Day 4

Author: WVerlaek

src/main/kotlin/common/datastructures/Grid.kt

@@ -11,6 +11,7 @@ private val neighsYDiags = listOf(0, 1, 1, 1, 0, -1, -1, -1)
 data class Dir(val dRow: Int, val dCol: Int)
 fun Dir.toPoint() = Point(dCol, dRow)
 val directions: List<Dir> = (0..3).map { i -> Dir(neighsY[i], neighsX[i]) }
+val directionsWithDiagonals: List<Dir> = neighsYDiags.zip(neighsXDiags) { y, x -> Dir(y, x) }
 
 class Grid<T>(val rows: List<List<Cell<T>>>) {
     constructor(rows: Int, cols: Int, constructor: (row: Int, col: Int) -> T) : this(Array(rows) { row ->

src/main/kotlin/y24/Day4.kt

@@ -0,0 +1,63 @@
+package y24
+
+import common.puzzle.solvePuzzle
+import common.puzzle.Input
+import common.puzzle.Puzzle
+import common.datastructures.*
+import common.ext.*
+import common.util.*
+import java.util.*
+import kotlin.math.*
+import kotlin.system.exitProcess
+
+
+fun main() = solvePuzzle(year = 2024, day = 4) { Day4(it) }
+
+class Day4(val input: Input) : Puzzle {
+
+    override fun solveLevel1(): Any {
+        val grid = Grid(input.lines.size, input.lines[0].length) { r, c ->
+            input.lines[r][c]
+        }
+
+        return grid.cells()
+            .sumOf { cell ->
+                grid.countAt(cell.row, cell.col)
+            }
+    }
+
+    private fun Grid<Char>.countAt(row: Int, col: Int, text: String = "XMAS"): Int {
+        return directionsWithDiagonals.count { dir ->
+            var r = row
+            var c = col
+            text.all { ch ->
+                if (!withinBounds(r, c)) return@count false
+                val matches = this[r][c].value == ch
+
+                r += dir.dRow
+                c += dir.dCol
+                matches
+            }
+        }
+    }
+
+    private fun Grid<Char>.isXMas(row: Int, col: Int): Boolean {
+        if (this[row][col].value != 'A') return false
+
+        if (row == 0 || row == numRows - 1 || col == 0 || col == numCols - 1) return false
+
+        return setOf(this[row - 1][col - 1].value, this[row + 1][col + 1].value) == setOf('M', 'S') &&
+                setOf(this[row - 1][col + 1].value, this[row + 1][col - 1].value) == setOf('M', 'S')
+    }
+
+    override fun solveLevel2(): Any {
+        val grid = Grid(input.lines.size, input.lines[0].length) { r, c ->
+            input.lines[r][c]
+        }
+
+        return grid.cells()
+            .count { cell ->
+                grid.isXMas(cell.row, cell.col)
+            }
+    }
+}

src/test/kotlin/y24/Day4Test.kt

@@ -0,0 +1,32 @@
+package y24
+
+import common.puzzle.Input
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+
+internal class Day4Test {
+    private val sample = Input("""
+    MMMSXXMASM
+    MSAMXMSMSA
+    AMXSXMAAMM
+    MSAMASMSMX
+    XMASAMXAMM
+    XXAMMXXAMA
+    SMSMSASXSS
+    SAXAMASAAA
+    MAMMMXMMMM
+    MXMXAXMASX
+    """.trimIndent())
+
+    private val day = Day4(sample)
+
+    @Test
+    fun solveLevel1() {
+        assertEquals(18, day.solveLevel1())
+    }
+
+    @Test
+    fun solveLevel2() {
+         assertEquals(9, day.solveLevel2())
+    }
+}