day(18): separate by rows

Allows to paralelize by rows.

Signed-off-by: Matej Focko <me@mfocko.xyz>

src/bin/day18.rs

@@ -1,7 +1,6 @@
-use std::{
-    collections::BTreeMap,
-    str::FromStr,
-};
+use std::{collections::BTreeMap, str::FromStr};
+
+use rayon::iter::{IntoParallelRefIterator, ParallelIterator};
 
 use aoc_2023::*;
 
@@ -64,32 +63,44 @@ fn opposite(u: (isize, isize)) -> (isize, isize) {
 }
 
 fn solve(plan: &[Step]) -> usize {
-    let mut hole: BTreeMap<Vector2D<isize>, u8> = BTreeMap::new();
+    let mut hole: BTreeMap<isize, BTreeMap<isize, u8>> = BTreeMap::new();
 
     let mut position = Vector2D::new(0, 0);
     for s in plan.iter() {
         let direction = s.direction();
 
         for _ in 0..s.count {
-            *hole.entry(position).or_insert(0) |= dir_to_u8(s._direction);
+            *(*hole.entry(position.y()).or_default())
+                .entry(position.x())
+                .or_default() |= dir_to_u8(s._direction);
             position = position + direction;
-            *hole.entry(position).or_insert(0) |= dir_to_u8(opposite(s._direction));
+            *(*hole.entry(position.y()).or_default())
+                .entry(position.x())
+                .or_default() |= dir_to_u8(opposite(s._direction));
         }
     }
 
-    let mut counter = 0;
-    let mut in_between = 0;
-    let mut x_prev = 0;
-    for (v, directions) in hole.iter() {
-        if in_between != 0 && x_prev < v.x() {
-            // debug!("{} {}", v.x(), x_prev);
-            counter += (v.x() - x_prev) as usize - 1;
-        }
-        in_between ^= directions & 3;
-        x_prev = v.x();
-    }
+    hole.par_iter().map(|(_, row)| row.len()).sum::<usize>()
+        + hole
+            .par_iter()
+            .map(|(_y, row)| {
+                let mut in_between = 0;
+                let mut x_prev = 0;
 
-    counter + hole.len()
+                let mut counter = 0;
+                for (x, directions) in row.iter() {
+                    if in_between != 0 {
+                        let cells_between = (x - x_prev) as usize - 1;
+
+                        counter += cells_between;
+                    }
+                    in_between ^= directions & 3;
+                    x_prev = *x;
+                }
+
+                counter
+            })
+            .sum::<usize>()
 }
 
 struct Day18 {