From 285cefa67002e2615e54d23d080e0ae781773818 Mon Sep 17 00:00:00 2001
From: Jan-Bulthuis <git@bulthuis.dev>
Date: Sat, 21 Dec 2024 22:45:13 +0100
Subject: [PATCH] day21 faster than Dirk >:)

---
 aoc_2024/src/day21.rs | 119 ++++++++++++++++++++++++++++++++----------
 1 file changed, 92 insertions(+), 27 deletions(-)

diff --git a/aoc_2024/src/day21.rs b/aoc_2024/src/day21.rs
index 6377118..ea5f532 100644
--- a/aoc_2024/src/day21.rs
+++ b/aoc_2024/src/day21.rs
@@ -1,12 +1,12 @@
 use aoc_runner_derive::{aoc, aoc_generator};
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
-struct Path<T> {
+struct Path {
     steps: usize,
-    path: [Option<T>; 6],
+    path: [Option<DirButton>; 6],
 }
 
-impl<T: Copy> Path<T> {
+impl Path {
     fn new() -> Self {
         Self {
             steps: 0,
@@ -14,7 +14,7 @@ impl<T: Copy> Path<T> {
         }
     }
 
-    fn push(&mut self, button: T) {
+    fn push(&mut self, button: DirButton) {
         self.path[self.steps] = Some(button);
         self.steps += 1;
     }
@@ -24,11 +24,30 @@ impl<T: Copy> Path<T> {
             self.push(other.path[i].unwrap());
         }
     }
+
+    fn reversed(&self) -> Self {
+        let mut path = [None; 6];
+        (0..self.steps).for_each(|i| {
+            path[i] = match self.path[self.steps - i - 1] {
+                Some(DirButton::Up) => Some(DirButton::Down),
+                Some(DirButton::Down) => Some(DirButton::Up),
+                Some(DirButton::Left) => Some(DirButton::Right),
+                Some(DirButton::Right) => Some(DirButton::Left),
+                Some(DirButton::A) => Some(DirButton::A),
+                None => None,
+            };
+        });
+
+        Self {
+            steps: self.steps,
+            path,
+        }
+    }
 }
 
-impl<T> IntoIterator for Path<T> {
-    type Item = T;
-    type IntoIter = std::iter::Flatten<std::array::IntoIter<Option<T>, 6>>;
+impl IntoIterator for Path {
+    type Item = DirButton;
+    type IntoIter = std::iter::Flatten<std::array::IntoIter<Option<DirButton>, 6>>;
 
     fn into_iter(self) -> Self::IntoIter {
         self.path.into_iter().flatten()
@@ -65,7 +84,7 @@ impl DirButton {
         }
     }
 
-    fn to(&self, other: &DirButton) -> [Option<Path<DirButton>>; 2] {
+    fn to(&self, other: &DirButton) -> [Option<Path>; 2] {
         let start = self.pos();
         let end = other.pos();
 
@@ -134,7 +153,8 @@ impl NumButton {
         }
     }
 
-    fn to(&self, other: &NumButton) -> [Option<Path<DirButton>>; 2] {
+    #[inline(always)]
+    fn to(&self, other: &NumButton) -> [Option<Path>; 2] {
         let start = self.pos();
         let end = other.pos();
 
@@ -203,26 +223,67 @@ fn parse(input: &str) -> Input {
 
 type DistanceMatrix = [[usize; 5]; 5];
 
-fn calculate_steps(start: DirButton, end: DirButton, steps: &mut [DistanceMatrix]) -> usize {
-    if steps.is_empty() {
-        return 1;
+fn precompute_steps(steps: &mut [DistanceMatrix]) {
+    if steps.len() == 1 {
+        return;
     }
 
-    if steps[0][start.index()][end.index()] == 0 {
-        let extension = start
-            .to(&end)
-            .into_iter()
-            .flatten()
-            .map(|path| shortest_dir_path(path, &mut steps[1..]))
-            .min()
-            .unwrap();
-        steps[0][start.index()][end.index()] = extension;
-    }
+    precompute_steps(&mut steps[1..]);
 
-    steps[0][start.index()][end.index()]
+    let buttons = [
+        DirButton::Up,
+        DirButton::Down,
+        DirButton::Left,
+        DirButton::Right,
+        DirButton::A,
+    ];
+
+    buttons.iter().enumerate().for_each(|(i, start)| {
+        buttons[i + 1..].iter().for_each(|end| {
+            let (forward, reversed) = start
+                .to(end)
+                .into_iter()
+                .flatten()
+                .map(|mut path| {
+                    path.steps -= 1;
+                    let mut reversed = path.reversed();
+                    let mut forward = path;
+                    reversed.push(DirButton::A);
+                    forward.push(DirButton::A);
+
+                    (
+                        forward
+                            .path
+                            .into_iter()
+                            .flatten()
+                            .fold((0, DirButton::A), |acc, next| {
+                                let extension = steps[1][acc.1.index()][next.index()];
+
+                                (acc.0 + extension, next)
+                            })
+                            .0,
+                        reversed
+                            .path
+                            .into_iter()
+                            .flatten()
+                            .fold((0, DirButton::A), |acc, next| {
+                                let extension = steps[1][acc.1.index()][next.index()];
+
+                                (acc.0 + extension, next)
+                            })
+                            .0,
+                    )
+                })
+                .fold((usize::MAX, usize::MAX), |acc, next| {
+                    (acc.0.min(next.0), acc.1.min(next.1))
+                });
+            steps[0][start.index()][end.index()] = forward;
+            steps[0][end.index()][start.index()] = reversed;
+        });
+    });
 }
 
-fn shortest_dir_path(path: Path<DirButton>, steps: &mut [DistanceMatrix]) -> usize {
+fn shortest_dir_path(path: Path, steps: &mut [DistanceMatrix]) -> usize {
     path.path
         .into_iter()
         .flatten()
@@ -230,7 +291,7 @@ fn shortest_dir_path(path: Path<DirButton>, steps: &mut [DistanceMatrix]) -> usi
             let sum = acc.0;
             let pos = acc.1;
 
-            let extension = calculate_steps(pos, next, steps);
+            let extension = steps[0][pos.index()][next.index()];
 
             (sum + extension, next)
         })
@@ -257,7 +318,9 @@ fn shortest_path(path: Vec<NumButton>, steps: &mut [DistanceMatrix]) -> usize {
 
 #[aoc(day21, part1)]
 fn part1(input: &Input) -> usize {
-    let mut steps = vec![[[0; 5]; 5]; 3 - 1];
+    let mut steps = vec![[[1; 5]; 5]; 3];
+
+    precompute_steps(&mut steps);
 
     input
         .iter()
@@ -268,7 +331,9 @@ fn part1(input: &Input) -> usize {
 
 #[aoc(day21, part2)]
 fn part2(input: &Input) -> usize {
-    let mut steps = vec![[[0; 5]; 5]; 26 - 1];
+    let mut steps = vec![[[1; 5]; 5]; 26];
+
+    precompute_steps(&mut steps);
 
     input
         .iter()