Complete day 13

day13/Cargo.lock

@@ -0,0 +1,14 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "anyhow"
+version = "1.0.35"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2c0df63cb2955042487fad3aefd2c6e3ae7389ac5dc1beb28921de0b69f779d4"
+
+[[package]]
+name = "day13"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+]

day13/Cargo.toml

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+[package]
+name = "day13"
+version = "0.1.0"
+authors = ["Tyler Hallada <tyler@hallada.net>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+anyhow = "1.0"

day13/input/input.txt

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+1000511
+29,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,409,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,17,13,19,x,x,x,23,x,x,x,x,x,x,x,353,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,41

day13/input/test.txt

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+939
+7,13,x,x,59,x,31,19

day13/src/main.rs

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+use std::fs::File;
+use std::io::prelude::*;
+use std::io::BufReader;
+use std::time::Instant;
+
+use anyhow::{anyhow, Result};
+
+const INPUT: &str = "input/input.txt";
+
+// Shamelessly copied from: https://rosettacode.org/wiki/Chinese_remainder_theorem#Rust
+// I did not like this problem.
+
+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)
+    }
+}
+
+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
+    }
+}
+
+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)
+}
+
+fn solve_part1(input_path: &str) -> Result<u32> {
+    let file = File::open(input_path)?;
+    let reader = BufReader::new(file);
+    let mut lines = reader.lines();
+    let earliest: u32 = lines
+        .next()
+        .expect("first line contains earliest timestamp")?
+        .parse()?;
+    dbg!(earliest);
+    let (bus, min) = lines
+        .next()
+        .expect("second line contains bus ids")?
+        .split(",")
+        .filter_map(|id| id.parse().ok())
+        .map(|bus: u32| {
+            let mut time = bus;
+            while time < earliest {
+                time += bus;
+            }
+            (bus, time)
+        })
+        .min_by_key(|&(_, time)| time)
+        .expect("buses list is not empty");
+    Ok((min - earliest) * bus)
+}
+
+fn solve_part2(input_path: &str) -> Result<i64> {
+    let file = File::open(input_path)?;
+    let reader = BufReader::new(file);
+    let buses: Vec<(i64, i64)> = reader
+        .lines()
+        .skip(1)
+        .next()
+        .expect("second line contains bus ids")?
+        .split(",")
+        .enumerate()
+        .filter_map(|(index, bus)| {
+            if let Ok(bus) = bus.parse::<i64>() {
+                Some((index as i64, bus))
+            } else {
+                None
+            }
+        })
+        .collect();
+    dbg!(&buses);
+
+    let modulii: Vec<i64> = buses.iter().map(|&(_, bus)| bus).collect();
+    let residues: Vec<i64> = buses.iter().map(|&(index, bus)| bus - index).collect();
+
+    chinese_remainder(residues.as_slice(), modulii.as_slice()).ok_or_else(|| anyhow!("no result"))
+}
+
+fn main() {
+    let mut now = Instant::now();
+    println!("Part 1: {}", solve_part1(INPUT).unwrap());
+    println!("(elapsed: {:?})", now.elapsed());
+    now = Instant::now();
+    println!("");
+    println!("Part 2: {}", solve_part2(INPUT).unwrap());
+    println!("(elapsed: {:?})", now.elapsed());
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const TEST_INPUT: &str = "input/test.txt";
+
+    #[test]
+    fn solves_part1() {
+        assert_eq!(solve_part1(TEST_INPUT).unwrap(), 295);
+    }
+
+    #[test]
+    fn solves_part2() {
+        assert_eq!(solve_part2(TEST_INPUT).unwrap(), 1068781);
+    }
+}