day3: add clever solution

utils/misc/src/lib.rs

@@ -2,6 +2,19 @@ use num_traits::Signed;
 use std::fmt::Display;
 use std::ops::{Add, AddAssign};
 
+const POW10MAX: usize = u64::MAX.ilog10() as usize;
+pub const POW10: [u64; POW10MAX] = pow10_lut();
+
+const fn pow10_lut<const N: usize>() -> [u64; N] {
+    let mut res = [0; N];
+    let mut i = 0;
+    while i < N {
+        res[i] = 10u64.pow(i as u32);
+        i += 1;
+    }
+    res
+}
+
 /// Wrapped signed integer with custom upper bound with wrapping of 0s to the upper bound
 #[derive(Eq, Clone, Copy)]
 pub struct CustomWrapped<T: Signed + Copy> {
@@ -91,8 +104,9 @@ mod tests {
     use super::*;
 
     #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
+    fn test_pow10() {
+        for i in 0..POW10MAX {
+            assert_eq!(POW10[i], 10u64.pow(i as u32))
+        }
     }
 }