diff --git a/day-13/Cargo.lock b/day-13/Cargo.lock
new file mode 100644
index 0000000..d0515ae
--- /dev/null
+++ b/day-13/Cargo.lock
@@ -0,0 +1,78 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "day-13"
+version = "0.1.0"
+dependencies = [
+ "indoc",
+]
+
+[[package]]
+name = "indoc"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "47741a8bc60fb26eb8d6e0238bbb26d8575ff623fdc97b1a2c00c050b9684ed8"
+dependencies = [
+ "indoc-impl",
+ "proc-macro-hack",
+]
+
+[[package]]
+name = "indoc-impl"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ce046d161f000fffde5f432a0d034d0341dc152643b2598ed5bfce44c4f3a8f0"
+dependencies = [
+ "proc-macro-hack",
+ "proc-macro2",
+ "quote",
+ "syn",
+ "unindent",
+]
+
+[[package]]
+name = "proc-macro-hack"
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+checksum = "dbf0c48bc1d91375ae5c3cd81e3722dff1abcf81a30960240640d223f59fe0e5"
+
+[[package]]
+name = "proc-macro2"
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+checksum = "1e0704ee1a7e00d7bb417d0770ea303c1bccbabf0ef1667dae92b5967f5f8a71"
+dependencies = [
+ "unicode-xid",
+]
+
+[[package]]
+name = "quote"
+version = "1.0.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
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+dependencies = [
+ "proc-macro2",
+]
+
+[[package]]
+name = "syn"
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+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-xid",
+]
+
+[[package]]
+name = "unicode-xid"
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+checksum = "f7fe0bb3479651439c9112f72b6c505038574c9fbb575ed1bf3b797fa39dd564"
+
+[[package]]
+name = "unindent"
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diff --git a/day-13/Cargo.toml b/day-13/Cargo.toml
new file mode 100644
index 0000000..a4e1f5b
--- /dev/null
+++ b/day-13/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "day-13"
+version = "0.1.0"
+authors = ["Tristan Daniël Maat <tm@tlater.net>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+indoc = "0.3"
diff --git a/day-13/input b/day-13/input
new file mode 100644
index 0000000..e614056
--- /dev/null
+++ b/day-13/input
@@ -0,0 +1,2 @@
+1000391
+19,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,383,x,x,x,x,x,x,x,23,x,x,x,x,13,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,29,x,457,x,x,x,x,x,x,x,x,x,41,x,x,x,x,x,x,17
diff --git a/day-13/src/main.rs b/day-13/src/main.rs
new file mode 100644
index 0000000..be4bd15
--- /dev/null
+++ b/day-13/src/main.rs
@@ -0,0 +1,172 @@
+use std::fs;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+ let input = fs::read_to_string("input")?;
+ let (time, ids) = parse_notes(&input)?;
+
+ // Part 1
+ let (earliest, arrival) = find_closest_bus(time, &ids);
+ println!("{}", (arrival - time) * earliest);
+
+ // Part 2
+ let timestamp = find_gauss(&ids);
+ println!("{}", timestamp);
+
+ Ok(())
+}
+
+fn parse_notes(input: &str) -> Result<(u32, Vec<u32>), &str> {
+ let mut lines = input.lines();
+
+ let estimate = lines
+ .next()
+ .ok_or("Input too short")?
+ .parse()
+ .map_err(|_| "Estimate is not a number")?;
+
+ let ids = lines
+ .next()
+ .ok_or("Input too short")?
+ .split(',')
+ .map(|id| id.parse().unwrap_or(1))
+ .collect();
+
+ Ok((estimate, ids))
+}
+
+fn find_closest_bus(time: u32, ids: &Vec<u32>) -> (u32, u32) {
+ ids.iter()
+ .filter(|id| **id != 1)
+ .map(|id| {
+ let mut arrival = *id;
+
+ while arrival < time {
+ arrival += id
+ }
+
+ (*id, arrival)
+ })
+ .min_by_key(|id| id.1)
+ .expect("Must have at least one bus ID")
+}
+
+fn find_gauss(ids: &Vec<u32>) -> u128 {
+ let ids: Vec<u128> = ids.iter().map(|id| *id as u128).collect();
+ let product: u128 = ids.iter().product();
+
+ let gauss = |mut n: u128, mut d: u128, m: u128| -> u128 {
+ while d != 1 {
+ if m == 1 {
+ return 0;
+ };
+
+ let multiplier = if m > d {
+ m / d + (m % d != 0) as u128
+ } else {
+ 1
+ };
+
+ n = (n * multiplier) % m;
+ d = (d * multiplier) % m;
+ }
+
+ n / d
+ };
+
+ (0..ids.len())
+ .map(|a| {
+ let b = product / ids[a];
+ let bi = gauss(1, b, ids[a]);
+ let a = (ids[a] * a as u128 - a as u128) % ids[a] as u128;
+
+ a * b * bi
+ })
+ .sum::<u128>()
+ % product
+}
+
+#[cfg(test)]
+mod tests {
+ use super::*;
+ use indoc::indoc;
+
+ #[test]
+ fn test_simple() -> Result<(), Box<dyn std::error::Error>> {
+ let input = indoc!(
+ "939
+ 7,13,x,x,59,x,31,19
+ "
+ );
+
+ let (time, ids) = parse_notes(input)?;
+ let (earliest, arrival) = find_closest_bus(time, &ids);
+
+ assert_eq!(arrival, 944);
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_simple2() -> Result<(), Box<dyn std::error::Error>> {
+ let input = indoc!(
+ "939
+ 17,x,13,19
+ "
+ );
+
+ let (time, ids) = parse_notes(input)?;
+ let timestamp = find_gauss(&ids);
+
+ assert_eq!(timestamp, 3417);
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_simple3() -> Result<(), Box<dyn std::error::Error>> {
+ let input = indoc!(
+ "939
+ 7,13,x,x,59,x,31,19
+ "
+ );
+
+ let (time, ids) = parse_notes(input)?;
+ let timestamp = find_gauss(&ids);
+
+ assert_eq!(timestamp, 1068781);
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_simple4() -> Result<(), Box<dyn std::error::Error>> {
+ let input = indoc!(
+ "939
+ 67,7,59,61
+ "
+ );
+
+ let (time, ids) = parse_notes(input)?;
+ let timestamp = find_gauss(&ids);
+
+ assert_eq!(timestamp, 754018);
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_simple5() -> Result<(), Box<dyn std::error::Error>> {
+ let input = indoc!(
+ "939
+ 67,x,7,59,61
+ "
+ );
+
+ let (time, ids) = parse_notes(input)?;
+ let timestamp = find_gauss(&ids);
+
+ assert_eq!(timestamp, 779210);
+
+ Ok(())
+ }
+}