day2: fix arithmetic bruteforce algorithm & test

src/day2.rs

@@ -88,14 +88,22 @@ fn part2_arithmetic_brute(input: &[RangeInclusive<u64>]) -> u64 {
             let n_digits = product.ilog10() + 1;
 
             for n in 1..=n_digits / 2 {
+                let repetitions = n_digits / n;
                 if n_digits % n != 0 {
                     continue;
                 }
-                let decade = 10u64.pow(n);
+                let decade = 10u64.pow(n_digits - n);
                 let lhs = product / decade;
-                let rhs = product % decade;
-                if lhs == rhs {
-                    invalid_sum += product
+                let remainder = product % decade;
+
+                // for each repetition we multiply by 10^(rep * n)
+                let expected_remainder = (0..repetitions - 1)
+                    .map(|rep| lhs * 10u64.pow(rep * n))
+                    .sum();
+
+                if remainder == expected_remainder {
+                    invalid_sum += product;
+                    break;
                 }
             }
         }
@@ -155,7 +163,7 @@ mod tests {
     }
     #[test]
     fn part2_arithmetic_example() {
-        assert_eq!(part2_arithmetic_brute(&parse(EXAMPLE)), 1227775554);
+        assert_eq!(part2_arithmetic_brute(&parse(EXAMPLE)), 4174379265);
     }
     #[test]
     fn part1_clever_example() {