Check matching range remnants against other ranges

Author: maneatingape

README.md

@@ -103,7 +103,7 @@ pie
 | 2 | [Cube Conundrum](https://adventofcode.com/2023/day/2) | [Source](src/year2023/day02.rs) | 10 |
 | 3 | [Gear Ratios](https://adventofcode.com/2023/day/3) | [Source](src/year2023/day03.rs) | 55 |
 | 4 | [Scratchcards](https://adventofcode.com/2023/day/4) | [Source](src/year2023/day04.rs) | 18 |
-| 5 | [If You Give A Seed A Fertilizer](https://adventofcode.com/2023/day/5) | [Source](src/year2023/day05.rs) | 13 |
+| 5 | [If You Give A Seed A Fertilizer](https://adventofcode.com/2023/day/5) | [Source](src/year2023/day05.rs) | 19 |
 | 6 | [Wait For It](https://adventofcode.com/2023/day/6) | [Source](src/year2023/day06.rs) | 1 |
 | 7 | [Camel Cards](https://adventofcode.com/2023/day/7) | [Source](src/year2023/day07.rs) | 97 |
 | 8 | [Haunted Wasteland](https://adventofcode.com/2023/day/8) | [Source](src/year2023/day08.rs) | 34 |

src/year2023/day05.rs

@@ -47,39 +47,43 @@ pub fn part1(input: &Input) -> u64 {
 pub fn part2(input: &Input) -> u64 {
     let mut current = &mut Vec::new();
     let mut next = &mut Vec::new();
+    let next_stage = &mut Vec::new();
 
     // Convert input pairs to ranges.
     for [start, length] in input.seeds.iter().copied().chunk::<2>() {
         current.push([start, start + length]);
     }
 
     for stage in &input.stages {
-        'outer: for &[s1, e1] in current.iter() {
-            // Split ranges that overlap into 1, 2 or 3 new ranges.
-            // Assumes that seed ranges will only overlap with a single range in each stage.
-            for &[dest, s2, e2] in stage {
+        for &[dest, s2, e2] in stage {
+            while let Some([s1, e1]) = current.pop() {
+                // Split ranges that overlap into 1, 2 or 3 new ranges.
                 // x1 and x2 are the possible overlap.
                 let x1 = s1.max(s2);
                 let x2 = e1.min(e2);
 
-                if x1 < x2 {
+                if x1 >= x2 {
+                    // No overlap.
+                    next.push([s1, e1]);
+                } else {
+                    // Move overlap to new destination. Only compare with next range.
+                    next_stage.push([x1 - s2 + dest, x2 - s2 + dest]);
+
+                    // Check remnants with remaining ranges.
                     if s1 < x1 {
                         next.push([s1, x1]);
                     }
                     if x2 < e1 {
                         next.push([x2, e1]);
                     }
-                    // Move overlap to new destination.
-                    next.push([x1 - s2 + dest, x2 - s2 + dest]);
-                    continue 'outer;
                 }
             }
-            // No intersection with any range so pass to next stage unchanged.
-            next.push([s1, e1]);
+
+            (current, next) = (next, current);
         }
 
-        (current, next) = (next, current);
-        next.clear();
+        // Combine elements for the next stage.
+        current.append(next_stage);
     }
 
     current.iter().map(|r| r[0]).min().unwrap()