Day 3 part 1

src/main/scala/Day03.scala

@@ -0,0 +1,25 @@
+import cats.data.NonEmptyList
+import cats.parse.{Parser, Parser0}
+import cats.parse.Numbers.nonNegativeIntString
+import cats.parse.Parser.{char, end, string, until, until0, Error}
+import cats.syntax.all.*
+
+object Day03:
+
+  case class Mul(x: Int, y: Int):
+    def eval: Int = x * y
+
+  object Mul:
+    def parser: Parser[Mul] =
+      val natLt1k = nonNegativeIntString.collect { case s if 1 <= s.length && s.length <= 3 => s.toInt }
+      val natLt1kPair = (
+        char('(') *> natLt1k.repSep(sep = char(',')) <* char(')')
+      ).collect { case NonEmptyList(x, y :: Nil) => (x, y) }
+      string("mul") *> natLt1kPair.map(Mul.apply.tupled)
+
+  val mulsOnlyParser: Parser0[List[Mul]] =
+    val mulRep = Mul.parser.backtrack.rep.map(_.toList)
+    until0(Mul.parser) *> mulRep.repSep0(sep = until(Mul.parser)).map(_.flatten) <* until0(end)
+
+  def evalMulsOnly(inputs: List[String]): Either[Error, Int] =
+    inputs.traverse(s => mulsOnlyParser.parseAll(s).map(_.foldMap(_.eval))).map(_.sum)

src/test/scala/Day03Suite.scala

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+import Day03.*
+import Day03Suite.*
+import munit.ScalaCheckSuite
+
+class Day03Suite extends ScalaCheckSuite:
+
+  test("parsing \"mul(42,1)\""):
+    assertEquals(Mul.parser.parse("mul(42,1)"), Right(("", Mul(42, 1))))
+
+  test("parsing Muls only from small input"):
+    assertEquals(
+      mulsOnlyParser.parseAll(smallInput.head),
+      Right(List(Mul(2, 4), Mul(5, 5), Mul(11, 8), Mul(8, 5)))
+    )
+
+  test("eval Muls only from small input returns 161"):
+    assertEquals(evalMulsOnly(smallInput), Right(161))
+
+  test("eval Muls only from big input returns 184_511_516"):
+    assertEquals(evalMulsOnly(bigInput), Right(184_511_516))
+
+object Day03Suite:
+
+  val bigInput: List[String] = getLinesFromFile("src/test/scala/day03_input.txt")
+
+  val smallInput: List[String] = List("xmul(2,4)%&mul[3,7]!@^do_not_mul(5,5)+mul(32,64]then(mul(11,8)mul(8,5))")