2021 day5/rust: avoid nightly for line point iterator

2021/day5/day5_rs/src/main.rs

@@ -1,6 +1,5 @@
 #![warn(clippy::pedantic)]
 use std::{
-    collections::HashMap,
     convert::Infallible,
     io::{stdin, BufRead},
     str::FromStr,
@@ -18,27 +17,8 @@ impl Line {
         self.start.x == self.end.x || self.start.y == self.end.y
     }
 
-    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()
+    pub fn expand(&self) -> impl Iterator<Item = Vector2D<usize>> {
+        LinePoints::new(self)
     }
 }
 
@@ -57,15 +37,66 @@ 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 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()
+    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
 }
 
 fn main() {