diff --git a/src/day3.rs b/src/day3.rs
new file mode 100644
index 0000000..f3a8c64
--- /dev/null
+++ b/src/day3.rs
@@ -0,0 +1,54 @@
+use aoc_runner_derive::{aoc, aoc_generator};
+
+type Bank = Vec<u8>;
+
+#[aoc_generator(day3)]
+fn parse(input: &str) -> Vec<Bank> {
+    input
+        .lines()
+        .map(|bank| bank.bytes().map(|c| c - b'0').collect())
+        .collect()
+}
+
+fn max_joltage(bank: &Bank, n: usize) -> u64 {
+    assert_eq!(n, 2, "unimplemented");
+    let mut max = 0;
+    for start in 0..bank.len() - 1 {
+        let cur = bank[start] as u64 * 10u64.pow(n as u32 - 1)
+            + *bank[start + 1..].iter().max().unwrap() as u64;
+        if cur > max {
+            max = cur
+        }
+    }
+    max
+}
+
+#[aoc(day3, part1)]
+fn part1(input: &Vec<Bank>) -> u64 {
+    input.iter().map(|bank| max_joltage(bank, 2)).sum()
+}
+
+#[aoc(day3, part2)]
+fn part2(input: &Vec<Bank>) -> u64 {
+    todo!()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const EXAMPLE: &str = "987654321111111
+811111111111119
+234234234234278
+818181911112111";
+
+    #[test]
+    fn part1_example() {
+        assert_eq!(part1(&parse(EXAMPLE)), 357);
+    }
+
+    #[test]
+    fn part2_example() {
+        assert_eq!(part2(&parse(EXAMPLE)), 0);
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index a228b5c..3a057fc 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,4 @@
+mod day3;
 mod day1;
 mod day2;