2021 day5/rust: use iterator to expand line points

2021/day5/day5_rs/src/main.rs

@@ -1,5 +1,7 @@
+#![feature(int_abs_diff)]
 #![warn(clippy::pedantic)]
 use std::{
+    cmp::Ordering,
     convert::Infallible,
     io::{stdin, BufRead},
     str::FromStr,
@@ -17,27 +19,8 @@ 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)
     }
 }
 
@@ -56,6 +39,54 @@ impl FromStr for Line {
     }
 }
 
+struct LinePoints {
+    pos: Vector2D<usize>,
+    x_change: Ordering,
+    y_change: Ordering,
+    points_left: usize,
+}
+
+impl LinePoints {
+    fn new(line: &Line) -> Self {
+        let x_change = line.end.x.cmp(&line.start.x);
+        let y_change = line.end.y.cmp(&line.start.y);
+
+        let delta_x = line.end.x.abs_diff(line.start.x);
+        let delta_y = line.end.y.abs_diff(line.start.y);
+
+        Self {
+            pos: line.start,
+            x_change,
+            y_change,
+            points_left: usize::max(delta_x, delta_y) + 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;
+        match self.x_change {
+            Ordering::Less => self.pos.x -= 1,
+            Ordering::Greater => self.pos.x += 1,
+            Ordering::Equal => {}
+        }
+        match self.y_change {
+            Ordering::Less => self.pos.y -= 1,
+            Ordering::Greater => self.pos.y += 1,
+            Ordering::Equal => {}
+        }
+        self.points_left -= 1;
+        Some(current)
+    }
+}
+
 fn count_overlaps<'a>(it: impl IntoIterator<Item = &'a Line>) -> usize {
     let points: Vec<(usize, usize)> = it
         .into_iter()