Day 13 part 1

But why oh why does part 2 not work?
Checks pass, termination not a problem but solution is rejected

src/solutions23/Day13.kt

@@ -0,0 +1,78 @@
+package solutions23
+
+import solutions22.transpose
+
+fun main() {
+
+    // returns number rows above the mirror
+    fun<A> findReflectionLine(grid: Array<Array<A>>): Int {
+        for(i in 1 until grid.size) {
+            // take i lines above and check if the next i lines below are mirrored
+            if((0 until i).all{ 2*i-(it+1) >= grid.size || grid[it].contentEquals(grid[2*i-(it+1)]) })
+                return i
+        }
+        return -1
+    }
+
+    fun part1(input: List<List<String>>): Int {
+        val grids = input.map { grid -> grid.map { row -> Array(row.length) {i -> row[i]} }.toTypedArray() }
+        val horizontalRefls = grids.map { findReflectionLine(it) }
+        var result = 0
+        for(i in grids.indices) {
+            if(horizontalRefls[i] != -1) {
+                result += horizontalRefls[i] * 100
+            } else {
+                val verticalRefl = findReflectionLine(transpose(grids[i]))
+                if(verticalRefl == -1) println("No reflection found")
+                else result += verticalRefl
+            }
+        }
+        return result
+    }
+
+    fun part2(input: List<List<String>>): Int {
+        // for each grid and each element in the grid try flipping it and find a reflection line different from the original
+        val grids = input.map { grid -> grid.map { row -> Array(row.length) {i -> row[i]} }.toTypedArray() }
+        // save the original reflections
+        val horizontalRefls = grids.map { findReflectionLine(it) }
+        val verticalRefls = grids.map { findReflectionLine(transpose(it)) }
+
+        var result = 0
+        for(i in grids.indices) {
+            val grid = grids[i]
+            rows@for(flipRow in grid.indices) {
+                for(flipCol in grid[0].indices) {
+                    when(grid[flipRow][flipCol]) {
+                        '#' -> grid[flipRow][flipCol] = '.'
+                        '.' -> grid[flipRow][flipCol] = '#'
+                    }
+                    val horizontalRefl = findReflectionLine(grid)
+                    if(horizontalRefl != -1 && horizontalRefl != horizontalRefls[i]) {
+                        result += horizontalRefl * 100
+                        break@rows
+                    }
+                    val verticalRefl = findReflectionLine(transpose(grid))
+                    if(verticalRefl != -1 && verticalRefl != verticalRefls[i]) {
+                        result += verticalRefl
+                        break@rows
+                    }
+                    // flip back
+                    when(grid[flipRow][flipCol]) {
+                        '#' -> grid[flipRow][flipCol] = '.'
+                        '.' -> grid[flipRow][flipCol] = '#'
+                    }
+                }
+            }
+        }
+        return result
+    }
+
+    val testInput = parseChunks("Day13_test")
+    check(part1(testInput) == 405)
+    check(part2(testInput) == 400)
+
+    val input = parseChunks("Day13")
+    println(part1(input))
+    println(part2(input))
+}
+