day 6 part 2 1.5 times faster

cleanup

cleanup

Cargo.toml

@@ -6,3 +6,7 @@ edition = "2021"
 [dependencies]
 itertools = "0.13.0"
 regex = "1.11.1"
+
+[profile.dev]
+opt-level = 3
+

src/day06.rs

@@ -64,7 +64,7 @@ fn parse(input: &str) -> (Grid<char>, Coord) {
 }
 
 fn is_loop(m: &Grid<char>, block: Coord, mut pos: Coord, mut dir: char) -> bool {
-    let mut visited = HashSet::new();
+    let mut visited: Grid<[bool; 5]> = Grid::from_default(m.content_width, m.content_height);
     loop {
         let (x, y) = pos;
 
@@ -79,10 +79,11 @@ fn is_loop(m: &Grid<char>, block: Coord, mut pos: Coord, mut dir: char) -> bool
             return false;
         } else if next == block || '#' == m[next] {
             // we only check for loops on a collision to speed things up
-            if visited.contains(&(pos, dir)) {
+            // this is our own little hash function to speed things up
+            if visited[pos][dir as usize % 5] {
                 return true;
             }
-            visited.insert((pos, dir));
+            visited[pos][dir as usize % 5] = true;
 
             dir = match dir {
                 '^' => '>',

src/day09.rs

@@ -1,4 +1,4 @@
-use std::fs::read_to_string;
+use std::{collections::BTreeMap, fs::read_to_string};
 
 pub fn day_main() {
     let input = read_to_string("input/day09.txt").unwrap();
@@ -53,48 +53,41 @@ fn part1(input: &str) -> RiddleResult {
 }
 
 fn part2(input: &str) -> RiddleResult {
-    let mut disk = Vec::with_capacity(input.len() * 10);
+    let mut free: BTreeMap<usize, usize> = BTreeMap::new();
     // (start_index, len, file_id)
-    let mut files: Vec<(usize, u32, usize)> = Vec::with_capacity(input.len() / 2 + 1);
+    let mut files: Vec<(usize, usize, usize)> = Vec::with_capacity(input.len() / 2 + 1);
+    let mut head = 0;
     for (i, l) in input.chars().enumerate() {
-        let l = l.to_digit(10).unwrap();
-        let content = if i % 2 == 0 {
-            // file
-            Some(i / 2) // id based on order of apperance and every second one is a file
-        } else {
-            None
-        };
+        let l = l.to_digit(10).unwrap() as usize;
         if i % 2 == 0 {
-            files.push((disk.len(), l, i / 2));
-        }
-        for _ in 0..l {
-            disk.push(content);
+            files.push((head, l, i / 2));
+        } else {
+            free.insert(head, l);
         }
+        head += l;
     }
-    while let Some((start_index, length, file_id)) = files.pop() {
-        let mut seeker = 0;
-        let mut found = None;
-        while disk[seeker] != Some(file_id) {
-            if disk[seeker..seeker + length as usize]
-                .iter()
-                .all(|v| v.is_none())
-            {
-                found = Some(seeker);
-                break;
-            }
-            seeker += 1;
-        }
-        if let Some(empty_start) = found {
-            for i in 0..length as usize {
-                disk[empty_start + i] = Some(file_id);
-                disk[start_index + i] = None;
+    for file in files.iter_mut().rev() {
+        let (start_index, length, _file_id) = *file;
+        let found = free
+            .iter()
+            .take_while(|f| *f.0 < start_index)
+            .find(|f| *f.1 >= length);
+
+        if let Some((&free_start, &free_length)) = found {
+            free.remove(&free_start);
+            free.insert(start_index, length);
+            file.0 = free_start;
+            if length < free_length {
+                free.insert(free_start + length, free_length - length);
             }
         }
     }
 
-    disk.into_iter()
-        .enumerate()
-        .map(|(i, v)| if let Some(value) = v { i * value } else { 0 })
+    files
+        .iter()
+        .map(|(start_index, l, file_id)| {
+            *file_id * (*start_index..start_index + l).sum::<RiddleResult>()
+        })
         .sum()
 }
 

src/day12.rs

@@ -56,12 +56,12 @@ fn part2(input: &str) -> RiddleResult {
         }
         let mut area = 0;
         let mut further = vec![p];
-        let mut convex_corners = 0;
-        let mut concave_corners = 0;
+        let mut group = HashSet::new();
         while let Some(current) = further.pop() {
             if processed.contains(&current) {
                 continue;
             }
+            group.insert(current);
             area += 1;
             for next in NEIGHBORS.iter().map(|d| (current.0 + d.0, current.1 + d.1)) {
                 let next_c = garden.get(next);
@@ -71,28 +71,88 @@ fn part2(input: &str) -> RiddleResult {
                 }
             }
             processed.insert(current);
-            for (da, db) in NEIGHBORS.iter().circular_tuple_windows() {
-                let a = (current.0 + da.0, current.1 + da.1);
-                let b = (current.0 + db.0, current.1 + db.1);
-                if garden.get(a) != Some(c) && garden.get(b) != Some(c) {
-                    convex_corners += 1;
-                } else if garden.get(a) == Some(c)
-                    && garden.get(b) == Some(c)
-                    && garden.get((current.0 + da.0 + db.0, current.1 + da.1 + db.1)) != Some(c)
-                {
-                    concave_corners += 1;
-                }
-            }
         }
-        let var_name = convex_corners + concave_corners;
-        let var_name = area * (var_name);
-        result += var_name;
+        result += area * sides(group);
     }
     result
 }
 
+fn sides(group: HashSet<(i64, i64)>) -> RiddleResult {
+    let x_range = group.iter().min_by_key(|it| it.0).unwrap().0
+        ..=group.iter().max_by_key(|it| it.0).unwrap().0;
+    let y_range = group.iter().min_by_key(|it| it.1).unwrap().1
+        ..=group.iter().max_by_key(|it| it.1).unwrap().1;
+    let mut result = 0;
+    for x in x_range {
+        // 1 for the ending of the last group, +1 for each time a line "stops", e.g. between the current point and next there is a gap
+        let points = group
+            .iter()
+            .filter(|p| p.0 == x)
+            .filter(|p| !group.contains(&(p.0 - 1, p.1)))
+            .sorted_by_key(|p| p.1)
+            .collect_vec();
+        if !points.is_empty() {
+            let count = points
+                .into_iter()
+                .tuple_windows()
+                .filter(|(a, b)| a.1 + 1 != b.1)
+                .count();
+            result += 1 + count;
+        }
+        let points = group
+            .iter()
+            .filter(|p| p.0 == x)
+            .filter(|p| !group.contains(&(p.0 + 1, p.1)))
+            .sorted_by_key(|p| p.1)
+            .collect_vec();
+        if !points.is_empty() {
+            let count = points
+                .into_iter()
+                .tuple_windows()
+                .filter(|(a, b)| a.1 + 1 != b.1)
+                .count();
+            result += 1 + count;
+        }
+    }
+    for y in y_range {
+        let points = group
+            .iter()
+            .filter(|p| p.1 == y)
+            .filter(|p| !group.contains(&(p.0, p.1 - 1)))
+            .sorted_by_key(|p| p.0)
+            .collect_vec();
+        if !points.is_empty() {
+            let count = points
+                .into_iter()
+                .tuple_windows()
+                .filter(|(a, b)| a.0 + 1 != b.0)
+                .count();
+            result += 1 + count;
+        }
+        let points = group
+            .iter()
+            .filter(|p| p.1 == y)
+            .filter(|p| !group.contains(&(p.0, p.1 + 1)))
+            .sorted_by_key(|p| p.0)
+            .collect_vec();
+        if !points.is_empty() {
+            let count = points
+                .into_iter()
+                .tuple_windows()
+                .filter(|(a, b)| a.0 + 1 != b.0)
+                .count();
+            result += 1 + count;
+        }
+    }
+    result as RiddleResult
+}
+
 #[cfg(test)]
 mod test {
+    use std::collections::HashSet;
+
+    use crate::day12::sides;
+
     use super::{part1, part2};
 
     const TEST_INPUT: &str = r"AAAA
@@ -110,4 +170,16 @@ EEEC
     fn test2() {
         assert_eq!(part2(TEST_INPUT), 80);
     }
+
+    #[test]
+    fn sides_one() {
+        let group = HashSet::from_iter([(3, 5)]);
+        assert_eq!(sides(group), 4);
+    }
+
+    #[test]
+    fn sides_plus() {
+        let group = HashSet::from_iter([(3, 3), (3, 4), (3, 2), (2, 3), (4, 3)]);
+        assert_eq!(sides(group), 12);
+    }
 }

src/day13.rs

@@ -0,0 +1,99 @@
+use std::fs::read_to_string;
+
+use itertools::Itertools;
+use regex::Regex;
+
+pub fn day_main() {
+    let input = read_to_string("input/day13.txt").unwrap();
+    let input = input.trim();
+    println!(" part1: {}", part1(input));
+    println!(" part2: {}", part2(input));
+}
+
+type RiddleResult = i64;
+
+fn part1(input: &str) -> RiddleResult {
+    // Button A: X+30, Y+84
+    // Button B: X+74, Y+60
+    // Prize: X=2358, Y=2628
+    let r = Regex::new(
+        r"Button A: X\+(\d+), Y\+(\d+)
+Button B: X\+(\d+), Y\+(\d+)
+Prize: X=(\d+), Y=(\d+)",
+    )
+    .unwrap();
+    r.captures_iter(input)
+        .map(|block| {
+            let (ax, ay, bx, by, px, py) = block
+                .iter()
+                .skip(1)
+                .map(|it| it.unwrap().as_str().parse::<i64>().unwrap())
+                .collect_tuple()
+                .unwrap();
+            solve((ax, ay, bx, by, px, py)).unwrap_or(0)
+        })
+        .sum()
+}
+
+fn part2(input: &str) -> RiddleResult {
+    let r = Regex::new(
+        r"Button A: X\+(\d+), Y\+(\d+)
+Button B: X\+(\d+), Y\+(\d+)
+Prize: X=(\d+), Y=(\d+)",
+    )
+    .unwrap();
+    r.captures_iter(input)
+        .map(|block| {
+            let (ax, ay, bx, by, px, py) = block
+                .iter()
+                .skip(1)
+                .map(|it| it.unwrap().as_str().parse::<i64>().unwrap())
+                .collect_tuple()
+                .unwrap();
+            let solution = solve((ax, ay, bx, by, px + 10000000000000, py + 10000000000000));
+            solution.unwrap_or(0)
+        })
+        .sum()
+}
+
+fn solve((ax, ay, bx, by, px, py): (i64, i64, i64, i64, i64, i64)) -> Option<RiddleResult> {
+    let b = (ay * px - ax * py) / (ay * bx - ax * by);
+    let a = (px - b * bx) / ax;
+    if a >= 0 && b >= 0 && a * ax + b * bx == px && a * ay + b * by == py {
+        Some(3 * a + b)
+    } else {
+        None
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::{part1, part2};
+
+    const TEST_INPUT: &str = r"Button A: X+94, Y+34
+Button B: X+22, Y+67
+Prize: X=8400, Y=5400
+
+Button A: X+26, Y+66
+Button B: X+67, Y+21
+Prize: X=12748, Y=12176
+
+Button A: X+17, Y+86
+Button B: X+84, Y+37
+Prize: X=7870, Y=6450
+
+Button A: X+69, Y+23
+Button B: X+27, Y+71
+Prize: X=18641, Y=10279
+";
+
+    #[test]
+    fn test1() {
+        assert_eq!(part1(TEST_INPUT), 480);
+    }
+
+    #[test]
+    fn test2() {
+        assert_eq!(part2(TEST_INPUT), 875318608908);
+    }
+}

src/day14.rs

@@ -0,0 +1,153 @@
+use std::fs::read_to_string;
+
+use itertools::Itertools;
+
+use crate::utils::grid::Grid;
+
+pub fn day_main() {
+    let input = read_to_string("input/day14.txt").unwrap();
+    let input = input.trim();
+    println!(" part1: {}", part1(input));
+    println!(" part2: {}", part2(input));
+}
+
+type RiddleResult = usize;
+
+fn part1(input: &str) -> RiddleResult {
+    solve_part1(input, 100, 101, 103)
+}
+
+fn solve_part1(input: &str, rounds: i64, width: i64, height: i64) -> RiddleResult {
+    let mut robots = parse(input);
+    for _ in 0..rounds {
+        robots.iter_mut().for_each(|((px, py), (vx, vy))| {
+            *px = (*px + width + *vx) % width;
+            *py = (*py + height + *vy) % height;
+        });
+    }
+    let upper_left = robots
+        .iter()
+        .filter(|&&((px, py), _)| px < width / 2 && py < height / 2)
+        .count();
+    let lower_left = robots
+        .iter()
+        .filter(|&&((px, py), _)| px < width / 2 && py > height / 2)
+        .count();
+    let upper_right = robots
+        .iter()
+        .filter(|&&((px, py), _)| px > width / 2 && py < height / 2)
+        .count();
+    let lower_right = robots
+        .iter()
+        .filter(|&&((px, py), _)| px > width / 2 && py > height / 2)
+        .count();
+
+    upper_left * lower_left * upper_right * lower_right
+}
+
+fn parse(input: &str) -> Vec<((i64, i64), (i64, i64))> {
+    input
+        .lines()
+        .map(|line| {
+            line.strip_prefix("p=")
+                .unwrap()
+                .split(" v=")
+                .map(|s| {
+                    s.split(",")
+                        .map(|d| d.parse::<i64>().unwrap())
+                        .collect_tuple::<(i64, i64)>()
+                        .unwrap()
+                })
+                .collect_tuple()
+                .unwrap()
+        })
+        .collect_vec()
+}
+
+#[allow(unused)]
+fn part2(input: &str) -> i64 {
+    let width = 101;
+    let height = 103;
+    let mut robots = parse(input);
+    let max_seconds = width * height;
+    for second in 0.. {
+        if second > max_seconds {
+            panic!("Seen all combinations but no christmas tree. So sad!");
+        }
+        let mut grid: Grid<u8> = Grid::from_default(101, 103);
+        robots.iter_mut().for_each(|((px, py), (vx, vy))| {
+            *px = (*px + width + *vx) % width;
+            *py = (*py + height + *vy) % height;
+            grid[(*px, *py)] += 1;
+            if let Some(v) = grid.get_mut((*px + 1, *py)) {
+                *v += 1
+            }
+            if let Some(v) = grid.get_mut((*px - 1, *py)) {
+                *v += 1
+            }
+            if let Some(v) = grid.get_mut((*px, *py + 1)) {
+                *v += 1
+            }
+            if let Some(v) = grid.get_mut((*px, *py - 1)) {
+                *v += 1
+            }
+        });
+
+        if robots.iter().filter(|(s, _)| grid[*s] > 1).count() > robots.len() * 70 / 100 {
+            // printr(&robots, width, height);
+            // println!("after {} seconds. Press enter to continue or type 'merry christmas' if you can spot a tree!", second + 1); //+1 because we look at it after they have changed
+            return second + 1;
+            // let stdin = io::stdin();
+            // let line = stdin.lock().lines().next().unwrap().unwrap();
+            // if line.as_str().eq_ignore_ascii_case("merry christmas") {
+            // return second + 1;
+            // }
+        }
+    }
+    unreachable!()
+}
+
+#[allow(unused)]
+type Robot = ((i64, i64), (i64, i64));
+
+#[allow(unused)]
+fn printr(robots: &[Robot], width: i64, height: i64) {
+    for y in 0..height {
+        for x in 0..width {
+            let c = robots
+                .iter()
+                .filter(|&&((px, py), _)| px == x && py == y)
+                .count();
+            if c == 0 {
+                print!(".");
+            } else {
+                print!("{c}");
+            }
+        }
+        println!();
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::day14::solve_part1;
+
+    const TEST_INPUT: &str = r"p=0,4 v=3,-3
+p=6,3 v=-1,-3
+p=10,3 v=-1,2
+p=2,0 v=2,-1
+p=0,0 v=1,3
+p=3,0 v=-2,-2
+p=7,6 v=-1,-3
+p=3,0 v=-1,-2
+p=9,3 v=2,3
+p=7,3 v=-1,2
+p=2,4 v=2,-3
+p=9,5 v=-3,-3
+";
+
+    #[test]
+    fn test1() {
+        assert_eq!(solve_part1(TEST_INPUT, 100, 11, 7), 12);
+    }
+}

src/day15.rs

@@ -0,0 +1,203 @@
+use std::{collections::HashSet, fs::read_to_string};
+
+use itertools::Itertools;
+
+use crate::utils::grid::{Coord, Grid};
+
+pub fn day_main() {
+    let input = read_to_string("input/day15.txt").unwrap();
+    let input = input.trim();
+    println!(" part1: {}", part1(input));
+    println!(" part2: {}", part2(input));
+}
+
+type RiddleResult = i64;
+
+fn part1(input: &str) -> RiddleResult {
+    let (grid, movements) = input.split_once("\n\n").unwrap();
+    let mut grid = Grid::parse(grid);
+    let mut robot = grid.entries().find(|(_r, c)| **c == '@').unwrap().0;
+    let directions = |d| match d {
+        '^' => (0, -1),
+        'v' => (0, 1),
+        '<' => (-1, 0),
+        '>' => (1, 0),
+        _ => panic!(),
+    };
+    for m in movements.chars().filter(|c| *c != '\n') {
+        let dir = directions(m);
+        let space = (1..)
+            .map(|i| (robot.0 + i * dir.0, robot.1 + i * dir.1))
+            .take_while(|p| grid.get(*p).is_some() && (grid[*p] == '.' || grid[*p] == 'O'))
+            .find(|p| grid[*p] == '.');
+        if let Some(p) = space {
+            if (p.0 - robot.0).abs() + (p.1 - robot.1).abs() > 1 {
+                // this means: the free spot is not a direct neighbor of the robot, i.e. there are boxes
+                grid[p] = 'O';
+            }
+            grid[robot] = '.';
+            robot = (robot.0 + dir.0, robot.1 + dir.1);
+            grid[robot] = '@';
+        }
+    }
+    grid.entries()
+        .filter(|(_, c)| **c == 'O')
+        .map(|((x, y), _)| y * 100 + x)
+        .sum()
+}
+
+fn part2(input: &str) -> RiddleResult {
+    let (grid, movements) = input.split_once("\n\n").unwrap();
+    let grid = grid
+        .lines()
+        .map(|line| {
+            {
+                line.chars().flat_map(|c| {
+                    match c {
+                        '.' => "..",
+                        '@' => "@.",
+                        '#' => "##",
+                        'O' => "[]",
+                        _ => panic!(),
+                    }
+                    .chars()
+                })
+            }
+            .join("")
+        })
+        .join("\n");
+
+    let mut grid = Grid::parse(grid.as_str());
+
+    let mut robot = grid.entries().find(|(_r, c)| **c == '@').unwrap().0;
+    let directions = |d| match d {
+        '^' => (0, -1),
+        'v' => (0, 1),
+        '<' => (-1, 0),
+        '>' => (1, 0),
+        _ => panic!(),
+    };
+    for m in movements.chars().filter(|c| *c != '\n') {
+        let dir = directions(m);
+        if let Some(tiles_to_move) = movable(robot, dir, &grid, false) {
+            let old = tiles_to_move
+                .iter()
+                .map(|tile| (*tile, grid[*tile]))
+                .collect_vec();
+            tiles_to_move.iter().for_each(|tile| grid[*tile] = '.');
+            old.into_iter()
+                .for_each(|(tile, c)| grid[(tile.0 + dir.0, tile.1 + dir.1)] = c);
+            robot = (robot.0 + dir.0, robot.1 + dir.1);
+        }
+    }
+    grid.entries()
+        .filter(|(_, c)| **c == '[')
+        .map(|((x, y), _)| y * 100 + x)
+        .sum()
+}
+
+fn movable(
+    from: (i64, i64),
+    dir: (i64, i64),
+    grid: &Grid<char>,
+    ignore_other_half: bool,
+) -> Option<HashSet<Coord>> {
+    match grid[from] {
+        '.' => Some(HashSet::new()),
+        '@' => {
+            let next = movable((from.0 + dir.0, from.1 + dir.1), dir, grid, false);
+            next.map(|mut v| {
+                v.insert(from);
+                v
+            })
+        }
+        '[' => {
+            if dir.0 != 0 {
+                // sideway movement is "regular"
+                let next = movable((from.0 + dir.0, from.1 + dir.1), dir, grid, false);
+                next.map(|mut v| {
+                    v.insert(from);
+                    v
+                })
+            } else {
+                // up/down always means the other part of the crate has to move in parallel
+                let mut next1 = movable((from.0 + dir.0, from.1 + dir.1), dir, grid, false)?;
+                if !ignore_other_half {
+                    let next2 = movable((from.0 + 1, from.1), dir, grid, true)?;
+                    next1.extend(next2);
+                }
+                next1.insert(from);
+                Some(next1)
+            }
+        }
+        ']' => {
+            if dir.0 != 0 {
+                // sideway movement is "regular"
+                let next = movable((from.0 + dir.0, from.1 + dir.1), dir, grid, false);
+                next.map(|mut v| {
+                    v.insert(from);
+                    v
+                })
+            } else {
+                // up/down always means the other part of the crate has to move in parallel
+                let mut next1 = movable((from.0 + dir.0, from.1 + dir.1), dir, grid, false)?;
+                if !ignore_other_half {
+                    let next2 = movable((from.0 - 1, from.1), dir, grid, true)?;
+                    next1.extend(next2);
+                }
+                next1.insert(from);
+                Some(next1)
+            }
+        }
+        _ => None,
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::{part1, part2};
+
+    const TEST_INPUT: &str = r"########
+#..O.O.#
+##@.O..#
+#...O..#
+#.#.O..#
+#...O..#
+#......#
+########
+
+<^^>>>vv<v>>v<<";
+
+    const TEST_LARGE: &str = r"##########
+#..O..O.O#
+#......O.#
+#.OO..O.O#
+#..O@..O.#
+#O#..O...#
+#O..O..O.#
+#.OO.O.OO#
+#....O...#
+##########
+
+<vv>^<v^>v>^vv^v>v<>v^v<v<^vv<<<^><<><>>v<vvv<>^v^>^<<<><<v<<<v^vv^v>^
+vvv<<^>^v^^><<>>><>^<<><^vv^^<>vvv<>><^^v>^>vv<>v<<<<v<^v>^<^^>>>^<v<v
+><>vv>v^v^<>><>>>><^^>vv>v<^^^>>v^v^<^^>v^^>v^<^v>v<>>v^v^<v>v^^<^^vv<
+<<v<^>>^^^^>>>v^<>vvv^><v<<<>^^^vv^<vvv>^>v<^^^^v<>^>vvvv><>>v^<<^^^^^
+^><^><>>><>^^<<^^v>>><^<v>^<vv>>v>>>^v><>^v><<<<v>>v<v<v>vvv>^<><<>^><
+^>><>^v<><^vvv<^^<><v<<<<<><^v<<<><<<^^<v<^^^><^>>^<v^><<<^>>^v<v^v<v^
+>^>>^v>vv>^<<^v<>><<><<v<<v><>v<^vv<<<>^^v^>^^>>><<^v>>v^v><^^>>^<>vv^
+<><^^>^^^<><vvvvv^v<v<<>^v<v>v<<^><<><<><<<^^<<<^<<>><<><^^^>^^<>^>v<>
+^^>vv<^v^v<vv>^<><v<^v>^^^>>>^^vvv^>vvv<>>>^<^>>>>>^<<^v>^vvv<>^<><<v>
+v^^>>><<^^<>>^v^<v^vv<>v^<<>^<^v^v><^<<<><<^<v><v<>vv>>v><v^<vv<>v^<<^
+";
+
+    #[test]
+    fn test1() {
+        assert_eq!(part1(TEST_INPUT), 2028);
+    }
+
+    #[test]
+    fn test2() {
+        assert_eq!(part2(TEST_LARGE), 9021);
+    }
+}

src/lib.rs

@@ -10,5 +10,8 @@ pub mod day09;
 pub mod day10;
 pub mod day11;
 pub mod day12;
+pub mod day13;
+pub mod day14;
+pub mod day15;
 // PLACEHOLDER
 pub mod utils;

src/main.rs

@@ -17,6 +17,9 @@ fn main() {
         (10, day10::day_main),
         (11, day11::day_main),
         (12, day12::day_main),
+        (13, day13::day_main),
+        (14, day14::day_main),
+        (15, day15::day_main),
         // PLACEHOLDER
     ]);
     let day: Option<u8> = args().nth(1).and_then(|a| a.parse().ok());

src/utils/grid.rs

@@ -89,6 +89,15 @@ impl<T> Grid<T> {
         })
     }
 
+    pub fn entries_mut(&mut self) -> impl Iterator<Item = (Coord, &mut T)> {
+        self.content.iter_mut().enumerate().map(|(i, val)| {
+            (
+                (i as i64 % self.content_width, i as i64 / self.content_width),
+                val,
+            )
+        })
+    }
+
     pub fn map_values<U>(self, f: fn(T) -> U) -> Grid<U> {
         let new_content = self.content.into_iter().map(f).collect_vec();
         Grid {
@@ -99,6 +108,21 @@ impl<T> Grid<T> {
     }
 }
 
+impl<T> Grid<T>
+where
+    T: Default,
+{
+    pub fn from_default(width: i64, height: i64) -> Grid<T> {
+        let mut content: Vec<T> = Vec::with_capacity((width * height) as usize);
+        content.resize_with((width * height) as usize, Default::default);
+        Grid {
+            content_width: width,
+            content_height: height,
+            content,
+        }
+    }
+}
+
 impl Grid<char> {
     pub fn parse(input: &str) -> Grid<char> {
         let content_width = input.lines().next().unwrap().len();
@@ -115,6 +139,15 @@ impl Grid<char> {
             content,
         }
     }
+
+    pub fn print(&self) {
+        for line in &self.content.iter().chunks(self.content_width as usize) {
+            for c in line.into_iter() {
+                print!("{c}");
+            }
+            println!();
+        }
+    }
 }
 
 impl<T> Index<Coord> for Grid<T> {