From 5ee921a0e4c6dd39e9d934c47ef777a796139ce9 Mon Sep 17 00:00:00 2001
From: Jens Peters <jp7677@gmail.com>
Date: Mon, 4 Dec 2023 20:36:06 +0100
Subject: [PATCH] 2023: Add performance hack for day 3

Not a good idea for all kinds of strings.
---
 2023/src/day03.rs        | 2 +-
 2023/src/util/mod.rs     | 2 ++
 2023/src/util/strings.rs | 5 +++++
 3 files changed, 8 insertions(+), 1 deletion(-)
 create mode 100644 2023/src/util/strings.rs

diff --git a/2023/src/day03.rs b/2023/src/day03.rs
index 0653b80..1c8e200 100644
--- a/2023/src/day03.rs
+++ b/2023/src/day03.rs
@@ -109,7 +109,7 @@ fn parse_map(input: &String) -> Vec<Part> {
                 })
                 .collect::<Vec<Adjacent>>();
 
-            if i >= 1 && input.chars().nth(i - 1).unwrap_or('_').is_digit(10) {
+            if i >= 1 && char_at(input, i - 1).is_digit(10) {
                 parts.last_mut().unwrap().number.push(c);
                 parts
                     .last_mut()
diff --git a/2023/src/util/mod.rs b/2023/src/util/mod.rs
index db858ee..fff6149 100644
--- a/2023/src/util/mod.rs
+++ b/2023/src/util/mod.rs
@@ -1,5 +1,7 @@
 mod days;
 mod input;
+mod strings;
 
 pub use days::DAYS;
 pub use input::read_input;
+pub use strings::char_at;
diff --git a/2023/src/util/strings.rs b/2023/src/util/strings.rs
new file mode 100644
index 0000000..8e57496
--- /dev/null
+++ b/2023/src/util/strings.rs
@@ -0,0 +1,5 @@
+#[inline(always)]
+pub fn char_at(str: &str, index: usize) -> char {
+    // Not a good idea for all kinds of strings.
+    *(&str[index..index + 1].chars().nth(0).unwrap())
+}