Day 03

src/day03.scala

@@ -0,0 +1,55 @@
+package aoc
+package day03
+
+import scala.annotation.tailrec
+
+val dayNumber = "03"
+
+@main def part1: Unit =
+  println(part1(loadInput(dayNumber)))
+
+@main def part2: Unit =
+  println(part2(loadInput(dayNumber)))
+
+def part1(input: String): String =
+  @tailrec def parseMul(s: String, instructions: List[Int]): List[Int] =
+    s match
+      case s"mul($l,$r)$rest"
+          if l.trim.forall(_.isDigit) && r.trim.forall(_.isDigit) =>
+        parseMul(rest, (l.trim.toInt * r.trim.toInt) :: instructions)
+      case "" => instructions
+      case _  => parseMul(s.drop(1), instructions)
+
+  parseMul(input, Nil).sum.toString
+
+def part2(input: String): String =
+  enum Instruction:
+    case Mul(l: Int, r: Int)
+    case Do
+    case Dont
+  import Instruction.*
+
+  @tailrec def parseInstructions(
+      s: String,
+      instructions: List[Instruction]
+  ): List[Instruction] =
+    s match
+      case s"mul($l,$r)$rest"
+          if l.trim.forall(_.isDigit) && r.trim.forall(_.isDigit) =>
+        parseInstructions(rest, Mul(l.trim.toInt, r.trim.toInt) :: instructions)
+      case s"do()$rest"    => parseInstructions(rest, Do :: instructions)
+      case s"don't()$rest" => parseInstructions(rest, Dont :: instructions)
+      case ""              => instructions
+      case _               => parseInstructions(s.drop(1), instructions)
+
+  parseInstructions(input, Nil)
+    .foldRight((0, false)) { case (inst, (sum, ignoreNext)) =>
+      inst match
+        case Mul(l, r) =>
+          if ignoreNext then (sum, ignoreNext)
+          else (sum + l * r, ignoreNext)
+        case Do   => (sum, false)
+        case Dont => (sum, true)
+    }
+    ._1
+    .toString