1 files changed, 40 insertions, 0 deletions
diff --git a/aoc-2021-kotlin/src/Day09.kt b/aoc-2021-kotlin/src/Day09.kt
new file mode 100644
index 0000000..9d0c41d
--- /dev/null
+++ b/aoc-2021-kotlin/src/Day09.kt
@@ -0,0 +1,40 @@
+object Day09 {
+    private fun Map<Pos2D, Int>.getLowPoints() =
+        filter { (pos, num) -> Pos2D.directions4.all { num < getOrDefault(pos + it, 9) } }
+
+    fun part1(input: List<String>) =
+        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.directions4
+                .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()
+    }
+}
+
+fun main() {
+    val testInput = readInputAsLines("Day09_test")
+    check(Day09.part1(testInput) == 15)
+    check(Day09.part2(testInput) == 1134)
+
+    val input = readInputAsLines("Day09")
+    println(Day09.part1(input))
+    println(Day09.part2(input))
+}