From 9de4d77a637844a48f374a4d669bdfb461b0ba40 Mon Sep 17 00:00:00 2001
From: Tyler Hallada <tyler@hallada.net>
Date: Fri, 5 Dec 2025 01:25:14 -0500
Subject: [PATCH] Improve perf of day 4

---
 README.md        |  2 +-
 src/day04/mod.rs | 38 +++++++++++++++++++++-----------------
 2 files changed, 22 insertions(+), 18 deletions(-)

diff --git a/README.md b/README.md
index 0268250..e30d754 100644
--- a/README.md
+++ b/README.md
@@ -35,4 +35,4 @@ Timings are given as: [lower-bound **best-estimate** upper-bound]
 | 01  | [101.34 µs **101.95 µs** 102.61 µs] | [105.90 µs **106.40 µs** 106.95 µs] |
 | 02  | [2.0990 ms **2.1113 ms** 2.1236 ms] | [2.0954 ms **2.1055 ms** 2.1157 ms] |
 | 03  | [38.717 µs **39.002 µs** 39.311 µs] | [175.35 µs **176.59 µs** 177.92 µs] |
-| 04  | [233.55 µs **234.88 µs** 236.20 µs] | [3.1026 ms **3.1174 ms** 3.1326 ms] |
+| 04  | [147.02 µs **147.75 µs** 148.53 µs] | [1.5579 ms **1.5737 ms** 1.5900 ms] |
diff --git a/src/day04/mod.rs b/src/day04/mod.rs
index 3d375fc..1d0a56f 100644
--- a/src/day04/mod.rs
+++ b/src/day04/mod.rs
@@ -8,6 +8,17 @@ use tracing::{debug, instrument};
 
 pub const INPUT: &str = include_str!("input/input.txt");
 
+const ADJACENT_DELTAS: [(isize, isize); 8] = [
+    (-1, -1),
+    (-1, 0),
+    (-1, 1),
+    (0, -1),
+    (0, 1),
+    (1, -1),
+    (1, 0),
+    (1, 1),
+];
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 enum Cell {
     Empty,
@@ -77,14 +88,6 @@ impl<const R: usize, const C: usize> Display for Grid<R, C> {
 }
 
 impl<const R: usize, const C: usize> Grid<R, C> {
-    fn get_cell(&self, row: isize, col: isize) -> Option<Cell> {
-        if row >= 0 && col >= 0 && (row as usize) < R && (col as usize) < C {
-            Some(self.cells[row as usize][col as usize])
-        } else {
-            None
-        }
-    }
-
     fn count_accessible_papers(&mut self, replace_with: Cell) -> usize {
         let mut count = 0;
         for row in 0..R {
@@ -93,15 +96,16 @@ impl<const R: usize, const C: usize> Grid<R, C> {
                     continue;
                 }
                 let mut adjacent_papers = 0;
-                for dr in -1..=1 {
-                    for dc in -1..=1 {
-                        if dr == 0 && dc == 0 {
-                            continue;
-                        }
-                        let adjacent = self
-                            .get_cell(row as isize + dr, col as isize + dc)
-                            .unwrap_or(Cell::Empty);
-                        if adjacent == Cell::Paper || adjacent == Cell::AccessiblePaper {
+                for &(dr, dc) in &ADJACENT_DELTAS {
+                    let adj_row = row as isize + dr;
+                    let adj_col = col as isize + dc;
+                    if adj_row >= 0
+                        && adj_col >= 0
+                        && (adj_row as usize) < R
+                        && (adj_col as usize) < C
+                    {
+                        let adjacent = self.cells[adj_row as usize][adj_col as usize];
+                        if matches!(adjacent, Cell::Paper | Cell::AccessiblePaper) {
                             adjacent_papers += 1;
                         }
                     }