Day 3

aoc/src/Day3.scala

@@ -0,0 +1,84 @@
+import scala.io.Source
+
+object Day3 extends App:
+
+  val input = Source
+    .fromURL(getClass.getResource("day3Input.txt"))
+    .mkString
+    .split('\n')
+    .map(_.toArray.map(Bit.parse))
+
+  enum Bit:
+    case One, Zero
+
+    def flip() = this match
+      case One  => Zero
+      case Zero => One
+
+    override def toString() = this match
+      case One  => "1"
+      case Zero => "0"
+
+  object Bit:
+    def parse(char: Char): Bit = char match
+      case '1' => One
+      case '0' => Zero
+
+  import Bit._
+
+  def findMostCommonBit(bits: Array[Bit]): Bit =
+    if (bits.count(_ == One) > bits.count(_ == Zero)) One
+    else Zero
+
+  val gamma = input.transpose
+    .map(findMostCommonBit)
+
+  val epsilon = gamma.map(_.flip())
+
+  val part1 =
+    Integer.parseInt(gamma.mkString, 2) * Integer.parseInt(epsilon.mkString, 2)
+
+  println(s"Part 1: $part1")
+
+  // Part 2
+
+  enum BitCriteria(val selectBitByCount: (Int, Int) => Bit):
+    case OxygenGeneratorRating
+        extends BitCriteria((onesCount, zerosCount) =>
+          if (onesCount >= zerosCount) One else Zero
+        )
+    case CO2ScrubberRating
+        extends BitCriteria((onesCount, zerosCount) =>
+          if (onesCount < zerosCount) One else Zero
+        )
+
+  import BitCriteria._
+
+  def filterByBitCriteria(
+      filteredBitIndex: Int,
+      numbers: Array[Array[Bit]],
+      bitCriteria: BitCriteria
+  ): Array[Bit] =
+    val filteredBits = numbers.map(_(filteredBitIndex))
+    val onesCount = filteredBits.count(_ == One)
+    val zeroesCount = filteredBits.count(_ == Zero)
+    val headToKeep = bitCriteria.selectBitByCount(onesCount, zeroesCount)
+    val filteredNumbers = numbers.filter(_(filteredBitIndex) == headToKeep)
+
+    if (filteredNumbers.length == 1)
+      filteredNumbers.head
+    else
+      filterByBitCriteria(filteredBitIndex + 1, filteredNumbers, bitCriteria)
+
+  val oxygenGeneratorRating =
+    Integer.parseInt(
+      filterByBitCriteria(0, input, OxygenGeneratorRating).mkString,
+      2
+    )
+  val cO2ScrubberRating =
+    Integer.parseInt(
+      filterByBitCriteria(0, input, CO2ScrubberRating).mkString,
+      2
+    )
+
+  println(s"Part 2: ${oxygenGeneratorRating * cO2ScrubberRating}")