From cdf14fdadabe4f0ef8252a965e5b758d2eb9de71 Mon Sep 17 00:00:00 2001
From: Christian <me@ctsk.xyz>
Date: Fri, 20 Dec 2024 16:38:26 +0100
Subject: [PATCH] Day 20 faster?

---
 aoc/src/dev/ctsk/aoc/days/Day20.scala | 30 +++++++++++----------------
 1 file changed, 12 insertions(+), 18 deletions(-)

diff --git a/aoc/src/dev/ctsk/aoc/days/Day20.scala b/aoc/src/dev/ctsk/aoc/days/Day20.scala
index 14890e9..7434764 100644
--- a/aoc/src/dev/ctsk/aoc/days/Day20.scala
+++ b/aoc/src/dev/ctsk/aoc/days/Day20.scala
@@ -5,12 +5,6 @@ import dev.ctsk.aoc.*
 import collection.parallel.CollectionConverters.*
 
 object Day20 extends Solver(20):
-  private def manhattanBall(dist: Int, center: Point): Array[(Point, Int)] =
-    (for
-      x <- -dist to dist
-      y <- -(dist - x.abs) to (dist - x.abs)
-    yield (Point(center.x + x, center.y + y), x.abs + y.abs)).toArray
-
   override def run(input: os.ReadablePath): (Timings, Solution) =
     val grid = Grid(os.read.lines(input).map(_.toArray).toArray)
     val start = grid.findFirst(_ == 'S').get
@@ -24,20 +18,20 @@ object Day20 extends Solver(20):
       .concat(Seq(start))
       .toArray
 
-    val distance = trace.zipWithIndex.toMap
-
     def cheats(radius: Int): Int =
-      trace.zipWithIndex.par
-        .map((from, fromDist) => {
-          (for
-            (to, dist) <- manhattanBall(radius, from)
-            toDist <- distance.get(to)
-            cheat = fromDist - toDist
-            if cheat >= 100 + dist
-          yield ()).length
-        })
-        .sum
+      trace.indices.par
+        .map(a =>
+          (a + 1 until trace.length)
+            .count(b =>
+              val (from, fromDist) = (trace(a), trace.length - a)
+              val (to, toDist) = (trace(b), trace.length - b)
+              val dist = (from - to).manhattan
 
+              dist <= radius && (fromDist - toDist >= 100 + dist)
+            )
+        )
+        .sum
+      
     val p1 = timed { cheats(2) }
     val p2 = timed { cheats(20) }