1 files changed, 70 insertions, 0 deletions
diff --git a/src/bin/day3.rs b/src/bin/day3.rs
new file mode 100644
index 0000000..3cc961d
--- /dev/null
+++ b/src/bin/day3.rs
@@ -0,0 +1,70 @@
+use std::collections::HashSet;
+
+static DAY: u8 = 3;
+
+fn main() {
+    let input = advent::read_lines(DAY);
+    println!("{DAY}a: {}", sum_priorities(&input));
+    println!("{DAY}b: {}", sum_badge_priorities(&input));
+}
+
+fn item_value(item: char) -> u32 {
+    if item.is_ascii_lowercase() {
+        item.to_digit(36).unwrap() - 9
+    } else if item.is_ascii_uppercase() {
+        item.to_digit(36).unwrap() + 17
+    } else {
+        panic!("unexpected common character: {}", item);
+    }
+}
+
+fn sum_priorities(input: &[String]) -> u32 {
+    let mut priorities = 0;
+
+    for line in input {
+        let (first, second) = line.split_at(line.len() / 2);
+        let first = first.chars().collect::<HashSet<_>>();
+        let second = second.chars().collect::<HashSet<_>>();
+        let common = first.intersection(&second).copied().collect::<Vec<_>>();
+        assert_eq!(common.len(), 1);
+        priorities += item_value(*common.first().unwrap());
+    }
+
+    priorities
+}
+
+fn sum_badge_priorities(input: &[String]) -> u32 {
+    let mut priorities = 0;
+
+    for group in input.chunks(3) {
+        let first = group[0].chars().collect::<HashSet<_>>();
+        let second = group[1].chars().collect::<HashSet<_>>();
+        let third = group[2].chars().collect::<HashSet<_>>();
+        let common = first.intersection(&second).copied().collect::<HashSet<_>>();
+        let common = common.intersection(&third).copied().collect::<Vec<_>>();
+        assert_eq!(common.len(), 1);
+        priorities += item_value(*common.first().unwrap());
+    }
+
+    priorities
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let input = [
+            "vJrwpWtwJgWrhcsFMMfFFhFp".to_string(),
+            "jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL".to_string(),
+            "PmmdzqPrVvPwwTWBwg".to_string(),
+            "wMqvLMZHhHMvwLHjbvcjnnSBnvTQFn".to_string(),
+            "ttgJtRGJQctTZtZT".to_string(),
+            "CrZsJsPPZsGzwwsLwLmpwMDw".to_string(),
+        ];
+
+        assert_eq!(sum_priorities(&input), 157);
+        assert_eq!(sum_badge_priorities(&input), 70);
+    }
+}