Day 4

src/Day04.kt

@@ -0,0 +1,84 @@
+import Direction.*
+
+enum class Direction(val x: Int, val y: Int) {
+    Left(-1, 0), Right(1, 0), Up(0, -1), Down(0, 1),
+    RightDown(1, 1), RightUp(1, -1), LeftDown(-1, 1), LeftUp(-1, -1)
+}
+
+fun main() {
+
+    data class Point(val row: Int, val col: Int) {
+        fun move(direction: Direction) = this.copy(row = row + direction.y, col = col + direction.x)
+    }
+
+    class CharTable(
+        private val table: Array<CharArray>,
+    ) {
+        constructor(list: List<String>) : this(list.map { it.toCharArray() }.toTypedArray())
+
+        val size = table.size
+
+        fun get(point: Point): Char? {
+            if (point.row in table.indices) {
+                if (point.col in table[point.row].indices) {
+                    return table[point.row][point.col]
+                }
+            }
+            return null
+        }
+    }
+
+    fun findMas(table: CharTable, curr: Point, direction: Direction, sequence: String = ""): Boolean {
+        val currChar = table.get(curr) ?: return false
+        val word = sequence + currChar
+        if (word.contentEquals("MAS")) return true
+        if ("MAS".startsWith(word)) return findMas(table, curr.move(direction), direction, word)
+        return false
+    }
+
+    fun findStarShapeMas(table: CharTable, curr: Point): Boolean {
+        val currChar = table.get(curr) ?: return false
+        if (currChar == 'A') {
+            // Check if this is the center of a star
+            val firstDiagonal = findMas(table, curr.move(LeftUp), RightDown) || findMas(table, curr.move(RightDown), LeftUp)
+            val secondDiagonal = findMas(table, curr.move(RightUp), LeftDown) || findMas(table, curr.move(LeftDown), RightUp)
+            return firstDiagonal && secondDiagonal
+        }
+        return false
+    }
+
+    fun part1(input: List<String>): Int {
+        val table = CharTable(input)
+        var count = 0
+        for (row in 0..<table.size) {
+            for (col in 0..<table.size) {
+                val currPoint = Point(row, col)
+                if (table.get(currPoint) == 'X') {
+                    for (direction in Direction.entries) {
+                        count += if (findMas(table, currPoint.move(direction), direction)) 1 else 0
+                    }
+                }
+            }
+        }
+        return count
+    }
+
+    fun part2(input: List<String>): Int {
+        val table = CharTable(input)
+        var count = 0
+        for (row in 0..<table.size) {
+            for (col in 0..<table.size) {
+                count += if (findStarShapeMas(table, Point(row, col))) 1 else 0
+            }
+        }
+        return count
+    }
+
+    val testInput1 = readInput("Day04_test")
+    check(part1(testInput1) == 18)
+    check(part2(testInput1) == 9)
+
+    val input = readInput("Day04")
+    part1(input).println()
+    part2(input).println()
+}