Clean up 2024 day 24 part 2

Author: sim642

src/main/scala/eu/sim642/adventofcode2024/Day24.scala

@@ -1,8 +1,9 @@
 package eu.sim642.adventofcode2024
 
-import scala.annotation.tailrec
 import scala.collection.mutable
 
+import eu.sim642.adventofcodelib.IteratorImplicits._
+
 object Day24 {
 
   enum Op {
@@ -16,15 +17,19 @@ object Day24 {
     case Gate(lhs: String, op: Op, rhs: String)
   }
 
-  class CyclicCircuit extends RuntimeException
+  /**
+   * Exception to indicate cyclic circuit evaluation.
+   * When trying for swaps in part 2, cycles may be introduced, which otherwise (slowly) lead to StackOverflowError.
+   */
+  class CircuitCycleException extends RuntimeException
 
   case class Circuit(wireMap: Map[String, Wire]) {
     def zValue: Long = {
       val memo = mutable.Map.empty[String, Boolean]
 
       def evalName(name: String, called: Set[String]): Boolean =
         if (called.contains(name))
-          throw new CyclicCircuit
+          throw new CircuitCycleException
         else
           memo.getOrElseUpdate(name, evalWire(wireMap(name), called + name))
 
@@ -54,7 +59,7 @@ object Day24 {
 
       def evalName(name: String, called: Set[String]): Set[String] =
         if (called.contains(name))
-          throw new CyclicCircuit
+          throw new CircuitCycleException
         else
           memo.getOrElseUpdate(name, evalWire(wireMap(name), called + name) + name)
 
@@ -91,62 +96,57 @@ object Day24 {
       withXValue(xValue).withYValue(yValue).zValue
   }
 
-  def findWrongBits(circuit: Circuit): Seq[(String, String)] = {
-
+  def findWireSwaps(circuit: Circuit): Seq[(String, String)] = {
     def isCorrect(circuit: Circuit, i: Int): Boolean = {
       (for {
+        // must also check previous bit to account for incoming carry
         xBit <- 0 to 3
         yBit <- 0 to 3
         xValue = xBit.toLong << i >> 1
         yValue = yBit.toLong << i >> 1
-        //if (try {circuit.dependencies("z45"); true} catch {case e: CyclicCircuit => false})
-      } yield try {circuit.add(xValue, yValue) == xValue + yValue} catch {case e: CyclicCircuit => false}).forall(identity)
+      } yield {
+        try circuit.add(xValue, yValue) == xValue + yValue
+        catch case _: CircuitCycleException => false
+      }).forall(identity)
     }
 
-    def helper(circuit: Circuit, i: Int, acc: Seq[(String, String)]): Seq[Seq[(String, String)]] = {
+    def helper(circuit: Circuit, i: Int, acc: List[(String, String)]): Iterator[List[(String, String)]] = {
       if (acc.sizeIs > 4)
-        Seq.empty
+        Iterator.empty
       else if (i > 44)
-        Seq(acc)
+        Iterator.single(acc)
       else if (isCorrect(circuit, i))
         helper(circuit, i + 1, acc)
       else {
-        println(i)
         val depsPrev = circuit.dependencies(s"z${i - 1}")
         val deps = circuit.dependencies(s"z$i")
         val depsNext = circuit.dependencies(s"z${i + 1}")
         val depsNext2 = circuit.dependencies(s"z${i + 2}")
         val wrong1 = ((deps -- depsPrev) ++ (depsNext -- deps)).filterNot(_.startsWith("x")).filterNot(_.startsWith("y"))
         val wrong2 = (depsNext2 -- depsPrev).filterNot(_.startsWith("x")).filterNot(_.startsWith("y"))
-        println(wrong1)
-        println(wrong2)
         val swaps =
           for {
             name1 <- wrong1
             name2 <- wrong2
+            // order names in swap to avoid duplicate checking
             minName = if (name1 < name2) name1 else name2
             maxName = if (name1 < name2) name2 else name1
           } yield (minName, maxName)
         for {
-          (name1, name2) <- swaps.toSeq
-          //name2 <- wrong2
+          swap@(name1, name2) <- swaps.iterator
           newCircuit = circuit.swapped(name1, name2)
-          //() = println((name1, name2))
-          if isCorrect(newCircuit, i - 1)
           if isCorrect(newCircuit, i)
-          swap = (name1, name2)
-          rest <- helper(newCircuit, i + 1, acc :+ swap)
-        } yield rest
+          newAcc <- helper(newCircuit, i + 1, swap :: acc)
+        } yield newAcc
       }
     }
 
-    val all = helper(circuit, 0, Seq.empty)
-    all.foreach(println)
-    all.head
+    val swapss = helper(circuit, 0, Nil)
+    swapss.head
   }
 
-  def findWrongBitsString(circuit: Circuit): String =
-    findWrongBits(circuit).flatMap({ case (a, b) => Seq(a, b)}).sorted.mkString(",")
+  def findWireSwapsString(circuit: Circuit): String =
+    findWireSwaps(circuit).flatMap({ case (name1, name2) => Seq(name1, name2) }).sorted.mkString(",")
 
   def parseInput(s: String): (String, Wire.Input) = s match {
     case s"$name: 0" => name -> Wire.Input(false)
@@ -186,16 +186,6 @@ object Day24 {
   def main(args: Array[String]): Unit = {
     val circuit = parseCircuit(input)
     println(circuit.zValue)
-    findWrongBits(circuit)
-    val circuit2 = circuit.swapped("z21", "nhn").swapped("tvb", "khg").swapped("z33", "gst").swapped("z12", "vdc")
-    //printCircuitDot(circuit2)
-    //println(circuit2.zValue)
-    //println("51401618891888")
-
-    val circuit3 = circuit2.withXValue(0)
-    //println(circuit3.zValue)
-
-    //println(Seq("z21", "nhn", "tvb", "khg", "z33", "gst", "z12", "vdc").sorted.mkString(","))
-    // part 2: gst,khg,nhn,tvb,vdc,z12,z21,z33 - correct
+    println(findWireSwapsString(circuit))
   }
 }

src/test/scala/eu/sim642/adventofcode2024/Day24Test.scala

@@ -76,6 +76,6 @@ class Day24Test extends AnyFunSuite {
   }
 
   test("Part 2 input answer") {
-    assert(findWrongBitsString(parseCircuit(input)) == "gst,khg,nhn,tvb,vdc,z12,z21,z33")
+    assert(findWireSwapsString(parseCircuit(input)) == "gst,khg,nhn,tvb,vdc,z12,z21,z33")
   }
 }