day 13 solution **after hints!**

Attempted a brute force solution.

After letting my i9 churn on it across all cores and hyperthreads for ~4 days, looked online and learned about Chinese Remainder Theorem. Applied code from Rosetta Code and success.

Signed-off-by: Stephen Waits <swaits@amazon.com>

data/input-13.txt

@@ -0,0 +1,2 @@
+1002392
+23,x,x,x,x,x,x,x,x,x,x,x,x,41,x,x,x,37,x,x,x,x,x,421,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,17,x,19,x,x,x,x,x,x,x,x,x,29,x,487,x,x,x,x,x,x,x,x,x,x,x,x,13

data/output-13.txt

@@ -0,0 +1,2 @@
+3789
+667437230788118

src/day13.rs

@@ -0,0 +1,121 @@
+// ==== day 13
+
+use crate::utils;
+use anyhow::Result;
+
+#[derive(Debug)]
+struct BusRoute {
+    index: usize,
+    time: usize,
+}
+
+fn parse_bus_routes(input: &str) -> Vec<BusRoute> {
+    input
+        .trim()
+        .split(',')
+        .enumerate()
+        .filter_map(|(i, s)| {
+            if let Ok(t) = s.parse::<usize>() {
+                Some(BusRoute { index: i, time: t })
+            } else {
+                None
+            }
+        })
+        .collect()
+}
+
+fn mins_to_next_departure(dep_time: usize, bus: &BusRoute) -> usize {
+    let scheduled = bus.time * (dep_time / bus.time);
+    if scheduled == dep_time {
+        0
+    } else {
+        scheduled + bus.time - dep_time
+    }
+}
+
+// after failing at brute force, looked online for hints and learned about
+// the Chinese Remainder Theorem.
+//
+// code from https://rosettacode.org/wiki/Chinese_remainder_theorem#Rust
+fn egcd(a: i64, b: i64) -> (i64, i64, i64) {
+    if a == 0 {
+        (b, 0, 1)
+    } else {
+        let (g, x, y) = egcd(b % a, a);
+        (g, y - (b / a) * x, x)
+    }
+}
+
+// after failing at brute force, looked online for hints and learned about
+// the Chinese Remainder Theorem.
+//
+// code from https://rosettacode.org/wiki/Chinese_remainder_theorem#Rust
+fn mod_inv(x: i64, n: i64) -> Option<i64> {
+    let (g, x, _) = egcd(x, n);
+    if g == 1 {
+        Some((x % n + n) % n)
+    } else {
+        None
+    }
+}
+
+// after failing at brute force, looked online for hints and learned about
+// the Chinese Remainder Theorem.
+//
+// code from https://rosettacode.org/wiki/Chinese_remainder_theorem#Rust
+fn chinese_remainder(residues: &[i64], modulii: &[i64]) -> Option<i64> {
+    let prod = modulii.iter().product::<i64>();
+
+    let mut sum = 0;
+
+    for (&residue, &modulus) in residues.iter().zip(modulii) {
+        let p = prod / modulus;
+        sum += residue * mod_inv(p, modulus)? * p
+    }
+
+    Some(sum % prod)
+}
+
+// main
+pub fn run() -> Result<(usize, i64)> {
+    let data = utils::read_file("data/input-13.txt")?;
+    let answers = utils::read_i64s("data/output-13.txt")?;
+
+    // parse initial start time and bus routes
+    let parsed: Vec<&str> = data.splitn(2, '\n').collect();
+    assert_eq!(parsed.len(), 2);
+    let departure_time = parsed[0].parse::<usize>().unwrap();
+    let routes: Vec<BusRoute> = parse_bus_routes(parsed[1]);
+
+    // find which of the bus routes departs most soon after departure_time
+    let mut next_departures: Vec<(usize, usize)> = routes
+        .iter()
+        .map(|bus| (mins_to_next_departure(departure_time, bus), bus.time))
+        .collect();
+    next_departures.sort_unstable();
+    let best = next_departures.first().unwrap();
+    let p1 = best.0 * best.1;
+
+    // use CRT to compute the congruence across all these modulii
+    //
+    // after failing at brute force, looked online for hints and learned about
+    // the Chinese Remainder Theorem.
+    //
+    // code from https://rosettacode.org/wiki/Chinese_remainder_theorem#Rust
+    let modulii: Vec<i64> = routes.iter().map(|bus| bus.time as i64).collect();
+    let residues: Vec<i64> = routes
+        .iter()
+        .map(|bus| {
+            if bus.index == 0 {
+                0
+            } else {
+                ((100 * bus.time - bus.index) % bus.time) as i64
+            }
+        })
+        .collect();
+    let p2 = chinese_remainder(&residues, &modulii).unwrap();
+
+    assert_eq!(p1, answers[0] as usize);
+    assert_eq!(p2, answers[1]);
+    Ok((p1, p2))
+}

src/main.rs

@@ -12,6 +12,7 @@ mod day09;
 mod day10;
 mod day11;
 mod day12;
+mod day13;
 mod day14;
 mod day15;
 mod utils;
@@ -39,6 +40,7 @@ fn main() -> Result<()> {
     println!("Day 10: {:?}", day10::run()?);
     println!("Day 11: {:?}", day11::run()?);
     println!("Day 12: {:?}", day12::run()?);
+    println!("Day 13: {:?}", day13::run()?);
     println!("Day 14: {:?}", day14::run()?);
     println!("Day 15: {:?}", day15::run()?);
     Ok(())