WIP: Re-implement using ndarray

This is to have a potentially nicer way to iterate neighbours, as well
as recursive directions.

While the former is neater than it was previously, the latter sadly
wasn't possible after all, and it's very inefficient.

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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+
+[[package]]
+name = "proc-macro2"
+version = "1.0.24"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+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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+
+[[package]]
+name = "unindent"
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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"

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

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(())
+    }
+}