Day 16 - Part 2

Signed-off-by: Anthony Oteri <anthony.oteri@gmail.com>

day-16/Cargo.toml

@@ -6,6 +6,8 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+rayon.workspace = true
+
 itertools = { workspace = true }
 nom = { workspace = true }
 tracing = { workspace = true }
@@ -14,7 +16,6 @@ miette = { workspace = true }
 thiserror = { workspace = true }
 dhat = { workspace = true }
 glam = { workspace = true }
-
 [dev-dependencies]
 divan = { workspace = true }
 env_logger = { workspace = true }

day-16/src/part1.rs

@@ -55,7 +55,7 @@ fn parse(input: &str) -> IResult<&str, HashMap<IVec2, Tile>> {
     Ok((input, grid))
 }
 
-fn step(grid: &HashMap<IVec2, Tile>, beams: &Vec<Beam>) -> Vec<Beam> {
+fn step(grid: &HashMap<IVec2, Tile>, beams: &[Beam]) -> Vec<Beam> {
     beams
         .iter()
         .flat_map(|beam| {
@@ -146,7 +146,7 @@ fn step(grid: &HashMap<IVec2, Tile>, beams: &Vec<Beam>) -> Vec<Beam> {
         .collect_vec()
 }
 
-fn check_bounds(_grid: &HashMap<IVec2, Tile>, beams: &Vec<Beam>, boundary: &IVec2) -> Vec<Beam> {
+fn check_bounds(_grid: &HashMap<IVec2, Tile>, beams: &[Beam], boundary: &IVec2) -> Vec<Beam> {
     beams
         .iter()
         .filter(|beam| {
@@ -159,7 +159,7 @@ fn check_bounds(_grid: &HashMap<IVec2, Tile>, beams: &Vec<Beam>, boundary: &IVec
         .collect_vec()
 }
 
-fn check_history(beams: &Vec<Beam>, visited: &HashSet<Beam>) -> Vec<Beam> {
+fn check_history(beams: &[Beam], visited: &HashSet<Beam>) -> Vec<Beam> {
     beams
         .iter()
         .filter(|beam| !visited.contains(beam))
@@ -167,6 +167,7 @@ fn check_history(beams: &Vec<Beam>, visited: &HashSet<Beam>) -> Vec<Beam> {
         .collect_vec()
 }
 
+#[cfg(test)]
 fn visualize(grid: &HashMap<IVec2, Tile>, visited: &HashSet<Beam>) {
     let boundary = grid.keys().fold(IVec2::new(0, 0), |max, position| {
         IVec2::new(max.x.max(position.x), max.y.max(position.y))

day-16/src/part2.rs

@@ -1,8 +1,235 @@
 use crate::error::AocError;
+use glam::IVec2;
+use itertools::Itertools;
+use nom::bytes::complete::is_a;
+use nom::character::complete::line_ending;
+use nom::multi::separated_list1;
+use nom::IResult;
+use rayon::prelude::*;
+use std::collections::HashMap;
+use std::collections::HashSet;
+
+#[derive(Debug, Clone)]
+enum Tile {
+    Empty,
+    RightMirror,
+    LeftMirror,
+    HorizontalSplit,
+    VerticalSplit,
+}
+#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+enum Direction {
+    North,
+    South,
+    East,
+    West,
+}
+
+#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+struct Beam {
+    direction: Direction,
+    position: IVec2,
+}
+
+fn parse(input: &str) -> IResult<&str, HashMap<IVec2, Tile>> {
+    let (input, rows) = separated_list1(line_ending, is_a(r".|\/-"))(input)?;
+
+    let grid = rows
+        .into_iter()
+        .enumerate()
+        .flat_map(|(y, row)| {
+            row.chars().enumerate().map(move |(x, tile)| {
+                let position = IVec2::new(x as i32, y as i32);
+                let tile = match tile {
+                    '.' => Tile::Empty,
+                    '|' => Tile::VerticalSplit,
+                    '/' => Tile::RightMirror,
+                    '\\' => Tile::LeftMirror,
+                    '-' => Tile::HorizontalSplit,
+                    _ => unreachable!(),
+                };
+                (position, tile)
+            })
+        })
+        .collect::<HashMap<IVec2, Tile>>();
+
+    Ok((input, grid))
+}
+
+fn step(grid: &HashMap<IVec2, Tile>, beams: &[Beam]) -> Vec<Beam> {
+    beams
+        .iter()
+        .flat_map(|beam| {
+            let position = match beam.direction {
+                Direction::North => beam.position + IVec2::new(0, -1),
+                Direction::South => beam.position + IVec2::new(0, 1),
+                Direction::East => beam.position + IVec2::new(1, 0),
+                Direction::West => beam.position + IVec2::new(-1, 0),
+            };
+
+            match grid.get(&position) {
+                Some(Tile::VerticalSplit) => match beam.direction {
+                    Direction::North | Direction::South => vec![Beam {
+                        direction: beam.direction.clone(),
+                        position,
+                    }],
+                    Direction::East | Direction::West => vec![
+                        Beam {
+                            direction: Direction::North,
+                            position,
+                        },
+                        Beam {
+                            direction: Direction::South,
+                            position,
+                        },
+                    ],
+                },
+
+                Some(Tile::HorizontalSplit) => match beam.direction {
+                    Direction::East | Direction::West => vec![Beam {
+                        direction: beam.direction.clone(),
+                        position,
+                    }],
+                    Direction::North | Direction::South => vec![
+                        Beam {
+                            direction: Direction::East,
+                            position,
+                        },
+                        Beam {
+                            direction: Direction::West,
+                            position,
+                        },
+                    ],
+                },
+                Some(Tile::RightMirror) => match beam.direction {
+                    Direction::North => vec![Beam {
+                        direction: Direction::East,
+                        position,
+                    }],
+                    Direction::South => vec![Beam {
+                        direction: Direction::West,
+                        position,
+                    }],
+                    Direction::East => vec![Beam {
+                        direction: Direction::North,
+                        position,
+                    }],
+                    Direction::West => vec![Beam {
+                        direction: Direction::South,
+                        position,
+                    }],
+                },
+                Some(Tile::LeftMirror) => match beam.direction {
+                    Direction::North => vec![Beam {
+                        direction: Direction::West,
+                        position,
+                    }],
+                    Direction::South => vec![Beam {
+                        direction: Direction::East,
+                        position,
+                    }],
+                    Direction::East => vec![Beam {
+                        direction: Direction::South,
+                        position,
+                    }],
+                    Direction::West => vec![Beam {
+                        direction: Direction::North,
+                        position,
+                    }],
+                },
+                Some(Tile::Empty) | None => vec![Beam {
+                    direction: beam.direction.clone(),
+                    position,
+                }],
+            }
+            .to_vec()
+        })
+        .collect_vec()
+}
+
+fn check_bounds(_grid: &HashMap<IVec2, Tile>, beams: &[Beam], boundary: &IVec2) -> Vec<Beam> {
+    beams
+        .iter()
+        .filter(|beam| {
+            beam.position.x <= boundary.x
+                && beam.position.y <= boundary.y
+                && beam.position.x >= 0
+                && beam.position.y >= 0
+        })
+        .cloned()
+        .collect_vec()
+}
+
+fn check_history(beams: &[Beam], visited: &HashSet<Beam>) -> Vec<Beam> {
+    beams
+        .iter()
+        .filter(|beam| !visited.contains(beam))
+        .cloned()
+        .collect_vec()
+}
+
+fn process_from(starting_position: &Beam, grid: &HashMap<IVec2, Tile>, boundary: &IVec2) -> u64 {
+    let mut visited: HashSet<Beam> = HashSet::new();
+
+    let mut beams: Vec<Beam> = vec![starting_position.clone()];
+
+    loop {
+        beams = step(grid, &beams);
+        beams = check_bounds(grid, &beams, boundary);
+        beams = check_history(&beams, &visited);
+        visited.extend(beams.iter().cloned());
+
+        if beams.is_empty() {
+            break;
+        }
+    }
+
+    let energized = visited
+        .iter()
+        .map(|node| node.position)
+        .collect::<HashSet<IVec2>>();
+
+    energized.len() as u64
+}
 
 #[tracing::instrument]
 pub fn process(input: &str) -> miette::Result<u64, AocError> {
-    Ok(0)
+    let (input, grid) = parse(input).unwrap();
+    debug_assert!(input.is_empty(), "Not all input was parsed");
+
+    let boundary = grid.keys().fold(IVec2::new(0, 0), |max, position| {
+        IVec2::new(max.x.max(position.x), max.y.max(position.y))
+    });
+
+    let mut starting_positions = vec![];
+    for y in 0..=boundary.y {
+        starting_positions.push(Beam {
+            position: IVec2::new(-1, y),
+            direction: Direction::East,
+        });
+        starting_positions.push(Beam {
+            position: IVec2::new(boundary.x + 1, y),
+            direction: Direction::West,
+        });
+    }
+    for x in 0..boundary.x {
+        starting_positions.push(Beam {
+            position: IVec2::new(x, -1),
+            direction: Direction::South,
+        });
+        starting_positions.push(Beam {
+            position: IVec2::new(x, boundary.y + 1),
+            direction: Direction::North,
+        });
+    }
+
+    let energized: u64 = starting_positions
+        .par_iter()
+        .map(|s| process_from(s, &grid, &boundary))
+        .max()
+        .unwrap();
+
+    Ok(energized)
 }
 
 #[cfg(test)]
@@ -12,7 +239,7 @@ mod tests {
     #[test_log::test]
     fn test_process() -> miette::Result<()> {
         let input = include_str!("../test-input.txt");
-        assert_eq!(0, process(input)?);
+        assert_eq!(51, process(input)?);
         Ok(())
     }
 }