordinate) -> (Grid, Coordinate) {
+  let Coordinate(width, height) = end;
+  let mut larger_grid = grid.clone();
+
+  for (Coordinate(x, y), risk) in grid {
+    for step_x in 0..5 {
+      for step_y in 0..5 {
+        let risk = match (risk + step_x + step_y) % 9 {
+          0 => 9,
+          n => n,
+        };
+        let x = x + (width * step_x);
+        let y = y + (height * step_y);
+        larger_grid.insert(Coordinate(x, y), risk);
+      }
+    }
+  }
+
+  (larger_grid, Coordinate(width * 5, height * 5))
+}
+
+fn run(grid: Grid, end: Coordinate) -> Result<isize> {
+  Ok(
+    astar(
+      &Coordinate(1, 1),
+      |p| p.successors(&grid),
+      |p| p.distance(&end),
+      |p| p == &end,
+    )
+    .ok_or_else(|| eyre!("No path found"))?
+    .1,
+  )
+}
+
+fn part_1(input: &str) -> Result<isize> {
+  let (grid, end) = parse(input)?;
+  run(grid, end)
+}
+
+fn part_2(input: &str) -> Result<isize> {
+  let (grid, end) = parse(input)?;
+  let (grid, end) = enlarge_grid(grid, end);
+  run(grid, end)
+}
diff --git a/source/main.rs b/source/main.rs
index 1860461..57f5cff 100644
--- a/source/main.rs
+++ b/source/main.rs
@@ -14,6 +14,7 @@ mod day_09;
 mod day_10;
 mod day_13;
 mod day_14;
+mod day_15;
 
 fn main() -> Result<()> {
   color_eyre::install()?;
@@ -33,6 +34,7 @@ fn main() -> Result<()> {
     day_10::solve,
     day_13::solve,
     day_14::solve,
+    day_15::solve,
   ];
 
   for day in days {