Solve 2024 day 17 part 1

Author: sim642

src/main/resources/eu/sim642/adventofcode2024/day17.txt

@@ -0,0 +1,5 @@
+Register A: 30344604
+Register B: 0
+Register C: 0
+
+Program: 2,4,1,1,7,5,1,5,4,5,0,3,5,5,3,0

src/main/scala/eu/sim642/adventofcode2024/Day17.scala

@@ -0,0 +1,74 @@
+package eu.sim642.adventofcode2024
+
+object Day17 {
+
+  case class Registers(a: Int, b: Int, c: Int)
+
+  type Program = Seq[Int]
+
+  case class Input(registers: Registers, program: Program)
+
+  def runOutput(input: Input): String = {
+    val Input(registers, program) = input
+
+    def helper(ip: Int, registers: Registers): LazyList[Int] = {
+      println(ip)
+      println(registers)
+
+      def combo(operand: Int): Int = operand match {
+        case 0 | 1 | 2 | 3 => operand
+        case 4 => registers.a
+        case 5 => registers.b
+        case 6 => registers.c
+        case 7 => throw new IllegalArgumentException("illegal combo operand")
+      }
+
+      // 0, 5, 3
+      // 2 1 7 4
+
+      if (program.indices.contains(ip)) {
+        lazy val literalOperand = program(ip + 1)
+        lazy val comboOperand = combo(literalOperand)
+
+        program(ip) match {
+          case 0 => // adv
+            helper(ip + 2, registers.copy(a = registers.a / (1 << comboOperand)))
+          case 1 => // bxl
+            helper(ip + 2, registers.copy(b = registers.b ^ literalOperand))
+          case 2 => // bst
+            helper(ip + 2, registers.copy(b = comboOperand & 0b111))
+          case 3 => // jnz
+            helper(if (registers.a != 0) literalOperand else ip + 2, registers)
+          case 4 => // bxc
+            helper(ip + 2, registers.copy(b = registers.b ^ registers.c))
+          case 5 => // out
+            (comboOperand & 0b111) +: helper(ip + 2, registers)
+          case 6 => // bdv
+            helper(ip + 2, registers.copy(b = registers.a / (1 << comboOperand)))
+          case 7 => // cdv
+            helper(ip + 2, registers.copy(c = registers.a / (1 << comboOperand)))
+          case _ => throw new IllegalArgumentException("illegal instruction")
+        }
+      }
+      else
+        LazyList.empty
+    }
+
+    helper(0, registers).mkString(",")
+  }
+
+  def parseInput(input: String): Input = input match {
+    case s"Register A: $a\nRegister B: $b\nRegister C: $c\n\nProgram: $programStr" =>
+      val registers = Registers(a.toInt, b.toInt, c.toInt)
+      val program = programStr.split(",").map(_.toInt).toSeq
+      Input(registers, program)
+  }
+
+  lazy val input: String = scala.io.Source.fromInputStream(getClass.getResourceAsStream("day17.txt")).mkString.trim
+
+  def main(args: Array[String]): Unit = {
+    println(runOutput(parseInput(input)))
+
+    // part 1: 4,5,0,4,7,4,3,0,0 - wrong (bst used literal not combo operand)
+  }
+}

src/test/scala/eu/sim642/adventofcode2024/Day17Test.scala

@@ -0,0 +1,40 @@
+package eu.sim642.adventofcode2024
+
+import Day17._
+import org.scalatest.funsuite.AnyFunSuite
+
+class Day17Test extends AnyFunSuite {
+
+  // TODO: small examples
+
+  val exampleInput =
+    """Register A: 729
+      |Register B: 0
+      |Register C: 0
+      |
+      |Program: 0,1,5,4,3,0""".stripMargin
+
+  val exampleInput1 =
+    """Register A: 10
+      |Register B: 0
+      |Register C: 0
+      |
+      |Program: 5,0,5,1,5,4""".stripMargin
+
+  val exampleInput2 =
+    """Register A: 2024
+      |Register B: 0
+      |Register C: 0
+      |
+      |Program: 0,1,5,4,3,0""".stripMargin
+
+  test("Part 1 examples") {
+    assert(runOutput(parseInput(exampleInput)) == "4,6,3,5,6,3,5,2,1,0")
+    assert(runOutput(parseInput(exampleInput1)) == "0,1,2")
+    assert(runOutput(parseInput(exampleInput2)) == "4,2,5,6,7,7,7,7,3,1,0")
+  }
+
+  test("Part 1 input answer") {
+    assert(runOutput(parseInput(input)) == "4,3,2,6,4,5,3,2,4")
+  }
+}