1 files changed, 49 insertions, 0 deletions
diff --git a/src/bin/day6.rs b/src/bin/day6.rs
new file mode 100644
index 0000000..e782931
--- /dev/null
+++ b/src/bin/day6.rs
@@ -0,0 +1,49 @@
+use std::collections::HashSet;
+
+static DAY: u8 = 6;
+
+fn main() {
+    let input = advent::read_file(DAY);
+    println!("{DAY}a: {}", start_of_packet(&input));
+    println!("{DAY}b: {}", start_of_message(&input));
+}
+
+fn start_of_prefix(input: &str, prefix_len: usize) -> usize {
+    input.trim()
+         .chars()
+         .collect::<Vec<_>>()
+         .windows(prefix_len)
+         .enumerate()
+         .map(|(pos, window)| (pos + prefix_len, window.iter().collect::<HashSet<_>>()))
+         .find(|(_, set)| set.len() == prefix_len)
+         .unwrap_or_else(|| panic!("set should contain {} different characters", prefix_len))
+         .0
+}
+
+fn start_of_packet(input: &str) -> usize {
+    start_of_prefix(input, 4)
+}
+
+fn start_of_message(input: &str) -> usize {
+    start_of_prefix(input, 14)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        assert_eq!(start_of_packet("mjqjpqmgbljsphdztnvjfqwrcgsmlb"), 7);
+        assert_eq!(start_of_packet("bvwbjplbgvbhsrlpgdmjqwftvncz"), 5);
+        assert_eq!(start_of_packet("nppdvjthqldpwncqszvftbrmjlhg"), 6);
+        assert_eq!(start_of_packet("nznrnfrfntjfmvfwmzdfjlvtqnbhcprsg"), 10);
+        assert_eq!(start_of_packet("zcfzfwzzqfrljwzlrfnpqdbhtmscgvjw"), 11);
+
+        assert_eq!(start_of_message("mjqjpqmgbljsphdztnvjfqwrcgsmlb"), 19);
+        assert_eq!(start_of_message("bvwbjplbgvbhsrlpgdmjqwftvncz"), 23);
+        assert_eq!(start_of_message("nppdvjthqldpwncqszvftbrmjlhg"), 23);
+        assert_eq!(start_of_message("nznrnfrfntjfmvfwmzdfjlvtqnbhcprsg"), 29);
+        assert_eq!(start_of_message("zcfzfwzzqfrljwzlrfnpqdbhtmscgvjw"), 26);
+    }
+}