Solved day 6

src/day6.rs

@@ -1,11 +1,13 @@
-use std::collections::HashSet;
+use itertools::Itertools;
+
 use crate::day_solver::DaySolver;
 use crate::util::{Coord, Grid};
 #[cfg(test)]
 use crate::util::read_file;
 
 pub struct Day6 {
-    maze: Grid<char>
+    maze: Grid<char>,
+    initial_path: Option<Vec<Coord<usize>>>
 }
 
 const DIRECTIONS: [Coord<isize>; 4] = [
@@ -15,6 +17,12 @@ const DIRECTIONS: [Coord<isize>; 4] = [
     Coord { x: -1, y: 0 }
 ];
 
+enum GuardResult {
+    Loop,
+    Walkout(Vec<Coord<usize>>),
+    Illegal
+}
+
 impl Day6 {
 
     pub fn create(input: String) -> Self {
@@ -22,46 +30,101 @@ impl Day6 {
 
         // Put the input into the day struct
         Day6 {
-            maze: Grid::parse(input, None)
+            maze: Grid::parse(input, None),
+            initial_path: None
         }
     }
+
+    fn encode_dirs(prev_dirs: &u8, to_add: usize) -> u8 {
+        prev_dirs | 1 << to_add
+    }
+
+    fn has_dir(dirs: &u8, dir_to_check: usize) -> bool {
+        (dirs & 1 << dir_to_check) != 0
+    }
+
+    fn simulate_guard(maze: &Grid<char>) -> GuardResult {
+        let mut cur_pos = match maze.find(&'^') {
+            Some(pos) => pos,
+            None => return GuardResult::Illegal
+        };
+        let mut cur_direction_index = 0;
+        let mut cur_direction = DIRECTIONS[cur_direction_index];
+        let mut next_pos = cur_pos.add_signed(cur_direction);
+
+        let mut visited_positions = Vec::new();
+        let mut previous_directions: Grid<u8> = Grid {
+            data: vec![0; maze.width * maze.height()],
+            width: maze.width,
+            default: Some(0)
+        };
+
+        while maze.in_bounds(next_pos.x, next_pos.y) {
+
+            // println!("pos: {:?}", cur_pos);
+            let c = maze.get(next_pos.x, next_pos.y);
+            if c == &'#' {
+                cur_direction_index = (cur_direction_index + 1) % DIRECTIONS.len();
+                cur_direction = DIRECTIONS[cur_direction_index];
+                next_pos = cur_pos.add_signed(cur_direction);
+            } else {
+                let existing_dirs = previous_directions.get(cur_pos.x, cur_pos.y);
+                if Self::has_dir(existing_dirs, cur_direction_index) {
+                    // println!("Loop detected at {:?}", cur_pos);
+                    return GuardResult::Loop
+                }
+                previous_directions.set(cur_pos.x, cur_pos.y, Self::encode_dirs(existing_dirs, cur_direction_index));
+                cur_pos = next_pos;
+                visited_positions.push(cur_pos);
+                next_pos = cur_pos.add_signed(cur_direction);
+            }
+        }
+
+        GuardResult::Walkout(visited_positions)
+    }
 }
 
 impl DaySolver for Day6 {
 
 
     fn solve_part1(&mut self) -> String {
-        let mut cur_pos = self.maze.find(&'^').unwrap();
-        let mut cur_direction_index = 0;
-        let mut cur_direction = DIRECTIONS[cur_direction_index];
-        let mut next_pos = cur_pos.add_signed(cur_direction);
-
-        let mut visited_positions = HashSet::new();
-
-        while self.maze.in_bounds(next_pos.x, next_pos.y) {
-
-            // println!("pos: {:?}", cur_pos);
-            let c = self.maze.get(next_pos.x, next_pos.y);
-            if c == &'#' {
-                cur_direction_index = (cur_direction_index + 1) % DIRECTIONS.len();
-                cur_direction = DIRECTIONS[cur_direction_index];
-                next_pos = cur_pos.add_signed(cur_direction);
-            } else {
-                cur_pos = next_pos;
-                visited_positions.insert(cur_pos);
-                next_pos = cur_pos.add_signed(cur_direction);
-            }
+        if let GuardResult::Walkout(path) = Self::simulate_guard(&self.maze) {
+            self.initial_path = Some(path.to_owned());
+            path.iter().unique().count().to_string()
+        } else {
+            panic!("The guard got stuck in the maze")
         }
-
-        visited_positions.len().to_string()
     }
 
     fn solve_part2(&mut self) -> String {
-        // Idea so far:
-        // When walking the path as in part 1, for each position, check if the "next position" has been visited before, but while walking 1 rotation to the left
-        // If that is the case, then if we block the next position, the guard would rotate to go back into his original path
 
-        return 0.to_string();
+        let initial_path = match &self.initial_path {
+            Some(path) => path.to_owned(),
+            None => match Self::simulate_guard(&self.maze) {
+                GuardResult::Walkout(path) => {
+                    self.initial_path = Some(path.to_owned());
+                    path.to_owned()
+                },
+                GuardResult::Loop => panic!("The initial path does not leave the maze!"),
+                GuardResult::Illegal => panic!("You are BANDIT!")
+            } 
+        };
+
+        let mut loop_count = 0;
+
+        // For each position that the guard visits, we check what will happen if it is blocked by a wall
+        for pos in initial_path.iter().unique() {
+
+            let mut new_maze= self.maze.clone();
+            new_maze.set(pos.x, pos.y, '#');
+
+            if let GuardResult::Loop = Self::simulate_guard(&new_maze) {
+                loop_count += 1;
+            }
+
+        }
+
+        loop_count.to_string()
     }
 }