day(14): refactor based on Google Chat suggestions

• get rid of the ‹lines›
• simplify free falling check

Signed-off-by: Matej Focko <mfocko@redhat.com>

src/bin/day14.rs

@@ -21,36 +21,6 @@ struct Line {
     to: Coord,
 }
 
-impl Line {
-    fn direction(&self) -> Coord {
-        self.to - self.from
-    }
-
-    fn extend_to_x(&self, x: i32) -> Option<Coord> {
-        let d = self.direction();
-
-        match d.x() {
-            0 => None,
-            _ => {
-                let k = (self.from.x() - x) / d.x();
-                let y = self.from.y() + k * d.y();
-
-                Some(Coord::new(x, y))
-            }
-        }
-    }
-
-    fn contains(&self, coord: &Coord) -> bool {
-        let mut xs = [self.from.x(), self.to.x()];
-        xs.sort();
-
-        let mut ys = [self.from.y(), self.to.y()];
-        ys.sort();
-
-        (xs[0]..=xs[1]).contains(&coord.x()) && (ys[0]..=ys[1]).contains(&coord.y())
-    }
-}
-
 #[derive(Debug)]
 struct Multiline {
     lines: BTreeSet<Line>,
@@ -83,19 +53,11 @@ impl FromStr for Multiline {
     }
 }
 
-fn free_falling(grain: &Coord, lines: &[Line], set: &BTreeSet<Coord>) -> bool {
-    set.iter().all(|g| g.x() != grain.x() || g.y() < grain.y())
-        && lines
-            .iter()
-            .filter_map(|l| l.extend_to_x(*grain.x()))
-            .all(|g| g.y() < grain.y())
-}
-
-fn can_move(grain: &Coord, lines: &[Line], set: &BTreeSet<Coord>) -> Option<Coord> {
+fn can_move(grain: &Coord, set: &BTreeSet<Coord>) -> Option<Coord> {
     for direction in [Coord::new(0, 1), Coord::new(-1, 1), Coord::new(1, 1)] {
         let new_position = *grain + direction;
 
-        if !set.contains(&new_position) && lines.iter().all(|l| !l.contains(&new_position)) {
+        if !set.contains(&new_position) {
             return Some(new_position);
         }
     }
@@ -105,17 +67,33 @@ fn can_move(grain: &Coord, lines: &[Line], set: &BTreeSet<Coord>) -> Option<Coor
 
 fn simulate_falling<F>(lines: &Input, terminate: F) -> usize
 where
-    F: Fn(&Coord, &Input, &BTreeSet<Coord>, bool) -> bool
+    F: Fn(&Coord, bool) -> bool,
 {
     let mut grains: VecDeque<Coord> = VecDeque::new();
     let mut set_grains: BTreeSet<Coord> = BTreeSet::new();
 
+    // convert lines to points
+    for line in lines {
+        let d = Coord::new(
+            (line.to.x() - line.from.x()).signum(),
+            (line.to.y() - line.from.y()).signum(),
+        );
+
+        let mut p = line.from;
+        while p != line.to {
+            set_grains.insert(p);
+            p = p + d;
+        }
+        set_grains.insert(p);
+    }
+    let lines_count = set_grains.len();
+
     loop {
         for i in (0..grains.len()).rev() {
             let grain = grains[i];
 
             let mut is_set = false;
-            if let Some(new_grain) = can_move(&grain, lines, &set_grains) {
+            if let Some(new_grain) = can_move(&grain, &set_grains) {
                 grains[i] = new_grain;
             } else {
                 let grain = grains.pop_back().unwrap();
@@ -123,8 +101,8 @@ where
                 is_set = true;
             }
 
-            if terminate(&grain, lines, &set_grains, is_set) {
-                return set_grains.len();
+            if terminate(&grain, is_set) {
+                return set_grains.len() - lines_count;
             }
         }
 
@@ -142,15 +120,19 @@ impl Solution<Input, Output> for Day14 {
     }
 
     fn part_1(input: &Input) -> Output {
-        fn terminate(grain: &Coord, lines: &Input, set_grains: &BTreeSet<Coord>, _is_set: bool) -> bool {
-            free_falling(grain, lines, set_grains)
-        }
+        let max_y = *input
+            .iter()
+            .map(|l| max(l.from.y(), l.to.y()))
+            .max()
+            .unwrap();
+
+        let terminate = |grain: &Coord, _is_set: bool| *grain.y() >= max_y;
 
         simulate_falling(input, terminate)
     }
 
     fn part_2(input: &Input) -> Output {
-        fn terminate(grain: &Coord, _lines: &Input, _set_grains: &BTreeSet<Coord>, is_set: bool) -> bool {
+        fn terminate(grain: &Coord, is_set: bool) -> bool {
             is_set && grain == &Coord::new(500, 0)
         }