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+/// --- Day 14: Regolith Reservoir ---
+///
+/// The distress signal leads you to a giant waterfall!  Actually, hang on - the signal seems like
+/// it's coming from the waterfall itself, and that doesn't make any sense.  However, you do notice
+/// a little path that leads behind the waterfall.
+///
+/// Correction: the distress signal leads you behind a giant waterfall!  There seems to be a large
+/// cave system here, and the signal definitely leads further inside.
+///
+/// As you begin to make your way deeper underground, you feel the ground rumble for a moment.
+/// Sand begins pouring into the cave!  If you don't quickly figure out where the sand is going,
+/// you could quickly become trapped!
+///
+/// Fortunately, your familiarity with analyzing the path of falling material will come in handy
+/// here.  You scan a two-dimensional vertical slice of the cave above you (your puzzle input) and
+/// discover that it is mostly air with structures made of rock.
+///
+/// Your scan traces the path of each solid rock structure and reports the x,y coordinates that
+/// form the shape of the path, where x represents distance to the right and y represents distance
+/// down.  Each path appears as a single line of text in your scan.  After the first point of each
+/// path, each point indicates the end of a straight horizontal or vertical line to be drawn from
+/// the previous point.  For example:
+///
+/// ```
+/// 498,4 -> 498,6 -> 496,6
+/// 503,4 -> 502,4 -> 502,9 -> 494,9
+/// ```
+///
+/// This scan means that there are two paths of rock; the first path consists of two straight
+/// lines, and the second path consists of three straight lines.  (Specifically, the first path
+/// consists of a line of rock from 498,4 through 498,6 and another line of rock from 498,6 through
+/// 496,6.)
+///
+/// The sand is pouring into the cave from point 500,0.
+///
+/// Drawing rock as #, air as ., and the source of the sand as +, this becomes:
+///
+///
+/// ```
+///   4     5  5
+///   9     0  0
+///   4     0  3
+/// 0 ......+...
+/// 1 ..........
+/// 2 ..........
+/// 3 ..........
+/// 4 ....#...##
+/// 5 ....#...#.
+/// 6 ..###...#.
+/// 7 ........#.
+/// 8 ........#.
+/// 9 #########.
+/// ```
+///
+/// Sand is produced one unit at a time, and the next unit of sand is not produced until the
+/// previous unit of sand comes to rest.  A unit of sand is large enough to fill one tile of air in
+/// your scan.
+///
+/// A unit of sand always falls down one step if possible.  If the tile immediately below is
+/// blocked (by rock or sand), the unit of sand attempts to instead move diagonally one step down
+/// and to the left.  If that tile is blocked, the unit of sand attempts to instead move diagonally
+/// one step down and to the right.  Sand keeps moving as long as it is able to do so, at each step
+/// trying to move down, then down-left, then down-right.  If all three possible destinations are
+/// blocked, the unit of sand comes to rest and no longer moves, at which point the next unit of
+/// sand is created back at the source.
+///
+/// So, drawing sand that has come to rest as o, the first unit of sand simply falls straight down
+/// and then stops:
+///
+/// ```
+/// ......+...
+/// ..........
+/// ..........
+/// ..........
+/// ....#...##
+/// ....#...#.
+/// ..###...#.
+/// ........#.
+/// ......o.#.
+/// #########.
+/// ```
+///
+/// The second unit of sand then falls straight down, lands on the first one, and then comes to
+/// rest to its left:
+///
+/// ```
+/// ......+...
+/// ..........
+/// ..........
+/// ..........
+/// ....#...##
+/// ....#...#.
+/// ..###...#.
+/// ........#.
+/// .....oo.#.
+/// #########.
+/// ```
+///
+/// After a total of five units of sand have come to rest, they form this pattern:
+///
+/// ```
+/// ......+...
+/// ..........
+/// ..........
+/// ..........
+/// ....#...##
+/// ....#...#.
+/// ..###...#.
+/// ......o.#.
+/// ....oooo#.
+/// #########.
+/// ```
+///
+/// After a total of 22 units of sand:
+///
+/// ```
+/// ......+...
+/// ..........
+/// ......o...
+/// .....ooo..
+/// ....#ooo##
+/// ....#ooo#.
+/// ..###ooo#.
+/// ....oooo#.
+/// ...ooooo#.
+/// #########.
+/// ```
+///
+/// Finally, only two more units of sand can possibly come to rest:
+///
+/// ```
+/// ......+...
+/// ..........
+/// ......o...
+/// .....ooo..
+/// ....#ooo##
+/// ...o#ooo#.
+/// ..###ooo#.
+/// ....oooo#.
+/// .o.ooooo#.
+/// #########.
+/// ```
+///
+/// Once all 24 units of sand shown above have come to rest, all further sand flows out the bottom,
+/// falling into the endless void.  Just for fun, the path any new sand takes before falling
+/// forever is shown here with ~:
+///
+/// ```
+/// .......+...
+/// .......~...
+/// ......~o...
+/// .....~ooo..
+/// ....~#ooo##
+/// ...~o#ooo#.
+/// ..~###ooo#.
+/// ..~..oooo#.
+/// .~o.ooooo#.
+/// ~#########.
+/// ~..........
+/// ~..........
+/// ~..........
+/// ```
+///
+/// Using your scan, simulate the falling sand.  How many units of sand come to rest before sand
+/// starts flowing into the abyss below?
+use clap::Parser;
+use itertools::Itertools;
+use nom::bytes::complete::tag;
+use nom::character::complete::i32;
+use nom::combinator::{map, opt};
+use nom::error::{ContextError, ErrorKind as NomErrorKind, ParseError};
+use nom::multi::many1;
+use nom::sequence::{terminated, tuple};
+use nom::IResult;
+
+use std::fs::File;
+use std::io::prelude::*;
+use std::io::BufReader;
+use std::path::PathBuf;
+
+const FILEPATH: &'static str = "examples/input.txt";
+const SAND_SOURCE: Coords = Coords { x: 500, y: 0 };
+
+pub type Input<'a> = &'a str;
+pub type Result<'a, T> = IResult<Input<'a>, T, Error<Input<'a>>>;
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum ErrorKind {
+    Nom(NomErrorKind),
+    Context(&'static str),
+    Custom(String),
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+enum Terrain {
+    Air,
+    Rock,
+    Sand,
+    SandSource,
+}
+
+#[derive(Parser, Debug)]
+#[clap(author, version, about, long_about = None)]
+struct Cli {
+    #[clap(short, long, default_value = FILEPATH)]
+    file: PathBuf,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct Error<I> {
+    pub errors: Vec<(I, ErrorKind)>,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+struct Coords {
+    x: i32,
+    y: i32,
+}
+
+impl Coords {
+    fn path_between(&self, other: &Coords) -> Vec<Coords> {
+        if self == other {
+            panic!()
+        }
+        if self.x == other.x {
+            let yrange = {
+                if self.y > other.y {
+                    other.y..=self.y
+                } else if other.y > self.y {
+                    self.y..=other.y
+                } else {
+                    panic!()
+                }
+            };
+            yrange.map(|y| Coords { x: self.x, y }).collect_vec()
+        } else {
+            let xrange = {
+                if self.x > other.x {
+                    other.x..=self.x
+                } else if other.x > self.x {
+                    self.x..=other.x
+                } else {
+                    panic!()
+                }
+            };
+            xrange.map(|x| Coords { x, y: self.y }).collect_vec()
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+struct Grid {
+    data: Vec<Vec<Terrain>>,
+    bounds: GridBoundaries,
+}
+
+impl Grid {
+    fn new(bounds: GridBoundaries) -> Self {
+        let xlen = (bounds.maxx - bounds.minx) as usize + 1;
+        let ylen = (bounds.maxy - bounds.miny) as usize + 1;
+        let data = vec![vec![Terrain::Air; ylen]; xlen];
+        Self { data, bounds }
+    }
+
+    fn get(&self, coord: &Coords) -> &Terrain {
+        &self.data[(coord.x - self.bounds.minx) as usize][(coord.y - self.bounds.miny) as usize]
+    }
+
+    fn set(&mut self, coord: &Coords, terrain: Terrain) {
+        self.data[(coord.x - self.bounds.minx) as usize][(coord.y - self.bounds.miny) as usize] =
+            terrain;
+    }
+
+    fn populate(&mut self, geography: Vec<RockStructure>) {
+        geography
+            .into_iter()
+            .map(|rstruct| {
+                rstruct
+                    .0
+                    .as_slice()
+                    .windows(2)
+                    .map(|window| window[0].path_between(&window[1]))
+                    .flatten()
+                    .collect_vec()
+            })
+            .flatten()
+            .scan(self, |state, coords| {
+                state.data[(coords.x - state.bounds.minx) as usize]
+                    [(coords.y - state.bounds.miny) as usize] = Terrain::Rock;
+                Some(())
+            })
+            .last()
+            .unwrap()
+    }
+
+    fn run(&mut self) -> usize {
+        let mut count = 0;
+        let mut pos = SAND_SOURCE;
+        let mut history = vec![SAND_SOURCE];
+        'block: loop {
+            for (dx, dy) in [(0, 1), (-1, 1), (1, 1)] {
+                let next_pos = Coords {
+                    x: &pos.x + dx,
+                    y: &pos.y + dy,
+                };
+
+                if (next_pos.x < self.bounds.minx)
+                    || (next_pos.x > self.bounds.maxx)
+                    || (next_pos.y > self.bounds.maxy)
+                {
+                    break 'block;
+                }
+
+                if self.get(&next_pos) == &Terrain::Air {
+                    history.push(next_pos);
+                    pos = next_pos;
+                    continue 'block;
+                }
+            }
+
+            self.set(&pos, Terrain::Sand);
+            pos = {
+                loop {
+                    let prev_pos = history[history.len() - 1];
+                    match self.get(&prev_pos) {
+                        Terrain::Air | Terrain::SandSource => break prev_pos,
+                        _ => {
+                            let _ = history.pop();
+                            continue;
+                        }
+                    };
+                }
+            };
+            count += 1;
+        }
+        count
+    }
+}
+
+#[derive(Clone, Debug)]
+struct GridBoundaries {
+    minx: i32,
+    maxx: i32,
+    miny: i32,
+    maxy: i32,
+}
+
+impl GridBoundaries {
+    fn new() -> Self {
+        GridBoundaries {
+            minx: i32::max_value(),
+            maxx: i32::min_value(),
+            miny: 0,
+            maxy: i32::min_value(),
+        }
+    }
+
+    fn combine(&mut self, other: &GridBoundaries) {
+        self.minx = i32::min(self.minx, other.minx);
+        self.maxx = i32::max(self.maxx, other.maxx);
+        self.miny = i32::min(self.miny, other.miny);
+        self.maxy = i32::max(self.maxy, other.maxy);
+    }
+
+    fn fold(&self, other: &Coords) -> Self {
+        let mut res = GridBoundaries::new();
+        res.minx = i32::min(self.minx, other.x);
+        res.maxx = i32::max(self.maxx, other.x);
+        res.miny = i32::min(self.miny, other.y);
+        res.maxy = i32::max(self.maxy, other.y);
+        res
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+struct RockStructure(Vec<Coords>);
+
+impl<I> ParseError<I> for Error<I> {
+    fn from_error_kind(input: I, kind: NomErrorKind) -> Self {
+        let errors = vec![(input, ErrorKind::Nom(kind))];
+        Self { errors }
+    }
+
+    fn append(input: I, kind: NomErrorKind, mut other: Self) -> Self {
+        other.errors.push((input, ErrorKind::Nom(kind)));
+        other
+    }
+}
+
+impl<I> ContextError<I> for Error<I> {
+    fn add_context(input: I, ctx: &'static str, mut other: Self) -> Self {
+        other.errors.push((input, ErrorKind::Context(ctx)));
+        other
+    }
+}
+
+fn parse_coord(input: &str) -> Result<Coords> {
+    map(tuple((terminated(i32, tag(",")), i32)), |(x, y)| Coords {
+        x,
+        y,
+    })(input)
+}
+
+fn parse_line(input: &str) -> Result<RockStructure> {
+    map(many1(terminated(parse_coord, opt(tag(" -> ")))), |v| {
+        RockStructure(v)
+    })(input)
+}
+
+fn main() {
+    let args = Cli::parse();
+
+    let file = File::open(&args.file).unwrap();
+    let reader = BufReader::new(file);
+
+    let mut bounds = GridBoundaries::new();
+    let geography = reader
+        .lines()
+        .map(|l| parse_line(l.unwrap().as_str()).unwrap().1)
+        .scan(&mut bounds, |state, rstruct| {
+            state.combine(
+                &rstruct
+                    .0
+                    .iter()
+                    .fold(GridBoundaries::new(), |acc, x| acc.fold(x)),
+            );
+            Some(rstruct)
+        })
+        .collect_vec();
+
+    let mut grid = Grid::new(bounds);
+    grid.set(&SAND_SOURCE, Terrain::SandSource);
+    grid.populate(geography);
+    assert_eq!(grid.get(&SAND_SOURCE), &Terrain::SandSource);
+
+    let res = grid.run();
+    println!("{res:?}");
+}