Fix early exit from validation before all terms are used

Author: markjfisher

src/year2024/day07.rs

@@ -81,10 +81,17 @@ pub fn part2(input: &Input) -> u64 {
 }
 
 fn valid(terms: &[u64], test_value: u64, index: usize, concat: bool) -> bool {
-    (test_value == 0)
-        || (concat
-            && test_value % next_power_of_ten(terms[index]) == terms[index]
-            && valid(terms, test_value / next_power_of_ten(terms[index]), index - 1, concat))
+    if test_value == 0 {
+        return index == 0;
+    }
+
+    if index == 0 {
+        return false;
+    }
+
+    (concat
+        && test_value % next_power_of_ten(terms[index]) == terms[index]
+        && valid(terms, test_value / next_power_of_ten(terms[index]), index - 1, concat))
         || (test_value % terms[index] == 0
             && valid(terms, test_value / terms[index], index - 1, concat))
         || (test_value >= terms[index]

tests/year2024/day07_test.rs

@@ -11,14 +11,23 @@ const EXAMPLE: &str = "\
 21037: 9 7 18 13
 292: 11 6 16 20";
 
+const EXAMPLE2: &str = "\
+190: 10 19
+11174: 15 8 9 79 74
+729: 6 6 7 37 650";
+
 #[test]
 fn part1_test() {
     let input = parse(EXAMPLE);
     assert_eq!(part1(&input), 3749);
+    let input2 = parse(EXAMPLE2);
+    assert_eq!(part1(&input2), 190);
 }
 
 #[test]
 fn part2_test() {
     let input = parse(EXAMPLE);
     assert_eq!(part2(&input), 11387);
+    let input2 = parse(EXAMPLE2);
+    assert_eq!(part2(&input2), 11364);
 }