Day 4 Part 2

2024/src/main/kotlin/day4.kt

@@ -3,12 +3,33 @@ fun main() {
 
     val part1 = grid.all().fold(0) { acc, line ->
         val string = line.joinToString(separator = "")
-        println(string)
-
         acc + string.countSubstring("XMAS") + string.countSubstring("SAMX")
     }
 
     println("Part 1: $part1")
+
+    var part2 = 0
+    for (y in 1..<(grid.height - 1)) {
+        val row = grid.rows[y]
+        part2 += row.indices.filter { it > 0 && it < grid.width - 1 && row[it] == 'A' }
+            .count { grid.xmasAt(it, y) }
+    }
+    println("Part 2: $part2")
+
+}
+
+fun Grid.xmasAt(x: Int, y: Int): Boolean {
+    assert(get(x, y) == 'A')
+
+    val tl = get(x - 1, y - 1)
+    val tr = get(x + 1, y - 1)
+    val bl = get(x - 1, y + 1)
+    val br = get(x + 1, y + 1)
+
+    val d1 = (tl == 'M' && br == 'S') || (tl == 'S' && br == 'M')
+    val d2 = (bl == 'M' && tr == 'S') || (bl == 'S' && tr == 'M')
+
+    return d1 && d2
 }
 
 fun String.countSubstring(substring: String): Int {