Day 20 part 1 complete.

Author: jwcarman

src/main/kotlin/adventofcode/day20/RaceCondition.kt

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2025 James Carman
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package adventofcode.day20
+
+import adventofcode.util.geom.plane.Point2D
+import adventofcode.util.graph.Graphs
+import adventofcode.util.grid.TextGrid
+
+fun String.countBestCheats(savingsThreshold: Int): Int {
+    val map = TextGrid(lines())
+    val start = map.coordinates().find { map[it] == 'S' }!!
+    val end = map.coordinates().find { map[it] == 'E' }!!
+    val vertices = map.coordinates().filter { map[it] != '#' }.toSet()
+    val neighbors: (Point2D) -> List<Point2D> =
+        { p: Point2D -> p.neighbors().filter { it in map }.filter { map[it] != '#' } }
+    val shortestPaths = Graphs.shortestPaths(
+        start,
+        vertices,
+        neighbors
+    )
+    val shortestDistance = shortestPaths.distanceTo(end)
+    val shortestPath = shortestPaths.pathTo(end)
+
+    val cheats = shortestPath.flatMap { p -> p.neighbors().filter { map[it] == '#' && !map.isEdge(it) } }.toSet()
+    return cheats.count { cheat ->
+        map[cheat] = '.'
+        val newShortestPaths = Graphs.shortestPaths(start, vertices + cheat, neighbors)
+        map[cheat] = '#'
+        newShortestPaths.distanceTo(end) <= shortestDistance - savingsThreshold
+    }
+}

src/main/kotlin/adventofcode/util/grid/Grid.kt

@@ -91,4 +91,6 @@ interface Grid<T : Any> {
     fun coordinatesWithValues() = coordinates().map { Pair(it, get(it)) }
 
     fun pathsFrom(coordinate: Point2D) = Direction.entries.map { Path(this, it, coordinate) }
+
+    fun isEdge(coordinate: Point2D) = coordinate.x == 0 || coordinate.y == 0 || coordinate.x == width() - 1 || coordinate.y == height() - 1
 }

src/test/kotlin/adventofcode/Day20Test.kt

@@ -16,19 +16,20 @@
 
 package adventofcode
 
+import adventofcode.day20.countBestCheats
 import io.kotest.matchers.shouldBe
 import org.junit.jupiter.api.Test
 
 class Day20Test {
 
     @Test
     fun example1() {
-        calculatePart1(readExample1()) shouldBe 0
+        calculatePart1(readExample1(), 20) shouldBe 5
     }
 
     @Test
     fun part1() {
-        calculatePart1(readInput()) shouldBe 0
+        calculatePart1(readInput(), 100) shouldBe 0
     }
 
     @Test
@@ -41,7 +42,7 @@ class Day20Test {
         calculatePart2(readInput()) shouldBe 0
     }
 
-    private fun calculatePart1(input: String): Int = 0
+    private fun calculatePart1(input: String, savingsThreshold:Int): Int = input.countBestCheats(savingsThreshold)
 
     private fun calculatePart2(input: String): Int = 0
 }

src/test/resources/day20-example1.txt

@@ -0,0 +1,15 @@
+###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############