Day 21

src/main/kotlin/y23/Day21.kt

@@ -0,0 +1,84 @@
+package y23
+
+import common.puzzle.solvePuzzle
+import common.puzzle.Input
+import common.puzzle.Puzzle
+import common.datastructures.*
+import common.ext.*
+import common.util.*
+import java.util.*
+import kotlin.math.*
+import kotlin.system.exitProcess
+
+
+fun main() = solvePuzzle(year = 2023, day = 21, dryRun = true) { Day21(it) }
+
+class Day21(val input: Input) : Puzzle {
+
+    private lateinit var start: Point
+    private val grid = Grid(input.lines.size, input.lines[0].length) { r, c ->
+        when (input.lines[r][c]) {
+            '.' -> false
+            '#' -> true
+            'S' -> {
+                start = Point(c, r)
+                false
+            }
+            else -> error("unknown char")
+        }
+    }
+
+    fun reachablePlots(steps: Int, grid: Grid<Boolean> = this.grid, start: Point = this.start): Int {
+        var plots = listOf(start)
+
+        repeat(steps) {
+            plots = plots.flatMap { p ->
+                directions.mapNotNull { (dx, dy) ->
+                    val neigh = Point(p.col + dx, p.row + dy)
+                    if (!grid.withinBounds(neigh.row, neigh.col)) {
+                        return@mapNotNull null
+                    }
+                    if (grid[neigh.row][neigh.col].value) {
+                        return@mapNotNull null
+                    }
+                    return@mapNotNull neigh
+                }
+            }.distinct()
+        }
+
+        return plots.size
+    }
+
+    override fun solveLevel1(): Any {
+        return reachablePlots(64)
+    }
+
+    override fun solveLevel2(): Any {
+        // 65 <- S -> 65 (131 total)
+        // (26501365 - 65) / 131 = 202300
+
+        // start = [65,65]
+
+        // 4 diamond corners
+        return reachablePlots(130, grid, Point(0, 65)) +
+                reachablePlots(130, grid, Point(130, 65)) +
+                reachablePlots(130, grid, Point(65, 0)) +
+                reachablePlots(130, grid, Point(65, 130)) +
+
+                // Edges (large area)
+                202299L * reachablePlots(131+64, grid, Point(130, 130)) +
+                202299L * reachablePlots(131+64, grid, Point(0, 130)) +
+                202299L * reachablePlots(131+64, grid, Point(0, 0)) +
+                202299L * reachablePlots(131+64, grid, Point(130, 0)) +
+
+                // Edges (small area)
+                202300L * reachablePlots(64, grid, Point(130, 130)) +
+                202300L * reachablePlots(64, grid, Point(0, 130)) +
+                202300L * reachablePlots(64, grid, Point(0, 0)) +
+                202300L * reachablePlots(64, grid, Point(130, 0)) +
+
+                // Full squares
+                (202299L * 202299L) * reachablePlots(141, grid, start) + // odd
+                (202300L * 202300L) * reachablePlots(140, grid, start) // even
+    }
+}

src/test/kotlin/y23/Day21Test.kt

@@ -0,0 +1,33 @@
+package y23
+
+import common.puzzle.Input
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+
+internal class Day21Test {
+    private val sample = Input("""
+    ...........
+    .....###.#.
+    .###.##..#.
+    ..#.#...#..
+    ....#.#....
+    .##..S####.
+    .##..#...#.
+    .......##..
+    .##.#.####.
+    .##..##.##.
+    ...........
+    """.trimIndent())
+
+    private val day = Day21(sample)
+
+    @Test
+    fun solveLevel1() {
+        assertEquals(16, day.reachablePlots(6))
+    }
+
+    @Test
+    fun solveLevel2() {
+        // Level 2 solution doesn't work on sample input.
+    }
+}