Finish day 9

4 files changed, 61 insertions, 9 deletions

diff --git a/README.md b/README.md
index 1f33ce2..32a01b7 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,7 @@
 # Advent of Code 2021 in Kotlin
 ![Kotlin](https://img.shields.io/badge/Kotlin-grey?logo=Kotlin)
-![](https://img.shields.io/badge/⭐%20stars-16-yellow)
-![](https://img.shields.io/badge/📅%20days-8-blue)
+![](https://img.shields.io/badge/⭐%20stars-18-yellow)
+![](https://img.shields.io/badge/📅%20days-9-blue)
 
 Welcome to the Advent of Code[^aoc] Kotlin project created by [tchojnacki][github] using the [Advent of Code Kotlin Template][template] delivered by JetBrains.
 
@@ -16,7 +16,7 @@ Welcome to the Advent of Code[^aoc] Kotlin project created by [tchojnacki][githu
 | Day 6: Lanternfish             |   🌟   |   🌟   |
 | Day 7: The Treachery of Whales |   🌟   |   🌟   |
 | Day 8: Seven Segment Search    |   🌟   |   🌟   |
-| Day 9: ???                     |        |        |
+| Day 9: Smoke Basin             |   🌟   |   🌟   |
 | Day 10: ???                    |        |        |
 | Day 11: ???                    |        |        |
 | Day 12: ???                    |        |        |
diff --git a/src/Day05.kt b/src/Day05.kt
index bc3f1bc..eaa7b66 100644
--- a/src/Day05.kt
+++ b/src/Day05.kt
@@ -2,14 +2,12 @@ import kotlin.math.absoluteValue
 import kotlin.math.max
 import kotlin.math.sign
 
-data class Pos(val x: Int, val y: Int)
-
-data class Line(val start: Pos, val end: Pos) {
+data class Line(val start: Pos2D, val end: Pos2D) {
     companion object {
         fun fromString(input: String): Line {
             val (start, end) = input.split(" -> ").map { coordinateString ->
                 val (x, y) = coordinateString.split(",").map(String::toInt)
-                Pos(x, y)
+                Pos2D(x, y)
             }
 
             return Line(start, end)
@@ -22,7 +20,7 @@ data class Line(val start: Pos, val end: Pos) {
     val isDiagonal: Boolean
         get() = (end.x - start.x).absoluteValue == (end.y - start.y).absoluteValue
 
-    val pointSequence: Sequence<Pos>
+    val pointSequence: Sequence<Pos2D>
         get() = sequence {
             val xOffset = end.x - start.x
             val yOffset = end.y - start.y
@@ -31,7 +29,7 @@ data class Line(val start: Pos, val end: Pos) {
                 val x = start.x + s * xOffset.sign
                 val y = start.y + s * yOffset.sign
 
-                yield(Pos(x, y))
+                yield(Pos2D(x, y))
             }
         }
 }
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))
+}
diff --git a/src/Utils.kt b/src/Utils.kt
index 7876c3a..492c71d 100644
--- a/src/Utils.kt
+++ b/src/Utils.kt
@@ -32,6 +32,14 @@ fun readInputAsBitLists(name: String): List<List<Int>> =
     readInputAsLines(name)
         .map { binaryString -> binaryString.toList().map { bit -> bit.toString().toInt() } }
 
+data class Pos2D(val x: Int, val y: Int) {
+    companion object {
+        val directions = listOf(Pos2D(0, 1), Pos2D(1, 0), Pos2D(0, -1), Pos2D(-1, 0))
+    }
+
+    operator fun plus(other: Pos2D) = Pos2D(x + other.x, y + other.y)
+}
+
 /**
  * Converts string to md5 hash.
  * @receiver a string