Optimizing Ram Run algorithm.

Author: jwcarman

README.md

@@ -37,6 +37,6 @@ Here are the solutions I have implemented along with the time it took to run eac
 | [Day 15: Warehouse Woes](src/test/kotlin/adventofcode/Day15Test.kt) | 129 |
 | [Day 16: Reindeer Maze](src/test/kotlin/adventofcode/Day16Test.kt) | 261 |
 | [Day 17: Chronospatial Computer](src/test/kotlin/adventofcode/Day17Test.kt) | 93 |
-| [Day 18: RAM Run](src/test/kotlin/adventofcode/Day18Test.kt) | 5,520 |
+| [Day 18: RAM Run](src/test/kotlin/adventofcode/Day18Test.kt) | 3,328 |
 | [Day 19: Linen Layout](src/test/kotlin/adventofcode/Day19Test.kt) | 60 |
 | [Day 20: Race Condition](src/test/kotlin/adventofcode/Day20Test.kt) | 0 |

src/main/kotlin/adventofcode/day18/RamRun.kt

@@ -28,15 +28,18 @@ fun List<Point2D>.generateMap() = TextGrid(List(maxOf { it.y } + 1) { ".".repeat
 
 fun String.findFirstByteThatBlocksExit(): Point2D {
     val points = parsePoints()
-    val map = points.generateMap()
-    val start = map.coordinates().first()
-    val end = map.coordinates().last()
-    val vertices = map.coordinates().filter { map[it] != '#' }.toSet()
-    val neighbors = { p: Point2D -> p.neighbors().filter { it in map }.filter { map[it] != '#' } }
-    return points.find { p ->
-        map[p] = '#'
-        !Graphs.shortestPaths(start, vertices, neighbors).pathExists(end)
-    } ?: throw IllegalStateException("No byte blocks exit")
+
+    val start = Point2D.origin()
+    val xRange = 0..points.maxOf { it.x }
+    val yRange = 0..points.maxOf { it.y }
+    val end = Point2D(xRange.last, yRange.last)
+    val fallen = mutableSetOf<Point2D>()
+    val neighbors =
+        { p: Point2D -> p.neighbors().filter { it.x in xRange && it.y in yRange }.filter { it !in fallen} }
+    return points.first { p ->
+        fallen.add(p)
+        Graphs.dfs(start, end, neighbors).isEmpty()
+    }
 }
 
 fun String.findShortestPathToExitAfter(nBytes: Int): Int {