advent-of-code/source/year_2020/day_11/grid.rs

#[derive(Debug, Eq, PartialEq, Clone)]
pub enum Cell {
  Edge,
  Floor,
  Empty,
  Occupied,
}

#[derive(Debug, Eq, PartialEq)]
pub enum Ruleset {
  PartOne,
  PartTwo,
}

impl From<char> for Cell {
  fn from(input: char) -> Self {
    match input {
      '@' => Self::Edge,
      '.' => Self::Floor,
      'L' => Self::Empty,
      '#' => Self::Occupied,
      _ => unreachable!("{input}"),
    }
  }
}

#[derive(Debug)]
pub struct Grid {
  pub line_length: isize,
  pub occupied_cell_count: usize,
  pub occupied_cell_tolerance: usize,
  pub cells: Vec<Cell>,
  pub ruleset: Ruleset,
}

impl Grid {
  pub fn new(input: &str, ruleset: Ruleset) -> Self {
    let line_length = input.find('\n').unwrap() + 2;

    // Deal with edge cases by just adding an extra cell to the outer edges.
    let extra_floor = "@".repeat(line_length - 2);
    let data = format!("{}\n{}\n{}", extra_floor, input, extra_floor);

    let cells = data
      .trim()
      .replace('\n', "@@")
      .chars()
      .map(Into::into)
      .collect();

    let occupied_cell_tolerance = match ruleset {
      Ruleset::PartOne => 4,
      Ruleset::PartTwo => 5,
    };

    Self {
      line_length: line_length as isize,
      occupied_cell_count: 0,
      occupied_cell_tolerance,
      cells,
      ruleset,
    }
  }

  pub fn simulate_step(&self) -> Vec<Cell> {
    let mut cells = self.cells.clone();

    for (index, cell) in self.cells.iter().enumerate() {
      let index = index as isize;
      let mut occupied_cells = 0;
      let adjacencies = vec![
        // Top Left
        -self.line_length - 1,
        // Above
        -self.line_length,
        // Top Right
        -self.line_length + 1,
        // Left
        -1,
        // Right
        1,
        // Bottom Left
        self.line_length - 1,
        // Below
        self.line_length,
        // Bottom Right
        self.line_length + 1,
      ];

      if self.ruleset == Ruleset::PartOne {
        for mut adjacency in adjacencies {
          adjacency += index;
          if adjacency < 0 {
            continue;
          }

          if self.cells.get(adjacency as usize) == Some(&Cell::Occupied) {
            occupied_cells += 1;
          }
        }
      } else if self.ruleset == Ruleset::PartTwo {
        for adjacency in adjacencies {
          let mut target_index = index;
          'inner: loop {
            target_index += adjacency;
            if target_index < 0 || target_index as usize > self.cells.len() {
              break 'inner;
            }

            match self.cells.get(target_index as usize) {
              Some(&Cell::Empty) | Some(&Cell::Edge) => break 'inner,
              Some(&Cell::Occupied) => {
                occupied_cells += 1;
                break 'inner;
              }
              _ => (),
            };
          }
        }
      }

      if cell == &Cell::Edge || cell == &Cell::Floor {
        continue;
      }

      if cell == &Cell::Empty && occupied_cells == 0 {
        cells[index as usize] = Cell::Occupied;
      }

      if cell == &Cell::Occupied
        && occupied_cells >= self.occupied_cell_tolerance
      {
        cells[index as usize] = Cell::Empty;
      }
    }

    cells
  }

  pub fn count_cells(&self, target: Cell) -> usize {
    self.cells.iter().filter(|cell| cell == &&target).count()
  }

  // Useful for debugging.
  pub fn _draw(&self) -> String {
    let mut result = String::new();
    for (index, cell) in self.cells.iter().enumerate() {
      if index % self.line_length as usize == 0 {
        result += "\n";
      }

      result += match cell {
        Cell::Edge => "@",
        Cell::Floor => ".",
        Cell::Empty => "L",
        Cell::Occupied => "#",
      };
    }

    result
  }
}