feat: 2015 day 19

2015/day19.scala

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+package `2015`.day19
+
+import prelude.*
+
+type Replacements = Map[String, Set[String]]
+
+extension (s: String)
+  def replaceMolecules(replacementsMap: Replacements) =
+    (for
+      (from, tos) <- replacementsMap
+      to <- tos
+    yield s.transition(from, to)).flatten.toSet
+
+  def transition(from: String, to: String) =
+    s.sliding(from.length)
+      .zipWithIndex
+      .collect { case (`from`, i) => s.patch(i, to, from.length) }
+      .toSet
+
+def part1(molecule: String, reqs: Replacements) =
+  molecule.replaceMolecules(reqs).size
+
+def part2(molecule: String, reqs: Replacements) =
+  val revReqs = reqs.toVector
+    .flatMap { (key, values) => values.map(value => value -> key) }
+    .groupMap(_._1)(_._2)
+    .view
+    .mapValues(_.toSet)
+    .toMap
+
+  val keepBest = 5
+  def step(candidates: Vector[String]) = candidates
+    .flatMap(_.replaceMolecules(revReqs))
+    .sortBy(_.length)
+    .take(keepBest)
+
+  LazyList.iterate(Vector(molecule))(step).takeWhile(!_.contains("e")).size
+
+@main def main() =
+  val input = fromFile(".cache/2015/19.txt").getLines.toVector
+
+  val molecule = input.last
+  val reqs = input
+    .dropRight(2)
+    .map(_.trim)
+    .collect { case s"$from => $to" => from -> to }
+    .groupMap(_._1)(_._2)
+    .view
+    .mapValues(_.toSet)
+    .toMap
+
+  println(part1(molecule, reqs))
+  println(part2(molecule, reqs))

2015/day19.test.scala

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+package `2015`.day19
+
+import prelude.*
+import munit.FunSuite
+
+class Part1 extends FunSuite:
+  test("char"):
+    val replacements = Map(
+      "H" -> Set("HO", "OH"),
+      "O" -> Set("HH"),
+    )
+    assertEquals(
+      "HOH".replaceMolecules(replacements),
+      Set("HOOH", "HOHO", "OHOH", "HOOH", "HHHH"),
+    )
+
+  test("string"):
+    val replacements = Map(
+      "HO" -> Set("!!!"),
+      "H" -> Set("HO", "OH"),
+      "O" -> Set("HH"),
+    )
+    assertEquals(
+      "HOH".replaceMolecules(replacements),
+      Set("!!!H", "HOOH", "HOHO", "OHOH", "HOOH", "HHHH"),
+    )
+
+  test("do not consider surrounding characters"):
+    assertEquals("H2O".replaceMolecules(Map("H" -> Set("OO"))), Set("OO2O"))
+
+class Part2 extends FunSuite:
+  test("example"):
+    val replacements = Map(
+      "e" -> Set("H", "O"),
+      "H" -> Set("HO", "OH"),
+      "O" -> Set("HH"),
+    )
+
+    assertEquals(part2("HOH", replacements), 3)
+    assertEquals(part2("HOHOHO", replacements), 6)