day2: add mostly arithmetic solution for part1

src/day2.rs

@@ -103,6 +103,36 @@ fn part2_arithmetic_brute(input: &[RangeInclusive<u64>]) -> u64 {
     invalid_sum
 }
 
+#[aoc(day2, part1, Clever)]
+fn part1_clever(input: &[RangeInclusive<u64>]) -> u64 {
+    let mut invalid_sum = 0;
+    for r in input {
+        let n_digits = r.start().ilog10() + 1;
+
+        let mut lhs = if n_digits % 2 != 0 {
+            // If the number of digits is odd, we start our guess at the first possibility in the next decade
+            10u64.pow(n_digits / 2)
+        } else {
+            let start_decade = 10u64.pow(n_digits / 2);
+            r.start() / start_decade
+        };
+
+        // we will guess the next repeater, and check if it's in range, if not we are done.
+        loop {
+            let repeater = lhs * 10u64.pow(lhs.ilog10() + 1) + lhs;
+            if repeater < *r.start() {
+                lhs += 1
+            } else if r.contains(&repeater) {
+                invalid_sum += repeater;
+                lhs += 1
+            } else {
+                break;
+            }
+        }
+    }
+    invalid_sum
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -127,4 +157,8 @@ mod tests {
     fn part2_arithmetic_example() {
         assert_eq!(part2_arithmetic_brute(&parse(EXAMPLE)), 1227775554);
     }
+    #[test]
+    fn part1_clever_example() {
+        assert_eq!(part1_clever(&parse(EXAMPLE)), 1227775554);
+    }
 }