Day 04

aoc/src/dev/ctsk/aoc/Main.scala

@@ -5,10 +5,10 @@ import dev.ctsk.aoc.days._
 val solvers = Map[Int, Solver](
   1 -> Day01,
   2 -> Day02,
-  3 -> Day03
+  3 -> Day03,
+  4 -> Day04
 )
 
-
 @main def main(day: String, input: String): Unit =
   solvers.get(day.toInt) match
     case Some(solver) =>

aoc/src/dev/ctsk/aoc/days/Day04.scala

@@ -0,0 +1,61 @@
+package dev.ctsk.aoc.days
+
+import dev.ctsk.aoc._
+
+object Day04 extends Solver(4):
+
+  def part1(lines: Array[Array[Char]]): Int =
+    def pad(front: Int, line: Array[Char], end: Int): Array[Char] =
+      Array.fill(front)('.') ++ line ++ Array.fill(end)('.')
+
+    def count(line: Array[Char]) =
+      (0 until line.length - 3)
+        .count(idx => {
+          (line.slice(idx, idx + 4) sameElements Array('X', 'M', 'A', 'S'))
+          || (line.slice(idx, idx + 4) sameElements Array('S', 'A', 'M', 'X'))
+        })
+
+    lines.map(count(_)).sum
+      + lines.transpose.map(count(_)).sum
+      + lines.zipWithIndex
+        .map((line, index) => pad(index, line, lines.length - index))
+        .transpose
+        .map(count(_))
+        .sum
+      + lines.zipWithIndex
+        .map((line, index) => pad(lines.length - index, line, index))
+        .transpose
+        .map(count(_))
+        .sum
+
+  def part2(grid: Array[Array[Char]]): Int =
+    def countXMAS(g: Array[Array[Char]]): Int =
+      (for
+        i <- 0 until g.length - 2
+        j <- 0 until g(i).length - 2
+        if g(i)(j) == 'M' && g(i)(j + 2) == 'M'
+          && g(i + 1)(j + 1) == 'A'
+          && g(i + 2)(j) == 'S' && g(i + 2)(j + 2) == 'S'
+      yield 1).sum
+
+    List(
+      grid,
+      grid.reverse,
+      grid.transpose,
+      grid.transpose.reverse
+    ).map(countXMAS).sum
+
+  def run(input: String): (Timings, Solution) =
+    var in = io.Source
+      .fromFile(input)
+      .getLines()
+      .map { line => line.toArray }
+      .toArray
+
+    val (p1_time, p1_solution) = timed { part1(in) }
+    val (p2_time, p2_solution) = timed { part2(in) }
+
+    (
+      Timings(0, p1_time, p2_time),
+      Solution(Int.box(p1_solution), Int.box(p2_solution))
+    )