2021 day5/rust: dedup using Vec matrix instead of HashMap

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,
@@ -58,14 +57,27 @@ impl FromStr for Line {
 }
 
 fn count_overlaps<'a>(it: impl IntoIterator<Item = &'a Line>) -> usize {
-    let counts: HashMap<(usize, usize), usize> =
-        it.into_iter()
+    let points: Vec<(usize, usize)> = it
+        .into_iter()
         .flat_map(Line::expand)
-            .fold(HashMap::new(), |mut map, point| {
-                *map.entry(point.into()).or_default() += 1;
-                map
+        .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))
     });
-    counts.into_iter().filter(|&(_k, v)| v >= 2).count()
+
+    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() {