day 3

input/2021/day3-sample1.txt

@@ -0,0 +1,12 @@
+00100
+11110
+10110
+10111
+10101
+01111
+00111
+11100
+10000
+11001
+00010
+01010

src/main/scala/de.qwertyuiop.aoc/2021/day3.scala

@@ -2,5 +2,53 @@ package de.qwertyuiop.aoc.`2021`
 
 import de.qwertyuiop.aoc.lib.*
 import cats.*, cats.implicits.given
+import java.lang.Integer.parseInt
+
+def day3(using InputSource): Unit =
+  val reportRaw = input()
+
+  // part 1
+  val onesPerDigit = countBits(reportRaw)
+  val gamma = decode(onesPerDigit, _ > reportRaw.size / 2)
+  val epsilon = decode(onesPerDigit, _ < reportRaw.size / 2)
+
+  println(s"""
+    γ: $gamma
+    ε: $epsilon
+    Part 1 solution: ${gamma * epsilon}
+    """)
+
+  // part 2
+  val (o2, co2) = lifeSupportRating(reportRaw)
+  val (o2dec, co2dec) = (parseInt(o2, 2), parseInt(co2, 2))
+
+  println(s"""
+    O2: $o2dec ($o2)
+    CO2: $co2dec ($co2)
+    Part 2 solution: ${o2dec * co2dec}
+    """)
+
+
+def countBits(bitWords: Iterable[String]): Vector[Int] =
+  bitWords.foldLeft(Vector.fill(bitWords.head.size)(0)) { (sum, word) =>
+    sum.zip(word.map(_ - '0').toVector).map(t => t._1 + t._2)
+  }
+
+def decode(bitCounts: Vector[Int], condition: Int => Boolean) =
+  parseInt(bitCounts.map(i => if condition(i) then '1' else '0').mkString, 2)
+
+
+def lifeSupportRating(bitWords: List[String]): (String, String) =
+  def filterOut(remaining: List[String], position: Int, keep: (Int, Int) => Boolean): String =
+    remaining match
+      case Nil => sys.error("Invalid input, life support rating not possible")
+      case last :: Nil => last
+      case rest =>
+        val byBit = rest.groupBy(_(position))
+        def amount[K](m: Map[K, List[_]], key: K): Int = m.get(key).map(_.size).getOrElse(0)
+        filterOut(
+          if keep(amount(byBit, '1'), amount(byBit, '0')) then byBit('1') else byBit('0'),
+          position + 1, keep)
+
+  (filterOut(bitWords, 0, _ >= _), filterOut(bitWords, 0, _ < _))
 
-def day3(using InputSource): Unit = ???