day(04): solve

Signed-off-by: Matej Focko <me@mfocko.xyz>

inputs/day04/sample.txt

@@ -0,0 +1,10 @@
+MMMSXXMASM
+MSAMXMSMSA
+AMXSXMAAMM
+MSAMASMSMX
+XMASAMXAMM
+XXAMMXXAMA
+SMSMSASXSS
+SAXAMASAAA
+MAMMMXMMMM
+MXMXAXMASX

src/Day04.kt

@@ -0,0 +1,66 @@
+class Day04(
+    inputType: String,
+) : Day<Int, Int>() {
+    private val wordSearch: List<String> = readInput(4, inputType)
+
+    companion object {
+        private val DIRECTIONS = product(-1..1, -1..1).filter { (x, y) -> x != 0 || y != 0 }
+    }
+
+    override fun precompute() {
+        // no-op
+    }
+
+    private fun checkForXmas(
+        y0: Int,
+        x0: Int,
+        dy: Int,
+        dx: Int,
+    ): Boolean =
+        "XMAS".withIndex().all { c ->
+            val (y, x) = y0 + dy * c.index to x0 + dx * c.index
+            y in wordSearch.indices && x in wordSearch[y].indices && wordSearch[y][x] == c.value
+        }
+
+    override fun part1(): Int =
+        product(wordSearch.indices, wordSearch[0].indices).sumOf { (y, x) ->
+            DIRECTIONS.count { (dy, dx) ->
+                checkForXmas(y, x, dy, dx)
+            }
+        }
+
+    private fun checkForMas(
+        y0: Int,
+        x0: Int,
+        dy: Int,
+        dx: Int,
+    ): Boolean =
+        "MAS".withIndex().all { c ->
+            val (y, x) = y0 + dy * c.index to x0 + dx * c.index
+            y in wordSearch.indices && x in wordSearch[y].indices && wordSearch[y][x] == c.value
+        }
+
+    override fun part2(): Int =
+        product(wordSearch.indices, wordSearch[0].indices).sumOf { (y, x) ->
+            when (wordSearch[y][x]) {
+                'A' ->
+                    listOf(
+                        (1 to 1) to (1 to -1),
+                        (1 to -1) to (-1 to -1),
+                        (-1 to -1) to (-1 to 1),
+                        (-1 to 1) to (1 to 1),
+                    ).count { (d0, d1) ->
+                        val (dy0, dx0) = d0
+                        val (dy1, dx1) = d1
+                        checkForMas(y - dy0, x - dx0, dy0, dx0) && checkForMas(y - dy1, x - dx1, dy1, dx1)
+                    }
+
+                else -> 0
+            }
+        }
+}
+
+fun main() {
+    Day04("sample").test(18, 9)
+    Day04("input").run()
+}