✨ day 4

src/main/scala/org/lemon/advent/lib/2d/Area.scala

@@ -56,6 +56,22 @@ case class Area(xRange: Range, yRange: Range):
   def topRow: Iterator[Coord] = row(top)
   def bottomRow: Iterator[Coord] = row(bottom)
 
+  def upDiagonal(startAt: Coord): Iterator[Coord] =
+    assert(contains(startAt), s"$this  does not contain startAt  $startAt")
+    Iterator.iterate(startAt)(_.up.right).takeWhile(contains)
+
+  def downDiagonal(startAt: Coord): Iterator[Coord] =
+    assert(contains(startAt), s"$this  does not contain startAt  $startAt")
+    Iterator.iterate(startAt)(_.down.right).takeWhile(contains)
+
+  def rows: Iterator[Iterator[Coord]] = yRange.iterator.map(row)
+  def cols: Iterator[Iterator[Coord]] = xRange.iterator.map(col)
+  def upDiagonals: Iterator[Iterator[Coord]] =
+    xRange.iterator.map(x => upDiagonal((x, bottom))) ++ yRange.reverseIterator.drop(1).map(y => upDiagonal((left, y)))
+  def downDiagonals: Iterator[Iterator[Coord]] =
+    xRange.iterator.map(x => downDiagonal((x, top))) ++ yRange.iterator.drop(1).map(y => downDiagonal((left, y)))
+  def diagonals: Iterator[Iterator[Coord]] = upDiagonals ++ downDiagonals
+
   def encloses(area: Area): Boolean =
     left <= area.left && right >= area.right && top <= area.top && bottom >= area.bottom
 

src/main/scala/org/lemon/advent/lib/2d/Coord.scala

@@ -19,7 +19,10 @@ object Coord:
 
     def hasCoord(coord: Coord): Boolean =
       seq.indices.contains(coord.row) && seq(coord.row).indices.contains(coord.col)
-
+
+  extension (seq: Seq[String])
+    def apply(coord: Coord): Char = seq(coord.row)(coord.col)
+
   extension [V](grid: Map[Coord, V])
     def toGridAdjacencyList: UnitGraph[Coord] =
       grid.map((coord, _) => (coord, coord.adjacent.filter(grid.contains)))

src/main/scala/org/lemon/advent/year2024/Day04.scala

@@ -0,0 +1,34 @@
+package org.lemon.advent.year2024
+
+import org.lemon.advent.lib.`2d`._
+
+private object Day04:
+
+  def phrase(wordsearch: Seq[String], coords: Seq[Coord]) = coords.map(wordsearch(_)).mkString
+
+  def part1(input: String) =
+    val lines = input.linesIterator.toSeq
+    val area = Area(lines)
+
+    (area.rows ++ area.cols ++ area.diagonals)
+      .map(
+        _.sliding(4)
+          .map(phrase(lines, _))
+          .count(s => s == "XMAS" || s == "XMAS".reverse)
+      )
+      .sum
+
+  def part2(input: String) =
+    val lines = input.linesIterator.toSeq
+    val area = Area(lines)
+    val mas = Set("MAS", "MAS".reverse)
+
+    area.upDiagonals
+      .map(
+        _.sliding(3)
+          .map(coords => (coords.head, phrase(lines, coords)))
+          .filter((_, s) => mas.contains(s))
+          .filter((start, _) => mas.contains(phrase(lines, area.downDiagonal(start.up.up).take(3).toSeq)))
+          .size
+      )
+      .sum

src/test/scala/org/lemon/advent/year2024/Day04Test.scala

@@ -0,0 +1,42 @@
+package org.lemon.advent.year2024
+
+import org.lemon.advent._
+import org.lemon.advent.year2024.Day04._
+
+class Day04Test extends UnitTest:
+
+  test("part 1 example") {
+    val in = """|MMMSXXMASM
+                |MSAMXMSMSA
+                |AMXSXMAAMM
+                |MSAMASMSMX
+                |XMASAMXAMM
+                |XXAMMXXAMA
+                |SMSMSASXSS
+                |SAXAMASAAA
+                |MAMMMXMMMM
+                |MXMXAXMASX""".stripMargin
+    part1(in) shouldBe 18
+  }
+
+  test("part 1") {
+    part1(read(file(2024)(4))) shouldBe 2514
+  }
+
+  test("part 2 example") {
+    val in = """|MMMSXXMASM
+                |MSAMXMSMSA
+                |AMXSXMAAMM
+                |MSAMASMSMX
+                |XMASAMXAMM
+                |XXAMMXXAMA
+                |SMSMSASXSS
+                |SAXAMASAAA
+                |MAMMMXMMMM
+                |MXMXAXMASX""".stripMargin
+    part2(in) shouldBe 9
+  }
+
+  test("part 2") {
+    part2(read(file(2024)(4))) shouldBe 1888
+  }