Refactor the Grid class to be able to pass a list of special coordinates

Pass the nullElement and overrideElement as constructor parameters.
Otherwise they don't have the value overridden in sub-classes while
constructor execution. Kotlin initializes the super class first (including
its properties and constructors) and then the sub class(es).

Author: rhaendel

src/main/kotlin/Day04.kt

@@ -47,10 +47,9 @@ fun main() {
     printAndCheck(input, ::part2, 1933)
 }
 
-class XMasGrid(input: List<String>) : Grid<Char>(input) {
+class XMasGrid(input: List<String>) : Grid<Char>(input, ' ') {
 
     private val word = "XMAS".toCharArray()
-    override val nullElement = ' '
     override fun Char.toElementType() = this
 
     fun searchForXMAS() = forEachIndex(::searchForXMASAt).sum()

src/main/kotlin/Day06.kt

@@ -81,13 +81,12 @@ suspend fun main() {
     printAndCheck(input, ::part2, 1480)
 }
 
-private class Lab(input: List<String>) : Grid<Char>(input) {
+private class Lab(input: List<String>) : Grid<Char>(input, ' ') {
 
     private val guard = '^'
     private val obstruction = '#'
     private val free = '.'
-    private val offMap = ' '
-    override val nullElement = offMap
+    private val offMap = nullElement
     override fun Char.toElementType() = this
 
     private fun findTheGuard(): Coordinates = forEachElement { row, col, element ->

src/main/kotlin/Day08.kt

@@ -52,9 +52,8 @@ fun main() {
     printAndCheck(input, ::part2, 809)
 }
 
-private class CityMap(input: List<String>) : Grid<Char>(input) {
+private class CityMap(input: List<String>) : Grid<Char>(input, '.') {
 
-    override val nullElement = '.'
     override fun Char.toElementType() = this
 
     private val colIndices = input[0].indices

src/main/kotlin/Day10.kt

@@ -51,10 +51,9 @@ fun main() {
     printAndCheck(input, ::part2, 1960)
 }
 
-private class TopographicMap(input: List<String>) : Grid<Int>(input) {
+private class TopographicMap(input: List<String>) : Grid<Int>(input, Int.MIN_VALUE) {
 
-    override val nullElement = Int.MIN_VALUE
-    override fun Char.toElementType() = if (isDigit()) digitToInt() else Int.MIN_VALUE
+    override fun Char.toElementType() = if (isDigit()) digitToInt() else nullElement
 
     private fun heights(): Sequence<Pair<Coordinates, Int>> = forEachElement { row, col, height ->
         Coordinates(row, col) to height

src/main/kotlin/Day12.kt

@@ -98,9 +98,7 @@ fun main() {
 
 private data class Region(val area: Int, val perimeter: Int, val numberOfSides: Int)
 
-private class Farm(input: List<String>) : Grid<Char>(input) {
-    private val offMap = ' '
-    override val nullElement = offMap
+private class Farm(input: List<String>) : Grid<Char>(input, ' ') {
     override fun Char.toElementType() = this
 
     val assigned = mutableSetOf<Coordinates>()

src/main/kotlin/Day15_part1.kt

@@ -55,12 +55,11 @@ fun List<String>.toDay15Movements(): List<Direction> = takeLastWhile { it.isNotB
     }
 
 
-abstract class Warehouse(input: List<String>) : Grid<Char>(input) {
-    protected val wall = '#'
+abstract class Warehouse(input: List<String>) : Grid<Char>(input, '#') {
+    protected val wall = nullElement
     protected val robot = '@'
     protected val free = '.'
 
-    override val nullElement = wall
     override fun Char.toElementType(): Char = this
 
     protected abstract val leftGoodsChar: Char

src/main/kotlin/Day16.kt

@@ -112,9 +112,8 @@ fun main() {
     printAndCheck(input, ::part2, 504)
 }
 
-private class ReindeerMaze(input: List<String>) : Grid<Char>(input) {
-    private val wall = '#'
-    override val nullElement = wall
+private class ReindeerMaze(input: List<String>) : Grid<Char>(input, '#') {
+    private val wall = nullElement
     override fun Char.toElementType() = this
 
     data class Node(val direction: Direction, val position: Coordinates) {

src/main/kotlin/de/ronny_h/extensions/Grid.kt

@@ -1,22 +1,39 @@
 package de.ronny_h.extensions
 
-abstract class Grid<T>(input: List<String>) {
-    val height = input.size
-    val width = input[0].length
+import kotlin.collections.MutableList
+
+/**
+ * @param nullElement The initial element within the grid
+ * @param overrideElement Used for out of bound positions. Defaults to `nullElement`
+ */
+abstract class Grid<T>(
+    val height: Int,
+    val width: Int,
+    protected val nullElement: T,
+    private val overrideElement: T = nullElement,
+) {
+
+    private val grid: MutableList<MutableList<T>> = MutableList(height) { MutableList(width) { nullElement } }
 
-    abstract val nullElement: T
     abstract fun Char.toElementType(): T
 
-    private val grid = MutableList(height) { MutableList(width) { nullElement } }
+    constructor(height: Int, width: Int, nullElement: T, overrideElement: T = nullElement, overrides: List<Coordinates> = emptyList()) : this(height, width, nullElement, overrideElement) {
+        overrides.forEach {
+            grid[it.row][it.col] = overrideElement
+        }
+    }
 
-    init {
+    constructor(input: List<String>, nullElement: T, overrideElement: T = nullElement) : this(input.size, input[0].length, nullElement, overrideElement, emptyList()){
         input.forEachIndexed { row, line ->
             line.forEachIndexed { col, char -> grid[row][col] = char.toElementType() }
         }
     }
 
     operator fun get(row: Int, col: Int): T {
-        return grid.getOrNull(row)?.getOrNull(col) ?: nullElement
+        return grid
+            .getOrNull(row)
+            ?.getOrNull(col)
+            ?: overrideElement
     }
 
     fun getAt(position: Coordinates) = get(position.row, position.col)

src/test/kotlin/de/ronny_h/extensions/GridTest.kt

@@ -7,8 +7,7 @@ import io.kotlintest.specs.StringSpec
 
 class GridTest : StringSpec() {
 
-    private fun simpleCharGridOf(input: List<String>) = object : Grid<Char>(input) {
-        override val nullElement = ' '
+    private fun simpleCharGridOf(input: List<String>) = object : Grid<Char>(input, ' ') {
         override fun Char.toElementType() = this
     }
 
@@ -36,8 +35,7 @@ class GridTest : StringSpec() {
         }
 
         "toElementType converts the input values" {
-            val grid = object : Grid<Int>(listOf("12", "34")) {
-                override val nullElement = Int.MIN_VALUE
+            val grid = object : Grid<Int>(listOf("12", "34"), Int.MIN_VALUE) {
                 override fun Char.toElementType() = digitToInt()
             }
             grid[0, 0] shouldBe 1
@@ -63,8 +61,7 @@ class GridTest : StringSpec() {
         }
 
         "charAt with index out of the input values returns the nullElement of an Int Grid" {
-            val grid = object : Grid<Int>(listOf("12", "34")) {
-                override val nullElement = Int.MIN_VALUE
+            val grid = object : Grid<Int>(listOf("12", "34"), Int.MIN_VALUE) {
                 override fun Char.toElementType() = digitToInt()
             }
             grid[-1, 0] shouldBe Int.MIN_VALUE