Solve 2024 day 8 part 1

Author: sim642

src/main/resources/eu/sim642/adventofcode2024/day8.txt

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+...............3................d.................
+.........................s..7......i.....e........
+................C.......................e.........
+...............Z.......m....................e.....
+....................gC.....q......................
+...............Q....s..........................A..
+................................s........A........
+...........n.....3.C..F......w..m...d.............
+..E...............3.....m......d.i................
+............f.3.......C....d........A.............
+.........Z...........................n..A.........
+....Q......p..............g.i.....................
+.r......n...Q....p............S.7...........O.....
+..........r......K....p.....M..........7....G.....
+....................Fs...................G........
+..z.........D..........G.g........................
+rR.............F................M...............G.
+.........I..c.nr...............M................O.
+...I..............................................
+...................f......I.......................
+z.I...............f..K..........0................7
+k...................K......u.........O............
+.........Q...z.................ga......0.......o..
+....E.5..F..................u..b.P......a.1.......
+..........k9..................K.........H......1..
+.E.........h..........................0......a...H
+..........9...h..e........i......M....1...........
+.c.............z.......................j.........T
+c..D......................Pb.................2....
+....................w.y......W......j.........T.2.
+......ph...N..................y.......W.t.2.......
+............9.................................o..1
+.................Vq.......u....Pb.................
+.......6R.........................................
+........5............w...a.W.............H.j......
+......Z.......Y..........V............H..2........
+..........D.................v..y.........t...T..o.
+.......5...................t......................
+........8k...l...............v.........S....T...4.
+......6....U......PR........b.B....y..............
+..........6.V...U........................L........
+.......8.........N....4.Vq.v..t......oJ.....L.....
+N...........R.................w.JY................
+............N.....................................
+.....5Y.....................................j.....
+.98........Y.....l.............B...........S...L..
+.8...............U...............4................
+..................W.........U4....................
+...E........l..........B......................L..u
+.....D............l....J..q.....................S.

src/main/scala/eu/sim642/adventofcode2024/Day8.scala

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+package eu.sim642.adventofcode2024
+
+import eu.sim642.adventofcodelib.Grid
+import eu.sim642.adventofcodelib.pos.Pos
+import eu.sim642.adventofcodelib.GridImplicits._
+
+object Day8 {
+
+  def findAntennas(grid: Grid[Char]): Map[Char, Set[Pos]] = {
+    (for {
+      (row, y) <- grid.view.zipWithIndex
+      (cell, x) <- row.view.zipWithIndex
+      if cell != '.'
+    } yield cell -> Pos(x, y)).toSet.groupMap(_._1)(_._2)
+  }
+
+  def antennaAntinodes(antenna1: Pos, antenna2: Pos): Set[Pos] = {
+    val diff = antenna2 - antenna1
+    Set(antenna1 - diff, antenna2 + diff)
+  }
+
+  def countAntinodes(grid: Grid[Char]): Int = {
+    val antennas = findAntennas(grid)
+    val antinodes = (for {
+      (_, poss) <- antennas
+      antenna1 <- poss
+      antenna2 <- poss
+      if antenna1 != antenna2
+      antinode <- antennaAntinodes(antenna1, antenna2)
+      if grid.containsPos(antinode)
+    } yield antinode).toSet
+    antinodes.size
+  }
+
+  def parseGrid(input: String): Grid[Char] = input.linesIterator.map(_.toVector).toVector
+
+  lazy val input: String = scala.io.Source.fromInputStream(getClass.getResourceAsStream("day8.txt")).mkString.trim
+
+  def main(args: Array[String]): Unit = {
+    println(countAntinodes(parseGrid(input)))
+  }
+}

src/test/scala/eu/sim642/adventofcode2024/Day8Test.scala

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+package eu.sim642.adventofcode2024
+
+import Day8._
+import org.scalatest.funsuite.AnyFunSuite
+
+class Day8Test extends AnyFunSuite {
+
+  val exampleInput =
+    """............
+      |........0...
+      |.....0......
+      |.......0....
+      |....0.......
+      |......A.....
+      |............
+      |............
+      |........A...
+      |.........A..
+      |............
+      |............""".stripMargin
+
+  test("Part 1 examples") {
+    assert(countAntinodes(parseGrid(exampleInput)) == 14)
+  }
+
+  test("Part 1 input answer") {
+    assert(countAntinodes(parseGrid(input)) == 357)
+  }
+}