Slightly more natural part 1

2023/src/day01.rs

@@ -1,12 +1,5 @@
 use anyhow::Result;
 
-fn to_digit(c: u8) -> Option<u32> {
-    match c {
-        b'0'..=b'9' => Some(u32::from(c - b'0')),
-        _ => None,
-    }
-}
-
 pub fn part1(input: &[u8]) -> Result<String> {
     let mut it = input.iter();
     let mut sum = 0;
@@ -15,14 +8,17 @@ pub fn part1(input: &[u8]) -> Result<String> {
         let mut first = None;
         let mut last = 0;
 
-        for digit in it
-            .by_ref()
-            .take_while(|&&c| c != b'\n')
-            .filter_map(|&c| to_digit(c))
-        {
+        for &c in &mut it {
+            match c {
+                d @ b'0'..=b'9' => {
+                    let digit = u32::from(d - b'0');
                     first.get_or_insert(digit);
                     last = digit;
                 }
+                b'\n' => break,
+                _ => continue,
+            }
+        }
 
         if let Some(first) = first {
             sum += 10 * first + last;