From ff1a6851669f0548e8d5ea7eb844a9db2e66e902 Mon Sep 17 00:00:00 2001
From: Joscha <joscha@plugh.de>
Date: Thu, 15 Dec 2022 16:37:50 +0100
Subject: [PATCH] [rs] Optimize 2022_12

---
 rs/src/y2022/d12.rs | 71 ++++++++++++++++++++++++++++++++++++---------
 1 file changed, 58 insertions(+), 13 deletions(-)

diff --git a/rs/src/y2022/d12.rs b/rs/src/y2022/d12.rs
index 8ddc697..cb5eb05 100644
--- a/rs/src/y2022/d12.rs
+++ b/rs/src/y2022/d12.rs
@@ -1,16 +1,58 @@
-use std::collections::{HashMap, HashSet};
+use std::collections::HashSet;
+use std::ops::{Index, IndexMut};
 
-type Grid = HashMap<(i32, i32), u32>;
+struct Grid {
+    width: usize,
+    height: usize,
+    cells: Vec<u32>,
+}
 
-fn neighbours(grid: &Grid, (x, y): (i32, i32)) -> impl Iterator<Item = (i32, i32)> + '_ {
-    let height = grid.get(&(x, y)).cloned().unwrap_or(u32::MAX);
+impl Grid {
+    fn new(width: usize, height: usize, initial: u32) -> Self {
+        Self {
+            width,
+            height,
+            cells: vec![initial; width * height],
+        }
+    }
+
+    fn contains(&self, pos: (i32, i32)) -> bool {
+        let x_in_bounds = 0 <= pos.0 && (pos.0 as usize) < self.width;
+        let y_in_bounds = 0 <= pos.1 && (pos.1 as usize) < self.height;
+        x_in_bounds && y_in_bounds
+    }
+}
+
+impl Index<(i32, i32)> for Grid {
+    type Output = u32;
+
+    fn index(&self, index: (i32, i32)) -> &Self::Output {
+        assert!(index.0 >= 0);
+        assert!(index.1 >= 0);
+        let (x, y) = (index.0 as usize, index.1 as usize);
+        assert!(x < self.width);
+        assert!(y < self.height);
+        self.cells.index(y * self.width + x)
+    }
+}
+
+impl IndexMut<(i32, i32)> for Grid {
+    fn index_mut(&mut self, index: (i32, i32)) -> &mut Self::Output {
+        assert!(index.0 >= 0);
+        assert!(index.1 >= 0);
+        let (x, y) = (index.0 as usize, index.1 as usize);
+        assert!(x < self.width);
+        assert!(y < self.height);
+        self.cells.index_mut(y * self.width + x)
+    }
+}
+
+fn backwards_neighbours(grid: &Grid, (x, y): (i32, i32)) -> impl Iterator<Item = (i32, i32)> + '_ {
+    // One step down or arbitrarily many steps up are allowed
+    let height = grid[(x, y)];
     [(x - 1, y), (x + 1, y), (x, y - 1), (x, y + 1)]
         .into_iter()
-        .filter(move |n| {
-            let height2 = grid.get(n).cloned().unwrap_or(u32::MAX);
-            // One step down or arbitrarily many steps up are allowed
-            height2.saturating_add(1) >= height
-        })
+        .filter(move |n| grid.contains(*n) && grid[*n].saturating_add(1) >= height)
 }
 
 fn bfs(grid: &Grid, start: (i32, i32), until: impl Fn((i32, i32)) -> bool) -> usize {
@@ -27,7 +69,7 @@ fn bfs(grid: &Grid, start: (i32, i32), until: impl Fn((i32, i32)) -> bool) -> us
                 return steps;
             }
 
-            for neighbour in neighbours(grid, pos) {
+            for neighbour in backwards_neighbours(grid, pos) {
                 if !visited.contains(&neighbour) {
                     visited.insert(neighbour);
                     new_queue.insert(neighbour);
@@ -40,9 +82,12 @@ fn bfs(grid: &Grid, start: (i32, i32), until: impl Fn((i32, i32)) -> bool) -> us
 }
 
 pub fn solve(input: String) {
+    let width = input.lines().next().unwrap().len();
+    let height = input.lines().count();
+    let mut grid = Grid::new(width, height, 0);
+
     let mut start = (-1, -1);
     let mut end = (-1, -1);
-    let mut grid = HashMap::new();
     for (y, line) in input.lines().enumerate() {
         for (x, c) in line.chars().enumerate() {
             let pos = (x as i32, y as i32);
@@ -58,13 +103,13 @@ pub fn solve(input: String) {
                 _ => c,
             };
             let height = height as u32 - 'a' as u32;
-            grid.insert(pos, height);
+            grid[pos] = height;
         }
     }
 
     let part1 = bfs(&grid, end, |pos| pos == start);
     println!("Part 1: {part1}");
 
-    let part2 = bfs(&grid, end, |pos| grid.get(&pos) == Some(&0));
+    let part2 = bfs(&grid, end, |pos| grid[pos] == 0);
     println!("Part 2: {part2}");
 }