day18: problem 2 solution

18/src/main.rs

@@ -37,6 +37,14 @@ enum Direction {
     Down,
 }
 
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
+enum Turn {
+    LeftNinety,
+    RightNinety,
+    OneEighty,
+    None,
+}
+
 impl Direction {
     const fn all() -> &'static [Self; 4] {
         &[
@@ -62,6 +70,25 @@ impl Direction {
             Direction::Down => (0, 1),
         }
     }
+    fn turn_kind(&self, next_dir: Direction) -> Turn {
+        if *self == next_dir {
+            Turn::None
+        } else if self.opposite() == next_dir {
+            Turn::OneEighty
+        } else {
+            match self {
+                Direction::Left if next_dir == Direction::Up => Turn::RightNinety,
+                Direction::Left if next_dir == Direction::Down => Turn::LeftNinety,
+                Direction::Right if next_dir == Direction::Up => Turn::LeftNinety,
+                Direction::Right if next_dir == Direction::Down => Turn::RightNinety,
+                Direction::Up if next_dir == Direction::Left => Turn::LeftNinety,
+                Direction::Up if next_dir == Direction::Right => Turn::RightNinety,
+                Direction::Down if next_dir == Direction::Right => Turn::LeftNinety,
+                Direction::Down if next_dir == Direction::Left => Turn::RightNinety,
+                _ => unreachable!(),
+            }
+        }
+    }
 }
 
 impl From<&str> for Direction {
@@ -100,7 +127,13 @@ impl From<&str> for DigInstruction {
         let (dir, count, color) = (
             parts.next().unwrap(),
             parts.next().unwrap(),
-            parts.next().unwrap(),
+            parts
+                .next()
+                .unwrap()
+                .chars()
+                .skip(2)
+                .take(6)
+                .collect::<String>(),
         );
         Self {
             dir: dir.into(),
@@ -110,24 +143,42 @@ impl From<&str> for DigInstruction {
     }
 }
 
+impl DigInstruction {
+    fn part2_transform(&mut self) {
+        let (distance_s, direction_s) = self.color.split_at(5);
+        self.count = usize::from_str_radix(distance_s, 16).unwrap();
+        self.dir = match direction_s {
+            "0" => Direction::Right,
+            "1" => Direction::Down,
+            "2" => Direction::Left,
+            "3" => Direction::Up,
+            s => panic!("`{}` is not a valid direction code", s)
+        };
+    }
+}
+
 #[derive(Debug)]
 struct DigPlan {
     instructions: Vec<DigInstruction>,
 }
 
+impl DigPlan {
+    fn part2_transform(&mut self) {
+        for i in &mut self.instructions {
+            i.part2_transform();
+        }
+    }
+}
+
 #[derive(Debug, Clone)]
 struct DigTile {
     position: Position,
-    depth: usize,
-    color: String,
 }
 
 impl Default for DigTile {
     fn default() -> Self {
         Self {
             position: (0, 0),
-            depth: 0,
-            color: "000000".into(),
         }
     }
 }
@@ -140,6 +191,12 @@ struct DigHole {
     tiles_map: HashMap<Position, DigTile>,
     x_range: Range<isize>,
     y_range: Range<isize>,
+    area: u64,
+}
+
+// determinant of positions p1 and p2
+fn det(p1: Position, p2: Position) -> i64 {
+    ((p1.0 * p2.1) - (p1.1 * p2.0)) as i64
 }
 
 impl DigHole {
@@ -149,88 +206,63 @@ impl DigHole {
             tiles_map: HashMap::new(),
             x_range: 0..0,
             y_range: 0..0,
+            area: 0,
         }
     }
-    fn pos_offset(&self, pos: Position, offset: (isize, isize)) -> Position {
+    fn pos_offset_n(&self, pos: Position, offset: (isize, isize), n: usize) -> Position {
-        (pos.0 + offset.0, pos.1 + offset.1)
+        (pos.0 + offset.0 * n as isize, pos.1 + offset.1 * n as isize)
     }
     fn run_plan(&mut self, plan: &DigPlan) {
-        // start position is 1m deep
         let mut cur_pos = (0, 0);
-        let (mut max_x, mut max_y) = (0, 0);
 
         self.tiles_loop.push_back(DigTile {
             position: cur_pos,
-            depth: 1,
-            color: "000000".into(),
         });
-        for i in &plan.instructions {
+        let mut move_offset;
-            println!("instruction: {:?}", i);
+        for (idx, i) in plan.instructions.iter().enumerate() {
-            for _ in 0..i.count {
+            let prev_instruction = if idx > 0 {
-                println!("  cur_pos: {:?}", cur_pos);
+                &plan.instructions[idx - 1]
-                cur_pos = (cur_pos.0 + i.dir.offset().0, cur_pos.1 + i.dir.offset().1);
-                (max_x, max_y) = (
-                    std::cmp::max(cur_pos.0, max_x),
-                    std::cmp::max(cur_pos.1, max_y),
-                );
-                self.tiles_loop.push_back(DigTile {
-                    position: cur_pos,
-                    depth: 1,
-                    color: i.color.to_owned(),
-                });
-            }
-        }
-        // 2d vec dimension is (max_x+1, max_y+1)
-        for tile in &self.tiles_loop {
-            self.x_range.start = std::cmp::min(self.x_range.start, tile.position.0);
-            self.x_range.end = std::cmp::max(self.x_range.end, tile.position.0 + 1);
-            self.y_range.start = std::cmp::min(self.y_range.start, tile.position.1);
-            self.y_range.end = std::cmp::max(self.y_range.end, tile.position.1 + 1);
-            self.tiles_map.insert(tile.position, tile.clone());
-        }
-        println!("{}", self);
-        self.fill_at(self.find_first_inside().unwrap());
-    }
-    fn fill_at(&mut self, pos: Position) {
-        println!("filling at {:?}", pos);
-        let mut to_fill = vec![pos];
-
-        while let Some(fill_pos) = to_fill.pop() {
-            if self.tiles_map.contains_key(&fill_pos) {
-                continue;
             } else {
-                self.tiles_map.insert(
+                &plan.instructions[plan.instructions.len() - 1]
-                    fill_pos,
-                    DigTile {
-                        position: fill_pos,
-                        depth: 1,
-                        color: "000000".to_owned(),
-                    },
-                );
-            }
-            for new_pos in Direction::all()
-                .iter()
-                .map(|dir| self.pos_offset(fill_pos, dir.offset()))
-                .filter(|pos| !self.tiles_map.contains_key(&pos))
-            {
-                to_fill.push(new_pos);
-            }
-        }
-    }
-    fn find_first_inside(&self) -> Option<Position> {
-        for y in self.y_range.clone() {
-            let mut inside = false;
-            for x in self.x_range.clone() {
-                if self.tiles_map.contains_key(&(x, y)) && (!self.tiles_map.contains_key(&(x - 1, y)) || !self.tiles_map.contains_key(&(x+1, y)))
-                {
-                    inside = !inside;
-                }
-                if inside && !self.tiles_map.contains_key(&(x, y)) {
-                    return Some((x as isize, y as isize));
             };
+            let Some(next_instruction) = plan
+                .instructions
+                .get(idx + 1)
+                .or(Some(&plan.instructions[0]))
+            else {
+                panic!()
+            };
+            let cur_turn = prev_instruction.dir.turn_kind(i.dir);
+            let next_turn = i.dir.turn_kind(next_instruction.dir);
+            // point needs to live on the 'outside' corner of the character. to
+            // achieve this we need to offset the move by the following
+            move_offset = match (cur_turn, next_turn) {
+                (Turn::RightNinety, Turn::RightNinety) => 1,
+                (Turn::RightNinety, Turn::LeftNinety) => 0,
+                (Turn::LeftNinety, Turn::LeftNinety) => -1,
+                (Turn::LeftNinety, Turn::RightNinety) => 0,
+                t => panic!("turn {:?} not allowed here", t),
+            };
+
+            cur_pos = self.pos_offset_n(
+                cur_pos,
+                i.dir.offset(),
+                (i.count as isize + move_offset) as usize,
+            );
+            self.tiles_loop.push_back(DigTile {
+                position: cur_pos,
+            });
         }
-        }
+
-        None
+        // Shoelace formula
+        // https://en.wikipedia.org/wiki/Shoelace_formula
+        let double_area: i64 = self
+            .tiles_loop
+            .iter()
+            .tuple_windows()
+            .map(|(a, b)| det(a.position, b.position))
+            .sum();
+        self.area = (double_area / 2).abs() as u64;
     }
 }
 
@@ -266,16 +298,18 @@ impl<T: BufRead> From<Lines<T>> for DigPlan {
 
 fn problem1<T: BufRead>(input: Lines<T>) -> u64 {
     let plan = DigPlan::from(input);
-    println!("{:?}", plan);
     let mut dig = DigHole::new();
     dig.run_plan(&plan);
-    println!("{}", dig);
+    dig.area
-    dig.tiles_map.iter().count() as u64
 }
 
 // PROBLEM 2 solution
 fn problem2<T: BufRead>(input: Lines<T>) -> u64 {
-    0
+    let mut plan = DigPlan::from(input);
+    let mut dig = DigHole::new();
+    plan.part2_transform();
+    dig.run_plan(&plan);
+    dig.area
 }
 
 #[cfg(test)]
@@ -298,6 +332,17 @@ U 3 (#a77fa3)
 L 2 (#015232)
 U 2 (#7a21e3)";
 
+    const AREA16_SQUARE: &str = &"R 3 (#000000)
+D 3 (#000000)
+L 3 (#000000)
+U 4 (#000000";
+
+    #[test]
+    fn area16_square() {
+        let c = Cursor::new(AREA16_SQUARE);
+        assert_eq!(problem1(c.lines()), 16);
+    }
+
     #[test]
     fn problem1_example() {
         let c = Cursor::new(EXAMPLE);
@@ -307,6 +352,6 @@ U 2 (#7a21e3)";
     #[test]
     fn problem2_example() {
         let c = Cursor::new(EXAMPLE);
-        assert_eq!(problem2(c.lines()), 0);
+        assert_eq!(problem2(c.lines()), 952408144115);
     }
 }