diff --git a/day16/Cargo.lock b/day16/Cargo.lock
new file mode 100644
index 0000000..16dc480
--- /dev/null
+++ b/day16/Cargo.lock
@@ -0,0 +1,25 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "day16"
+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/day16/Cargo.toml b/day16/Cargo.toml
new file mode 100644
index 0000000..e6a15e1
--- /dev/null
+++ b/day16/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+name = "day16"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+itertools = "*"
diff --git a/day16/src/main.rs b/day16/src/main.rs
new file mode 100644
index 0000000..a663391
--- /dev/null
+++ b/day16/src/main.rs
@@ -0,0 +1,50 @@
+use itertools::Itertools;
+
+#[cfg(debug_assertions)]
+const INPUT: usize = 10000;
+#[cfg(debug_assertions)]
+const LENGTH: usize = 20;
+
+#[cfg(not(debug_assertions))]
+const INPUT: usize = 10011111011011001;
+#[cfg(not(debug_assertions))]
+const LENGTH: usize = 272;
+const LENGTH2: usize = 35651584;
+
+fn main() {
+    calculate_checksum(LENGTH);
+    calculate_checksum(LENGTH2);
+}
+
+fn calculate_checksum(length: usize) {
+    let mut a = INPUT.to_string();
+    while a.len() < length {
+        let mut b = a.clone();
+        b = b
+            .chars()
+            .rev()
+            .map(|c| if c == '0' { '1' } else { '0' })
+            .collect();
+
+        let mut result = a.clone();
+        result.push('0');
+        result.push_str(&b);
+        a = result;
+    }
+    a.truncate(length);
+
+    loop {
+        let possible_checksum: String = a
+            .chars()
+            .tuples()
+            .map(|(c0, c1)| c0 == c1)
+            .map(|b| if b { '1' } else { '0' })
+            .collect();
+        if possible_checksum.len() % 2 == 1 {
+            a = possible_checksum;
+            break;
+        }
+        a = possible_checksum;
+    }
+    println!("{a}");
+}