From 5664355bd6a550f744d3077d5c6ba583685ca508 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ren=C3=A9e=20Kooi?= <renee@kooi.me>
Date: Thu, 4 Jan 2024 19:32:06 +0100
Subject: [PATCH] Avoid small allocations when parsing short hex format

The `.repeat()` calls allocate a `String`, instead it's possible to
parse the single digit into an `u8` and then repeat the hex value with
bitwise operations.
---
 src/parser/mod.rs | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/src/parser/mod.rs b/src/parser/mod.rs
index 0d1c2da..9c59a14 100644
--- a/src/parser/mod.rs
+++ b/src/parser/mod.rs
@@ -349,16 +349,19 @@ fn parse_hex(s: &str) -> Result<Color, ParseColorError> {
 
     let n = s.len();
 
+    fn parse_single_digit(digit: &str) -> Result<u8, ParseColorError> {
+        u8::from_str_radix(digit, 16)
+            .map(|n| (n << 4) | n)
+            .map_err(|_| ParseColorError::InvalidHex)
+    }
+
     if n == 3 || n == 4 {
-        let r =
-            u8::from_str_radix(&s[0..1].repeat(2), 16).map_err(|_| ParseColorError::InvalidHex)?;
-        let g =
-            u8::from_str_radix(&s[1..2].repeat(2), 16).map_err(|_| ParseColorError::InvalidHex)?;
-        let b =
-            u8::from_str_radix(&s[2..3].repeat(2), 16).map_err(|_| ParseColorError::InvalidHex)?;
+        let r = parse_single_digit(&s[0..1])?;
+        let g = parse_single_digit(&s[1..2])?;
+        let b = parse_single_digit(&s[2..3])?;
 
         let a = if n == 4 {
-            u8::from_str_radix(&s[3..4].repeat(2), 16).map_err(|_| ParseColorError::InvalidHex)?
+            parse_single_digit(&s[3..4])?
         } else {
             255
         };