cleanup

Cargo.lock

@@ -12,6 +12,7 @@ dependencies = [
  "itertools",
  "num",
  "prev-iter",
+ "rand 0.8.5",
  "regex",
  "rust-crypto",
  "strum",
@@ -68,6 +69,12 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d468802bab17cbc0cc575e9b053f41e72aa36bfa6b7f55e3529ffa43161b97fa"
 
+[[package]]
+name = "cfg-if"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
+
 [[package]]
 name = "either"
 version = "1.9.0"
@@ -86,6 +93,17 @@ version = "0.3.55"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8f5f3913fa0bfe7ee1fd8248b6b9f42a5af4b9d65ec2dd2c3c26132b950ecfc2"
 
+[[package]]
+name = "getrandom"
+version = "0.2.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fe9006bed769170c11f845cf00c7c1e9092aeb3f268e007c3e760ac68008070f"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "wasi 0.11.0+wasi-snapshot-preview1",
+]
+
 [[package]]
 name = "heck"
 version = "0.4.1"
@@ -195,6 +213,12 @@ dependencies = [
  "autocfg",
 ]
 
+[[package]]
+name = "ppv-lite86"
+version = "0.2.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5b40af805b3121feab8a3c29f04d8ad262fa8e0561883e7653e024ae4479e6de"
+
 [[package]]
 name = "prev-iter"
 version = "0.1.2"
@@ -242,6 +266,27 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "rand"
+version = "0.8.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "34af8d1a0e25924bc5b7c43c079c942339d8f0a8b57c39049bef581b46327404"
+dependencies = [
+ "libc",
+ "rand_chacha",
+ "rand_core 0.6.4",
+]
+
+[[package]]
+name = "rand_chacha"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e6c10a63a0fa32252be49d21e7709d4d4baf8d231c2dbce1eaa8141b9b127d88"
+dependencies = [
+ "ppv-lite86",
+ "rand_core 0.6.4",
+]
+
 [[package]]
 name = "rand_core"
 version = "0.3.1"
@@ -257,6 +302,15 @@ version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9c33a3c44ca05fa6f1807d8e6743f3824e8509beca625669633be0acbdf509dc"
 
+[[package]]
+name = "rand_core"
+version = "0.6.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ec0be4795e2f6a28069bec0b5ff3e2ac9bafc99e6a9a7dc3547996c5c816922c"
+dependencies = [
+ "getrandom",
+]
+
 [[package]]
 name = "rdrand"
 version = "0.4.0"
@@ -405,7 +459,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1b797afad3f312d1c66a56d11d0316f916356d11bd158fbc6ca6389ff6bf805a"
 dependencies = [
  "libc",
- "wasi",
+ "wasi 0.10.0+wasi-snapshot-preview1",
  "winapi",
 ]
 
@@ -421,6 +475,12 @@ version = "0.10.0+wasi-snapshot-preview1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1a143597ca7c7793eff794def352d41792a93c481eb1042423ff7ff72ba2c31f"
 
+[[package]]
+name = "wasi"
+version = "0.11.0+wasi-snapshot-preview1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9c8d87e72b64a3b4db28d11ce29237c246188f4f51057d65a7eab63b7987e423"
+
 [[package]]
 name = "winapi"
 version = "0.3.9"

Cargo.toml

@@ -17,3 +17,4 @@ strum_macros = "0.25"
 prev-iter = "0.1.2"
 regex = "1.10.2"
 num = "0.4.1"
+rand = "0.8.5"

src/day15.rs

@@ -47,10 +47,10 @@ fn part2(input: &Vec<String>) -> usize {
             if let Some(position) = boxes[box_number].iter().position(|(l, _)| *l == *label) {
                 let _ = std::mem::replace(
                     &mut boxes[box_number][position],
-                    (label.clone().to_string(), *focal_length as usize),
+                    (label.to_string(), *focal_length as usize),
                 );
             } else {
-                boxes[box_number].push((label.clone().to_string(), *focal_length as usize));
+                boxes[box_number].push((label.to_string(), *focal_length as usize));
             }
         } else {
             panic!("invalid step AHHH {}", step)

src/day20.rs

@@ -1,5 +1,4 @@
 use std::collections::{HashMap, VecDeque};
-
 use aoc_runner_derive::{aoc, aoc_generator};
 use num::Integer;
 

src/day21.rs

@@ -0,0 +1,186 @@
+use std::collections::HashSet;
+use aoc_runner_derive::{aoc, aoc_generator};
+
+const STEPS: i32 = 64;
+
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+struct Position(i32, i32);
+
+#[derive(Debug, PartialEq)]
+struct Garden {
+    rocks: HashSet<Position>,
+    start: Position,
+    extents: (Position, Position),
+}
+
+#[aoc_generator(day21, part1)]
+fn parse(input: &str) -> Garden {
+    let mut rocks: HashSet<Position> = HashSet::new();
+    let mut start: Position = Position(0, 0);
+    for (row, line) in input.lines().enumerate() {
+        for (col, ch) in line.chars().enumerate() {
+            match ch {
+                '#' => {
+                    rocks.insert(Position(row as i32, col as i32));
+                }
+                'S' => {
+                    start = Position(row as i32, col as i32);
+                }
+                _ => {}
+            };
+        }
+    }
+    let mut trans_rocks: HashSet<Position> = HashSet::new();
+    for rock in rocks.iter() {
+        let trans_rock = Position(rock.0 - start.0, start.1 - rock.1);
+        trans_rocks.insert(trans_rock);
+    }
+    rocks = trans_rocks;
+    let num_rows = input.lines().count() as i32;
+    let num_cols = input.lines().next().unwrap().chars().count() as i32;
+    let extents = (
+        Position(-num_rows / 2, -num_cols / 2),
+        Position(num_rows / 2, num_cols / 2),
+    );
+    Garden {
+        rocks,
+        start: Position(0, 0),
+        extents,
+    }
+}
+
+#[aoc(day21, part1)]
+fn part1(map: &Garden) -> u32 {
+    let mut visited: HashSet<Position> = HashSet::new();
+    visited.insert(map.start);
+    for _ in 0..STEPS {
+        let mut new_visited = HashSet::new();
+        for pos in visited.iter().clone() {
+            let neighbors = [
+                Position(pos.0 - 1, pos.1),
+                Position(pos.0 + 1, pos.1),
+                Position(pos.0, pos.1 - 1),
+                Position(pos.0, pos.1 + 1),
+            ]
+            .into_iter()
+            .filter(|p| in_bounds(*p, map.extents) && !map.rocks.contains(p));
+            for neighbor in neighbors {
+                new_visited.insert(neighbor);
+            }
+        }
+        visited = new_visited;
+    }
+    visited.len() as u32
+}
+
+fn in_bounds(pos: Position, extents: (Position, Position)) -> bool {
+    pos.0 >= extents.0 .0 && pos.0 <= extents.1 .0 && pos.1 >= extents.0 .1 && pos.1 < extents.1 .1
+}
+
+fn in_bounds_part2(pos: Position, extents: Position) -> bool {
+    pos.0 >= 0 && pos.0 <= extents.0 && pos.1 >= 0 && pos.1 < extents.1
+}
+
+struct GardenPart2 {
+    rocks: HashSet<Position>,
+    start: Position,
+    extents: Position,
+}
+
+#[aoc_generator(day21, part2)]
+fn parse_part2(input: &str) -> GardenPart2 {
+    let mut rocks: HashSet<Position> = HashSet::new();
+    for (row, line) in input.lines().enumerate() {
+        for (col, ch) in line.chars().enumerate() {
+            match ch {
+                '#' => {
+                    rocks.insert(Position(row as i32, col as i32));
+                }
+                _ => {}
+            };
+        }
+    }
+    let num_rows = input.lines().count() as i32;
+    let num_cols = input.lines().next().unwrap().chars().count() as i32;
+    let mut expanded_rocks: HashSet<Position> = HashSet::new();
+    for rock in rocks.iter() {
+        for row_mul in 0..5 {
+            for col_mul in 0..5 {
+                let expanded_rock = Position(
+                    rock.0 + (num_rows * row_mul as i32),
+                    rock.1 + (num_cols * col_mul as i32),
+                );
+                expanded_rocks.insert(expanded_rock);
+            }
+        }
+    }
+    GardenPart2 {
+        rocks: expanded_rocks,
+        start: Position(num_rows * 5 / 2, num_cols * 5 / 2),
+        extents: Position(num_rows * 5, num_cols * 5),
+    }
+}
+
+#[aoc(day21, part2)]
+fn part2(map: &GardenPart2) -> i64 {
+    let b0: i64 = walk(&map, 65) as i64;
+    let b1: i64 = walk(&map, 65 + 131) as i64;
+    let b2: i64 = walk(&map, 65 + 2 * 131) as i64;
+    let n: i64 = 202300;
+    // below uses Cramer's Rule to solve for x0, x1, x2
+    let det_a: f64 = -2.0;
+    let det_a0: f64 = -b0 as f64 + 2.0 * b1 as f64 - b2 as f64;
+    let det_a1: f64 = 3.0 * b0 as f64 - 4.0 * b1 as f64 + b2 as f64;
+    let det_a2: f64 = -2.0 * b0 as f64;
+    let x0: i64 = (det_a0 / det_a) as i64;
+    let x1: i64 = (det_a1 / det_a) as i64;
+    let x2: i64 = (det_a2 / det_a) as i64;
+
+    x0 * n * n + x1 * n + x2
+}
+
+fn walk(garden: &GardenPart2, steps: u32) -> u32 {
+    let mut visited: HashSet<Position> = HashSet::new();
+    visited.insert(garden.start);
+    for _ in 0..steps {
+        let mut new_visited = HashSet::new();
+        for pos in visited.iter().clone() {
+            let neighbors = [
+                Position(pos.0 - 1, pos.1),
+                Position(pos.0 + 1, pos.1),
+                Position(pos.0, pos.1 - 1),
+                Position(pos.0, pos.1 + 1),
+            ]
+            .into_iter()
+            .filter(|p| in_bounds_part2(*p, garden.extents) && !garden.rocks.contains(p));
+            for neighbor in neighbors {
+                new_visited.insert(neighbor);
+            }
+        }
+        visited = new_visited;
+    }
+    visited.len() as u32
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const EX: &str = r"...........
+.....###.#.
+.###.##..#.
+..#.#...#..
+....#.#....
+.##..S####.
+.##..#...#.
+.......##..
+.##.#.####.
+.##..##.##.
+...........";
+
+    #[test]
+    fn part1_example() {
+        assert_eq!(part1(&parse(EX)), 16);
+    }
+}

src/day22.rs

@@ -0,0 +1,133 @@
+use std::collections::VecDeque;
+
+use aoc_runner_derive::{aoc, aoc_generator};
+use regex::Regex;
+
+pub struct Brick {
+    up: Vec<Vec<usize>>,
+    down: Vec<Vec<usize>>,
+}
+
+
+#[aoc_generator(day22)]
+fn parse(input: &str) -> Brick {
+    let re: Regex = Regex::new(r"\d+").unwrap();
+
+    let mut bricks: Vec<[usize; 6]> = input.lines().map(|line| {
+        re.captures_iter(line)
+            .map(|c| c.extract::<0>().0.parse::<usize>().unwrap())
+            .collect::<Vec<_>>().try_into().unwrap()
+    }).collect::<Vec<_>>();
+
+    let mut heights = [[0; 10]; 10];
+    let mut indices = [[usize::MAX; 10]; 10];
+    let mut up = vec![Vec::new(); bricks.len()];
+    let mut down = vec![Vec::new(); bricks.len()];
+
+    bricks.sort_unstable_by_key(|b| b[2]);
+
+    for (i, &[x1, y1, z1, x2, y2, z2]) in bricks.iter().enumerate() {
+        let height = z2 - z1 + 1;
+        let mut top = 0;
+        let mut previous = usize::MAX;
+
+        for x in x1..=x2 {
+            for y in y1..=y2 {
+                top = top.max(heights[x][y]);
+            }
+        }
+
+        for x in x1..=x2 {
+            for y in y1..=y2 {
+                if heights[x][y] == top {
+                    let index = indices[x][y];
+                    if index != previous {
+                        up[index].push(i);
+                        down[i].push(index);
+                        previous = index;
+                    }
+                }
+
+                heights[x][y] = top + height;
+                indices[x][y] = i;
+            }
+        }
+    }
+
+    Brick { up , down }
+}
+
+#[aoc(day22, part1)]
+fn part1(input: &Brick) -> usize {
+    let Brick { down, .. } = input;
+    let mut safe = vec![true; down.len()];
+
+    for underneath in down {
+        if underneath.len() == 1 {
+            safe[underneath[0]] = false;
+        }
+    }
+
+    safe.iter().filter(|&&b| b).count()
+}
+
+#[aoc(day22, part2)]
+fn part2(input: &Brick) -> usize {
+    let Brick { up, down } = input;
+    let mut safe = vec![true; down.len()];
+
+    for underneath in down {
+        if underneath.len() == 1 {
+            safe[underneath[0]] = false;
+        }
+    }
+
+    let mut result = 0;
+    let mut todo = VecDeque::new();
+    let mut removed = vec![usize::MAX; down.len()];
+
+    for (start, &safe) in safe.iter().enumerate() {
+        if safe {
+            continue;
+        }
+
+        todo.push_back(start);
+        removed[start] = start;
+
+        while let Some(current) = todo.pop_front() {
+            for &next in &up[current] {
+                if removed[next] != start && down[next].iter().all(|&i| removed[i] == start) {
+                    result += 1;
+                    removed[next] = start;
+                    todo.push_back(next);
+                }
+            }
+        }
+    }
+
+    result
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+const EX: &str = r"1,0,1~1,2,1
+0,0,2~2,0,2
+0,2,3~2,2,3
+0,0,4~0,2,4
+2,0,5~2,2,5
+0,1,6~2,1,6
+1,1,8~1,1,9";
+
+    #[test]
+    fn part1_example() {
+        assert_eq!(part1(&parse(EX)), 5);
+    }
+
+    #[test]
+    fn part2_example() {
+        assert_eq!(part2(&parse(EX)), 7);
+    }
+}

src/day23.rs

@@ -0,0 +1,156 @@
+use std::collections::HashMap;
+
+use aoc_runner_derive::{aoc, aoc_generator};
+
+const NEIGHBORS: &[(i64,i64)] = &[(-1,0),(0,1),(1,0),(0,-1)];
+use itertools::Itertools;
+
+#[aoc_generator(day23)]
+fn parse(input: &str) -> Vec<Vec<char>> {
+    input.lines().map(|x| x.chars().collect::<Vec<_>>()).collect::<Vec<_>>()
+}
+
+fn dfs(graph: &HashMap<(usize,usize), Vec<(usize,usize,usize)>>, seen: &mut Vec<Vec<bool>>, (r,c): (usize, usize)) -> Option<usize> {
+    if r == seen.len() - 1 {
+        return Some(0);
+    }
+    let mut max_dist = None;
+    for &(rr, cc, d) in &graph[&(r,c)] {
+        if !seen[rr][cc] {
+            seen[rr][cc] = true;
+            if let Some(dist) = dfs(graph, seen, (rr,cc)) {
+                max_dist = Some(max_dist.unwrap_or(0).max(d+dist))
+            }
+            seen[rr][cc] = false;
+        }
+    }
+
+    max_dist
+}
+
+#[aoc(day23, part1)]
+fn part1(grid: &Vec<Vec<char>>) -> usize {
+    let mut graph = HashMap::<_,Vec<_>>::new();
+    for (r, c) in (0..grid.len()).cartesian_product(0..grid[0].len()) {
+        let neighbors = match grid[r][c] {
+            '#' => continue,
+            '.' => NEIGHBORS,
+            '^' => &NEIGHBORS[0..][..1],
+            '>' => &NEIGHBORS[1..][..1],
+            'v' => &NEIGHBORS[2..][..1],
+            '<' => &NEIGHBORS[3..][..1],
+            _ => unreachable!(),
+        };
+        let e = graph.entry((r,c)).or_default();
+
+        for (dr, dc) in neighbors {
+            let rr = (r as i64 + dr) as usize;
+            let cc = (c as i64 + dc) as usize;
+            if grid.get(rr).and_then(|row| row.get(cc)).is_some_and(|&t| t != '#') {
+                e.push((rr,cc,1));
+            }
+        }
+    }
+
+    let corridors = graph.iter()
+        .filter(|(_, n)| n.len() == 2)
+        .map(|(&node, _)| node)
+        .collect::<Vec<_>>();
+
+    for (r,c) in corridors {
+        let neighbors = graph.remove(&(r,c)).unwrap();
+        let (r1, c1, d1) = neighbors[0];
+        let (r2, c2, d2) = neighbors[1];
+        let n1 = graph.get_mut(&(r1,c1)).unwrap();
+        if let Some(i) = n1.iter().position(|&(rr,cc,_)| (rr,cc) == (r,c)) {
+            n1[i] = (r2,c2,d1+d2);
+        }
+        let n2 = graph.get_mut(&(r2,c2)).unwrap();
+        if let Some(i) = n2.iter().position(|&(rr,cc,_)| (rr,cc) == (r,c)) {
+            n2[i] = (r1,c1,d1+d2);
+        }
+    }
+
+    dfs(&graph, &mut vec![vec![false; grid[0].len()]; grid.len()], (0,1)).unwrap()
+}
+
+#[aoc(day23, part2)]
+fn part2(grid: &Vec<Vec<char>>) -> usize {
+    let mut graph = HashMap::<_,Vec<_>>::new();
+    for (r, c) in (0..grid.len()).cartesian_product(0..grid[0].len()) {
+        let neighbors = match grid[r][c] {
+            '#' => continue,
+            _ => NEIGHBORS,
+        };
+        let e = graph.entry((r,c)).or_default();
+
+        for (dr, dc) in neighbors {
+            let rr = (r as i64 + dr) as usize;
+            let cc = (c as i64 + dc) as usize;
+            if grid.get(rr).and_then(|row| row.get(cc)).is_some_and(|&t| t != '#') {
+                e.push((rr,cc,1));
+            }
+        }
+    }
+
+    let corridors = graph.iter()
+        .filter(|(_, n)| n.len() == 2)
+        .map(|(&node, _)| node)
+        .collect::<Vec<_>>();
+
+    for (r,c) in corridors {
+        let neighbors = graph.remove(&(r,c)).unwrap();
+        let (r1, c1, d1) = neighbors[0];
+        let (r2, c2, d2) = neighbors[1];
+        let n1 = graph.get_mut(&(r1,c1)).unwrap();
+        if let Some(i) = n1.iter().position(|&(rr,cc,_)| (rr,cc) == (r,c)) {
+            n1[i] = (r2,c2,d1+d2);
+        }
+        let n2 = graph.get_mut(&(r2,c2)).unwrap();
+        if let Some(i) = n2.iter().position(|&(rr,cc,_)| (rr,cc) == (r,c)) {
+            n2[i] = (r1,c1,d1+d2);
+        }
+    }
+
+    dfs(&graph, &mut vec![vec![false; grid[0].len()]; grid.len()], (0,1)).unwrap()
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const EX: &str = r"#.#####################
+#.......#########...###
+#######.#########.#.###
+###.....#.>.>.###.#.###
+###v#####.#v#.###.#.###
+###.>...#.#.#.....#...#
+###v###.#.#.#########.#
+###...#.#.#.......#...#
+#####.#.#.#######.#.###
+#.....#.#.#.......#...#
+#.#####.#.#.#########v#
+#.#...#...#...###...>.#
+#.#.#v#######v###.###v#
+#...#.>.#...>.>.#.###.#
+#####v#.#.###v#.#.###.#
+#.....#...#...#.#.#...#
+#.#########.###.#.#.###
+#...###...#...#...#.###
+###.###.#.###v#####v###
+#...#...#.#.>.>.#.>.###
+#.###.###.#.###.#.#v###
+#.....###...###...#...#
+#####################.#";
+
+    #[test]
+    fn part1_example() {
+        assert_eq!(part1(&parse(EX)), 94);
+    }
+
+    #[test]
+    fn part2_example() {
+        assert_eq!(part2(&parse(EX)), 154);
+    }
+}

src/day24.rs

@@ -0,0 +1,144 @@
+use std::ops::RangeInclusive;
+
+use aoc_runner_derive::{aoc, aoc_generator};
+
+//const RANGE: RangeInclusive<f64> = 7.0..=27.0;
+const RANGE: RangeInclusive<f64> = 200_000_000_000_000.0..=400_000_000_000_000.0;
+
+#[aoc_generator(day24)]
+fn parse(input: &str) -> Vec<[f64; 6]> {
+    let input: Vec<[f64; 6]> = input.lines()
+        .map(|line| {
+            line.split("@")
+                .flat_map(|components| {
+                    components.split(",")
+                        .map(str::trim)
+                        .map(|num| num.parse::<f64>().unwrap())
+                })
+                .collect::<Vec<_>>().try_into().unwrap()
+        }).collect::<Vec<_>>();
+
+    input
+}
+
+#[aoc(day24, part1)]
+fn part1(input: &[[f64; 6]]) -> u32 {
+    let mut total = 0;
+
+    // It's just solve the linear equations. I'm using a matrix.
+    for i in 1..input.len() {
+        for j in 0..i {
+            let [a, b, _, d, e, _] = input[i];
+            let [g, h, _, j, k, _] = input[j];
+
+            let determinant = e * j - d * k;
+            if determinant == 0.0 {
+                continue;
+            }
+
+            let t = (j * (h - b) - k * (g - a)) / determinant;
+            let u = (d * (h - b) - e * (g - a)) / determinant;
+
+            let x = a + t * d;
+            let y = b + t * e;
+
+            if t >= 0.0 && u >= 0.0 && RANGE.contains(&x) && RANGE.contains(&y) {
+                total += 1;
+            }
+        }
+    }
+
+    total
+}
+
+#[aoc(day24, part2)]
+fn part2(input: &[[f64; 6]]) -> f64 {
+    let [a, b, c, d, e, f] = input[0];
+    let [g, h, i, j, k, l] = input[1];
+    let [m, n, o, p, q, r] = input[2];
+
+    let mut matrix = [
+        [0.0, l - f, e - k, 0.0, c - i, h - b, e * c - b * f + h * l - k * i],
+        [0.0, r - f, e - q, 0.0, c - o, n - b, e * c - b * f + n * r - q * o],
+        [f - l, 0.0, j - d, i - c, 0.0, a - g, a * f - d * c + j * i - g * l],
+        [f - r, 0.0, p - d, o - c, 0.0, a - m, a * f - d * c + p * o - m * r],
+        [k - e, d - j, 0.0, b - h, g - a, 0.0, d * b - a * e + g * k - j * h],
+        [q - e, d - p, 0.0, b - n, m - a, 0.0, d * b - a * e + m * q - p * n],
+    ];
+
+    // Use Gaussian elimination to solve for the 6 unknowns.
+    // Forward elimination, processing columns from left to right.
+    // This will leave a matrix in row echelon form.
+    // That's right, I got a D in applied linear algebra.
+    for pivot in 0..6 {
+        // Make leading coefficient of each row positive to make subsequent calculations easier.
+        for row in &mut matrix[pivot..] {
+            if row[pivot] < 0.0 {
+                row.iter_mut().for_each(|num| *num = -*num);
+            }
+        }
+
+        loop {
+            let column = matrix.map(|row| row[pivot]);
+
+            // If only one non-zero coefficient remaining in the column then we're done.
+            if column[pivot..].iter().filter(|&&c| c > 0.0).count() == 1 {
+                // Move this row into the pivot location
+                let index = column.iter().rposition(|&c| c > 0.0).unwrap();
+                matrix.swap(pivot, index);
+                break;
+            }
+
+            // Find the row with the lowest non-zero leading coefficient.
+            let min = *column[pivot..].iter().filter(|&&c| c > 0.0).min_by(|a, b| a.total_cmp(b)).unwrap();
+            let index = column.iter().rposition(|&c| c == min).unwrap();
+
+            // Subtract as many multiples of this minimum row from each other row as possible
+            // to shrink the coefficients of our column towards zero.
+            for row in pivot..6 {
+                if row != index && column[row] != 0.0 {
+                    let factor = column[row] / min;
+
+                    for col in pivot..7 {
+                        matrix[row][col] -= factor * matrix[index][col];
+                    }
+                }
+            }
+        }
+    }
+
+    // Back substitution, processing columns from right to left.
+    // This will leave the matrix in reduced row echelon form.
+    // The solved unknowns are then in the 7th column.
+    for pivot in (0..6).rev() {
+        matrix[pivot][6] /= matrix[pivot][pivot];
+
+        for row in 0..pivot {
+            matrix[row][6] -= matrix[pivot][6] * matrix[row][pivot];
+        }
+    }
+
+    (matrix[0][6] + matrix[1][6] + matrix[2][6]).floor()
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const EX: &str = r"19, 13, 30 @ -2,  1, -2
+18, 19, 22 @ -1, -1, -2
+20, 25, 34 @ -2, -2, -4
+12, 31, 28 @ -1, -2, -1
+20, 19, 15 @  1, -5, -3";
+
+    #[test]
+    fn part1_example() {
+        assert_eq!(part1(&parse(EX)), 2);
+    }
+
+    #[test]
+    fn part2_example() {
+        assert_eq!(part2(&parse(EX)), 47.0);
+    }
+}

src/day25.rs

@@ -0,0 +1,109 @@
+use std::collections::{HashSet, HashMap};
+use rand::Rng;
+
+use aoc_runner_derive::{aoc, aoc_generator};
+
+#[derive(Debug, Clone)]
+struct Graph {
+    edges: Vec<(String, String)>,
+    vertex_count: usize
+}
+
+impl Graph {
+    fn cut(&mut self) -> (usize, usize, usize) {
+        let mut contracted_edges = self.edges.clone();
+        let mut contracted_vertex_count = self.vertex_count;
+        let mut contracted: HashMap<String, Vec<String>> = HashMap::new();
+
+        while contracted_vertex_count > 2 {
+            let random = rand::thread_rng().gen_range(0..contracted_edges.len());
+            let edge_to_contract = contracted_edges[random].clone();
+            if contracted.contains_key(&edge_to_contract.0) {
+                contracted.get_mut(&edge_to_contract.0.clone()).unwrap().push(edge_to_contract.1.clone());
+            } else {
+                contracted.insert(edge_to_contract.0.clone(), vec![edge_to_contract.1.clone()]);
+            }
+
+            if contracted.contains_key(&edge_to_contract.1) {
+                let mut to_append = contracted.clone().get_mut(&edge_to_contract.1).unwrap().clone();
+                contracted.get_mut(&edge_to_contract.0).unwrap().append(&mut to_append);
+                contracted.remove(&edge_to_contract.1);
+            }
+
+            let mut new_edges: Vec<(String, String)> = vec![];
+
+            for edge in contracted_edges {
+                if edge.1 == edge_to_contract.1 {
+                    new_edges.push((edge.0.clone(), edge_to_contract.0.clone()));
+                } else if edge.0 == edge_to_contract.1 {
+                    new_edges.push((edge_to_contract.0.clone(), edge.1.clone()));
+                } else {
+                    new_edges.push(edge.clone());
+                }
+            }
+
+            let tmp = new_edges.iter().cloned().filter(|x| x.0 != x.1).collect::<Vec<_>>();
+
+            contracted_edges = tmp;
+            contracted_vertex_count -= 1;
+
+        }
+
+        let counts = contracted.iter().map(|x| x.1.len() + 1).collect::<Vec<_>>();
+
+        return (contracted_edges.len(), counts[0], *counts.last().unwrap());
+    }
+}
+
+#[aoc_generator(day25)]
+fn parse(input: &str) -> Graph {
+    let input = input.lines().flat_map(|line| {
+        let line = line.split(' ').map(|x| x.trim_end_matches(':').to_string()).collect::<Vec<_>>();
+        line[1..].iter().cloned().map(|x| (line[0].clone(), x)).collect::<Vec<_>>()
+    }).collect::<Vec<_>>();
+
+    let vertex_count = input.clone().into_iter().flat_map(|x| vec![x.0, x.1]).collect::<HashSet<_>>().len();
+    Graph {
+        edges: input,
+        vertex_count
+    }
+}
+
+#[aoc(day25, part1)]
+fn part1(graph: &Graph) -> usize {
+    let graph = &mut graph.clone();
+    let mut min_cut = usize::MAX;
+    let mut count1 = 0;
+    let mut count2 = 0;
+
+    while min_cut != 3 {
+        (min_cut, count1, count2) = graph.cut();
+    }
+
+    count1 * count2
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const EX: &str = r"jqt: rhn xhk nvd
+rsh: frs pzl lsr
+xhk: hfx
+cmg: qnr nvd lhk bvb
+rhn: xhk bvb hfx
+bvb: xhk hfx
+pzl: lsr hfx nvd
+qnr: nvd
+ntq: jqt hfx bvb xhk
+nvd: lhk
+lsr: lhk
+rzs: qnr cmg lsr rsh
+frs: qnr lhk lsr";
+
+    #[test]
+    fn part1_example() {
+        assert_eq!(part1(&parse(EX)), 54);
+    }
+}

src/lib.rs

@@ -1,3 +1,8 @@
+mod day25;
+mod day24;
+mod day23;
+mod day22;
+mod day21;
 mod day20;
 mod day19;
 mod day18;