diff --git a/days/src/main/kotlin/Day20.kt b/days/src/main/kotlin/Day20.kt
index 556b10d..5421d0f 100644
--- a/days/src/main/kotlin/Day20.kt
+++ b/days/src/main/kotlin/Day20.kt
@@ -19,7 +19,7 @@ class Day20(@Groups val input: Input<List<List<String>>>) {
         .map { it[0].let { it.substring(5 until it.indexOf(':')).toInt() } to it.drop(1) }
         .associate { (id, tile) -> id to gridOf(tile) }
 
-    private fun Tile.allEdges() = transformations().flatMap { it.edges() }
+    private fun Tile.allEdges() = listOf(edges(), edges().map { it.reversed() }).flatten()
 
     private fun edgesMatch(a: Tile, b: Tile): Boolean {
         val edges = b.allEdges()
@@ -70,13 +70,13 @@ class Day20(@Groups val input: Input<List<List<String>>>) {
         ?.let { top -> candidate.firstRow() == top.lastRow() }
         ?: true
 
-    private fun isBottomOk(grid: Grid<*>, x: Int, y: Int, candidate: Tile, neighbours: MutableMap<Int, MutableList<Tile>>) =
+    private fun isBottomOk(grid: Grid<*>, x: Int, y: Int, candidate: Tile, neighbours: Map<Int, List<Tile>>) =
         if (y == grid.lastRowIndex) true
         else neighbours[neighboursCount(grid, x, y + 1)]!!
             .filterNot { it == candidate }
             .count { it.transformations().any { candidate.lastColumn() == it.firstColumn() } } == 1
 
-    private fun isRightOk(grid: Grid<*>, x: Int, y: Int, candidate: Tile, neighbours: MutableMap<Int, MutableList<Tile>>) =
+    private fun isRightOk(grid: Grid<*>, x: Int, y: Int, candidate: Tile, neighbours: Map<Int, List<Tile>>) =
         if (x == grid.lastColumnIndex) true
         else neighbours[neighboursCount(grid, x + 1, y)]!!
             .filterNot { it == candidate }
@@ -148,9 +148,8 @@ class Day20(@Groups val input: Input<List<List<String>>>) {
         for (y in 0 until grid.height) {
             val row = grid.row(y)
             for (yy in 0 until row[0]!!.height) {
-                val combinedRow = ArrayList<Char>()
+                val combinedRow = ArrayList<Char>().also { newData.add(it) }
                 row.forEach { combinedRow.addAll(it!!.row(yy)) }
-                newData.add(combinedRow)
             }
         }
         return Tile(newData)
@@ -159,8 +158,7 @@ class Day20(@Groups val input: Input<List<List<String>>>) {
     private fun Tile.subGridData(startX: Int, startY: Int, width: Int, height: Int): List<List<Char>> {
         val newData = ArrayList<ArrayList<Char>>()
         for (y in startY until startY + height) {
-            val row = ArrayList<Char>()
-            newData.add(row)
+            val row = ArrayList<Char>().also { newData.add(it) }
             for (x in startX until startX + width) {
                 row.add(this[x, y]!!)
             }
@@ -174,7 +172,7 @@ class Day20(@Groups val input: Input<List<List<String>>>) {
         return monsterRe.matches(str)
     }
 
-    fun part2() {
+    fun part2(): Int {
         val grid = assembleGrid()
         removeGaps(grid)
         val megaTile = gridToTile(grid)
@@ -183,17 +181,14 @@ class Day20(@Groups val input: Input<List<List<String>>>) {
             |                  # |
             |#    ##    ##    ###|
             | #  #  #  #  #  #   |
-        """.trimMargin("|").lines().map { it.toCharArray().toList().dropLast(1) }
-
-        println(monster)
+        """.trimMargin("|").lines().map { it.toCharArray().dropLast(1) }
 
         val monsterWidth = monster[0].size
         val monsterHeight = 3
-        val monsterSquares = monster.joinToString("") { it.joinToString("") }.count { it == '#' }
+        val monsterSquares = monster.flatten().count { it == '#' }
+        val squares = megaTile.data.flatten().count { it == '#' }
 
-        val squares = megaTile.data.joinToString("") { it.joinToString("") }.count { it == '#' }
-
-        for (g: Tile in megaTile.transformations()) {
+        for (g in megaTile.transformations()) {
             var count = 0
             for (y in 0 until g.lastRowIndex - monsterHeight) {
                 for (x in 0 until g.lastColumnIndex - monsterWidth) {
@@ -203,11 +198,9 @@ class Day20(@Groups val input: Input<List<List<String>>>) {
                     }
                 }
             }
-            if (count != 0) {
-                println(count)
-                println(squares - (count * monsterSquares))
-            }
+            if (count != 0) return squares - (count * monsterSquares)
         }
+        error("No monsters found")
     }
 
 }