diff --git a/src/main/kotlin/no/rodland/advent_2023/Day21.kt b/src/main/kotlin/no/rodland/advent_2023/Day21.kt
index 66ee0d9..64b9bbc 100644
--- a/src/main/kotlin/no/rodland/advent_2023/Day21.kt
+++ b/src/main/kotlin/no/rodland/advent_2023/Day21.kt
@@ -1,26 +1,45 @@
 package no.rodland.advent_2023
 
 import no.rodland.advent.Day
+import no.rodland.advent.Pos
+
 
 // template generated: 21/12/2023
 // Fredrik Rødland 2023
 
-class Day21(val input: List<String>) : Day<Long, Long, List<String>> {
+class Day21(val input: List<String>) : Day<Int, Long, Map<Pos, Char>> {
 
     private val parsed = input.parse()
 
-    override fun partOne(): Long {
-        return 2
+    override fun partOne(): Int {
+//        val start = parsed.filterValues { it == 'S' }.map { it.key }.single()
+        var newMap = parsed
+        val repeat = if (parsed.size == 121) 6 else 64
+        repeat(repeat) { newMap = step(newMap) }
+        return newMap.count { it.value == 'O' }
     }
 
+    fun step(cave: Map<Pos, Char>): Map<Pos, Char> {
+        val previousSteps = cave.filterValues { it == 'O' || it == 'S' }.keys
+        val newOs = previousSteps
+            .flatMap { it.neighbourCellsUDLR() }
+            .filterNot { cave[it] == '#' }
+            .map { it to 'O' }
+        val nullOld = previousSteps.map { it to '.' }
+        return cave + nullOld + newOs
+    }
+
+
     override fun partTwo(): Long {
         return 2
     }
 
-    override fun List<String>.parse(): List<String> {
-        return map { line ->
-            line
-        }
+    override fun List<String>.parse(): Map<Pos, Char> {
+        return flatMapIndexed { y, line ->
+            line.mapIndexed() { x, c ->
+                Pos(x, y) to c
+            }
+        }.toMap()
     }
 
     override val day = "21".toInt()
diff --git a/src/test/kotlin/no/rodland/advent_2023/Day21Test.kt b/src/test/kotlin/no/rodland/advent_2023/Day21Test.kt
index 0726edd..aba7154 100644
--- a/src/test/kotlin/no/rodland/advent_2023/Day21Test.kt
+++ b/src/test/kotlin/no/rodland/advent_2023/Day21Test.kt
@@ -15,9 +15,9 @@ internal class Day21Test {
     private val data21 = "2023/input_21.txt".readFile()
     private val test21 = "2023/input_21_test.txt".readFile()
 
-    private val resultTestOne = 2L
+    private val resultTestOne = 16
     private val resultTestTwo = 2L
-    private val resultOne = 2L
+    private val resultOne = 3814
     private val resultTwo = 2L
 
     val test = defaultTestSuiteParseOnInit(
@@ -29,6 +29,7 @@ internal class Day21Test {
         resultTwo,
         { Day21(data21) },
         { Day21(test21) },
+        numTestPart1 = 2
     )
 
     @Nested