day 18

src/main/kotlin/me/gergo/Aoc18.kt

@@ -0,0 +1,62 @@
+package me.gergo
+
+import java.io.File
+import java.util.*
+
+fun main() {
+    val points = File("src/main/resources/input18.txt").readLines()
+        .map(::parseCoordinate)
+        .toSet()
+
+    // Part One: of all the 6 neighbors of this coordinate, how many are not in coordinates
+    fun surfaceArea(points: Set<Coordinate3D>): Int {
+        return points.sumOf { c ->
+            c.neighbors().count { !points.contains(it) }
+        }
+    }
+    println("Surface area: ${surfaceArea(points)}")
+
+    // Part Two: how many surfaces are on the inside?
+    // Dumb solution: fill the outside with "water" in 20x20x20 and count the surface area using that
+    // Note: this naive fill algo works clumsily unless the bounds are well around the object... so adding +2 to the max
+    val bounds = points.maxOf { c -> maxOf(c.x, c.y, c.z) } + 2
+    fun fill3D(start: Coordinate3D, points: Set<Coordinate3D>): Set<Coordinate3D> {
+        val result = mutableSetOf<Coordinate3D>()
+        val boundary = Stack<Coordinate3D>()
+        boundary.push(start)
+
+        while (boundary.isNotEmpty()) {
+            val c = boundary.pop()
+            result.add(c)
+            for (n in c.neighbors()) {
+                if (result.contains(n)) continue
+                else if (n.x <= -bounds || n.x >= bounds || n.y <= -bounds || n.y >= bounds || n.z <= -bounds || n.z >= bounds) continue
+                else if (points.contains(n)) continue
+                else boundary.push(n)
+            }
+        }
+
+        return result
+    }
+
+    val water = fill3D(Coordinate3D(0, 0, 0), points)
+    val width = water.maxOf { it.x } - water.minOf { it.x } + 1
+    println("Water volume: ${water.size} in ${width}x${width}x${width}")
+    println("Surface area excluding air bubbles: ${surfaceArea(water) - (width * width * 6)}")
+}
+
+private data class Coordinate3D(val x: Int, val y: Int, val z: Int) {
+    fun neighbors() = sequence {
+        yield(Coordinate3D(x - 1, y, z))
+        yield(Coordinate3D(x + 1, y, z))
+        yield(Coordinate3D(x, y - 1, z))
+        yield(Coordinate3D(x, y + 1, z))
+        yield(Coordinate3D(x, y, z - 1))
+        yield(Coordinate3D(x, y, z + 1))
+    }
+}
+
+private fun parseCoordinate(line: String): Coordinate3D {
+    val (x, y, z) = line.split(",")
+    return Coordinate3D(x.toInt(), y.toInt(), z.toInt())
+}