diff --git a/day11/Cargo.lock b/day11/Cargo.lock
new file mode 100644
index 0000000..73122b2
--- /dev/null
+++ b/day11/Cargo.lock
@@ -0,0 +1,25 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "day11"
+version = "0.1.0"
+dependencies = [
+ "itertools",
+]
+
+[[package]]
+name = "either"
+version = "1.16.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "91622ff5e7162018101f2fea40d6ebf4a78bbe5a49736a2020649edf9693679e"
+
+[[package]]
+name = "itertools"
+version = "0.15.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8b4baf93f58d4425749ca49a51c50ebab072c5df6994d08fed93541c331481dc"
+dependencies = [
+ "either",
+]
diff --git a/day11/Cargo.toml b/day11/Cargo.toml
new file mode 100644
index 0000000..e6ef091
--- /dev/null
+++ b/day11/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+name = "day11"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+itertools = "0.15.0"
diff --git a/day11/src/main.rs b/day11/src/main.rs
new file mode 100644
index 0000000..4aa36dd
--- /dev/null
+++ b/day11/src/main.rs
@@ -0,0 +1,128 @@
+use itertools::Itertools;
+
+// const INPUT: &str = "ghijklmn";
+// const INPUT: &str = "vzbxkghb";
+const INPUT: &str = "vzbxxyzz";
+
+fn main() {
+    part1(INPUT);
+}
+
+fn part1(input: &str) {
+    let mut input = increment(input);
+    loop {
+        if is_valid(&input) {
+            break;
+        }
+        input = increment(&input);
+    }
+    println!("{}", input);
+}
+
+fn increment(input: &str) -> String {
+    let chars = input.chars().collect_vec();
+    let mut carry = false;
+
+    let mut iter = chars.iter().rev();
+
+    let mut result = String::new();
+
+    let c = iter.next().unwrap();
+    let ret = handle_char(*c, true, carry);
+    result.push(ret.0);
+    carry = ret.1;
+
+    for c in iter {
+        let ret = handle_char(*c, false, carry);
+        result.push(ret.0);
+        carry = ret.1;
+    }
+
+    result.chars().rev().collect()
+}
+
+fn handle_char(c: char, increase: bool, carry: bool) -> (char, bool) {
+    let increase_input = if increase { 1 } else { 0 };
+    let carry_input = if carry { 1 } else { 0 };
+
+    let result = c as u8 + increase_input + carry_input;
+    let carry_result = result >= 123;
+
+    let result = (((result - 97) % 26) + 97) as char;
+
+    (result, carry_result)
+}
+
+fn is_valid(p: &str) -> bool {
+    for letter in ['i', 'o', 'l'] {
+        if p.contains(letter) {
+            return false;
+        }
+    }
+
+    let non_overlapping_double_letter_count = p
+        .chars()
+        .enumerate()
+        .tuple_windows()
+        .filter(|(a, b)| a.1 == b.1)
+        .map(|(a, _)| a.0)
+        .collect_vec();
+
+    let max = non_overlapping_double_letter_count.last().unwrap_or(&0);
+    let min = non_overlapping_double_letter_count.first().unwrap_or(&0);
+
+    if max - min < 2 {
+        return false;
+    }
+
+    let input = p.chars().collect_vec();
+
+    for i in 0..p.len() - 2 {
+        let s = i;
+        let e = s + 2;
+        if is_straight(&input[s..=e]) {
+            return true;
+        }
+    }
+    false
+}
+
+fn is_straight(s: &[char]) -> bool {
+    if s.len() != 3 {
+        panic!("must check exactly 3 letters");
+    }
+
+    let mut chars = s.iter();
+
+    let a = *chars.next().unwrap() as u8;
+    let b = *chars.next().unwrap() as u8;
+    let c = *chars.next().unwrap() as u8;
+
+    b == a + 1 && c == b + 1
+}
+
+#[cfg(test)]
+mod test {
+    use crate::{handle_char, is_valid};
+
+    #[test]
+    fn test_increase_char() {
+        assert_eq!(handle_char('a', true, false), ('b', false));
+        assert_eq!(handle_char('a', true, true), ('c', false));
+
+        assert_eq!(handle_char('i', true, false), ('j', false));
+
+        assert_eq!(handle_char('z', true, false), ('a', true));
+        assert_eq!(handle_char('z', true, true), ('b', true));
+    }
+
+    #[test]
+    fn test_is_valid() {
+        assert!(is_valid("abcdffaa"));
+        assert!(is_valid("ghjaabcc"));
+
+        assert!(!is_valid("hijklmmn"));
+        assert!(!is_valid("abbceffg"));
+        assert!(!is_valid("abbcegjk"));
+    }
+}