diff --git a/2021/Cargo.lock b/2021/Cargo.lock
new file mode 100644
index 0000000..28f50c9
--- /dev/null
+++ b/2021/Cargo.lock
@@ -0,0 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "advent-2021"
+version = "0.1.0"
diff --git a/2021/Cargo.toml b/2021/Cargo.toml
new file mode 100644
index 0000000..372ba32
--- /dev/null
+++ b/2021/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "advent-2021"
+version = "0.1.0"
+authors = ["Joseph Montanaro <jfmontanaro@modg.org>"]
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/2021/src/lib.rs b/2021/src/lib.rs
new file mode 100644
index 0000000..3c75893
--- /dev/null
+++ b/2021/src/lib.rs
@@ -0,0 +1,15 @@
+use std::io::{BufReader, BufRead};
+use std::fs::File;
+
+
+pub fn iter_nums(filename: &str) -> impl Iterator<Item=i32> {
+    let file = File::open(filename).expect("File not found! Make sure that your current directory is the project root.");
+    let reader = BufReader::new(file);
+
+    reader.lines().map(|line| 
+        line
+            .expect("Couldn't split on lines I guess?")
+            .parse::<i32>()
+            .expect("Could not parse integer!")
+    )
+}
\ No newline at end of file
diff --git a/2021/src/main.rs b/2021/src/main.rs
new file mode 100644
index 0000000..3d6561b
--- /dev/null
+++ b/2021/src/main.rs
@@ -0,0 +1,33 @@
+mod lib;
+use lib::iter_nums;
+
+
+fn main() {
+    day_one();
+}
+
+
+fn day_one() {
+    let mut single_increases = 0;
+    let mut window_increases = 0;
+    let mut prev = (None, None, None);
+    for n in iter_nums("data/01.txt") {
+        if let Some(p) = prev.2 {
+            // apparently we can't combine "if let" and regular boolean conditions
+            if n > p {
+                single_increases += 1;
+            }
+        }
+
+        if let (Some(first), Some(second), Some(third)) = prev {
+            if (n + second + third) > (first + second + third) {
+                window_increases += 1;
+            }
+        }
+        // would have forgotten this if not for the warning triggered by "let mut" without mutation
+        prev = (prev.1, prev.2, Some(n));
+    }
+
+    println!("One: {}", single_increases);
+    println!("Two: {}", window_increases);
+}