feat: more optimisation

use log10 to calculate num of digits instead of while loop (thx fgardt)

2025/src/days/two/mod.rs

···
 pub fn part_a(input: Vec<u64>) -> usize {
     input.iter().fold(0, |acc, x| {
         acc + if is_valid_id_a(x.clone()) {
-            0usize
+            0
         } else {
             *x as usize
         }
     })
 }
 
-fn num_digits(mut n: u64) -> u32 {
-    if n == 0 {
-        return 1;
-    }
-    let mut digits = 0;
-    while n > 0 {
-        digits += 1;
-        n /= 10;
-    }
-    digits
-}
-
 fn is_valid_id_a(id: u64) -> bool {
     // invalid ID's are identifiable due to them being made up from some sequences being repeated
     // eg: 11 -> [1, 1] invalid
     // eg: 123123 -> [123, 123] invalid
     // eg: 123456 -> [123456] valid
-    let digits = num_digits(id);
+    let digits = id.ilog10() + 1;
     if digits % 2 == 1 {
         return true;
     }
 
-    let half = digits / 2;
-    let div = 10u64.pow(half);
+    let div = 10u64.pow(digits / 2);
 
     let left = id / div;
     let right = id % div;