1 files changed, 46 insertions, 0 deletions
diff --git a/src/Day09.kt b/src/Day09.kt
new file mode 100644
index 0000000..0d22610
--- /dev/null
+++ b/src/Day09.kt
@@ -0,0 +1,46 @@
+fun parseToMap(input: List<String>): Map<Pos2D, Int> =
+    input.flatMapIndexed { y, line ->
+        line.mapIndexed { x, char ->
+            Pos2D(x, y) to char.toString().toInt()
+        }
+    }.toMap()
+
+fun Map<Pos2D, Int>.getLowPoints(): Map<Pos2D, Int> =
+    filter { (pos, num) -> Pos2D.directions.all { num < getOrDefault(pos + it, 9) } }
+
+fun main() {
+    fun part1(input: List<String>): Int =
+        parseToMap(input).getLowPoints().values.sumOf { it + 1 }
+
+    fun part2(input: List<String>): Int {
+        val map = parseToMap(input)
+
+        fun traverseBasin(pos: Pos2D, acc: MutableSet<Pos2D>) {
+            acc.add(pos)
+            Pos2D.directions
+                .map { pos + it }
+                .filter { !acc.contains(it) && map.getOrDefault(it, 9) < 9 }
+                .forEach { traverseBasin(it, acc) }
+        }
+
+        return map
+            .getLowPoints()
+            .map {
+                val visited = mutableSetOf<Pos2D>()
+                traverseBasin(it.key, visited)
+                visited.size
+            }
+            .sortedDescending()
+            .take(3)
+            .sum()
+    }
+
+
+    val testInput = readInputAsLines("Day09_test")
+    check(part1(testInput) == 15)
+    check(part2(testInput) == 1134)
+
+    val input = readInputAsLines("Day09")
+    println(part1(input))
+    println(part2(input))
+}