2021/day5/day5_rs/src/main.rs

@@ -1,5 +1,6 @@
 #![warn(clippy::pedantic)]
 use std::{
+    collections::HashMap,
     convert::Infallible,
     io::{stdin, BufRead},
     str::FromStr,
@@ -17,8 +18,27 @@ impl Line {
         self.start.x == self.end.x || self.start.y == self.end.y
     }
 
-    pub fn expand(&self) -> impl Iterator<Item = Vector2D<usize>> {
-        LinePoints::new(self)
+    pub fn expand(&self) -> Vec<Vector2D<usize>> {
+        let delta = self.end.as_isizes() - self.start.as_isizes();
+
+        #[allow(clippy::if_not_else)]
+        let len = if delta.x != 0 {
+            delta.x.abs()
+        } else if delta.y != 0 {
+            delta.y.abs()
+        } else {
+            unreachable!()
+        };
+
+        let step = delta / len;
+
+        (0..=len)
+            .scan(self.start, |acc, _| {
+                let prev = *acc;
+                *acc = (acc.as_isizes() + step).as_usizes();
+                Some(prev)
+            })
+            .collect()
     }
 }
 
@@ -37,66 +57,15 @@ impl FromStr for Line {
     }
 }
 
-struct LinePoints {
-    pos: Vector2D<isize>,
-    step: Vector2D<isize>,
-    points_left: usize,
-}
-
-impl LinePoints {
-    fn new(line: &Line) -> Self {
-        let start = line.start.as_isizes();
-        let end = line.end.as_isizes();
-        let delta = end - start;
-        let step = Vector2D::new(delta.x.signum(), delta.y.signum());
-
-        let len = usize::max(delta.x.unsigned_abs(), delta.y.unsigned_abs());
-
-        Self {
-            pos: start,
-            step,
-            points_left: len + 1,
-        }
-    }
-}
-
-impl Iterator for LinePoints {
-    type Item = Vector2D<usize>;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        if self.points_left == 0 {
-            return None;
-        }
-
-        let current = self.pos;
-        self.pos += self.step;
-        self.points_left -= 1;
-        Some(current.as_usizes())
-    }
-}
-
 fn count_overlaps<'a>(it: impl IntoIterator<Item = &'a Line>) -> usize {
-    let points: Vec<(usize, usize)> = it
-        .into_iter()
-        .flat_map(Line::expand)
-        .map(Into::into)
-        .collect();
-
-    let (max_x, max_y) = points.iter().fold((0, 0), |(acc_x, acc_y), &(x, y)| {
-        (usize::max(acc_x, x), usize::max(acc_y, y))
-    });
-
-    let mut field = vec![vec![0_usize; max_y + 1]; max_x + 1];
-
-    let mut hits = 0;
-    for (x, y) in points {
-        let point = &mut field[x][y];
-        if *point == 1 {
-            hits += 1;
-        }
-        *point += 1;
-    }
-    hits
+    let counts: HashMap<(usize, usize), usize> =
+        it.into_iter()
+            .flat_map(Line::expand)
+            .fold(HashMap::new(), |mut map, point| {
+                *map.entry(point.into()).or_default() += 1;
+                map
+            });
+    counts.into_iter().filter(|&(_k, v)| v >= 2).count()
 }
 
 fn main() {