a line integral for day 18 is slightly faster than using shoelace

README.md

@@ -52,8 +52,8 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 15](./src/bin/15.rs) | `20.4µs` | `85.9µs` |
 | [Day 16](./src/bin/16.rs) | `58.3µs` | `1.7ms` |
 | [Day 17](./src/bin/17.rs) | `1.6ms` | `3.7ms` |
-| [Day 18](./src/bin/18.rs) | `2.4µs` | `2.5µs` |
-| [Day 19](./src/bin/19.rs) | `167.6µs` | `156.8µs` |
+| [Day 18](./src/bin/18.rs) | `2.2µs` | `2.4µs` |
+| [Day 19](./src/bin/19.rs) | `167.2µs` | `155.1µs` |
 
 **Total: 14.40ms**
 <!--- benchmarking table --->

src/bin/18.rs

@@ -29,8 +29,14 @@ fn solve_part1_fast(input: &str) -> i64 {
             b'U' => Point::new(prev.x, prev.y - distance),
             _ => unreachable!(),
         };
+        area += match input[start] {
+            b'R' => -next.y * distance,
+            b'D' => next.x * distance,
+            b'L' => next.y * distance,
+            b'U' => -next.x * distance,
+            _ => unreachable!(),
+        };
         index += 15;
-        area += prev.x * next.y - prev.y * next.x;
         border_points += distance;
         prev = next;
     }
@@ -59,8 +65,14 @@ fn solve_part2_fast(input: &str) -> i64 {
             b'3' => Point::new(prev.x, prev.y - distance),
             _ => unreachable!(),
         };
+        area += match input[index + 5] {
+            b'0' => -next.y * distance,
+            b'1' => next.x * distance,
+            b'2' => next.y * distance,
+            b'3' => -next.x * distance,
+            _ => unreachable!(),
+        };
         index += 14;
-        area += prev.x * next.y - prev.y * next.x;
         border_points += distance;
         prev = next;
     }