[day5] solution

src/main/scala/Day5.scala

+import aoc2020.lib._
+
+def day5(input: List[String]): String =
+  val ids = for i <- input
+            yield seatId(seatCoords(i))
+
+  s"max id: ${ids.max}\nown id: ${findOwn(ids).map(_.toString).getOrElse("No free seat!")}"
+
+def seatCoords(code: String): (Int, Int) =
+  (
+    recursiveSeat('B', 'F')(code.substr(0,7).toList, 0, 127),
+    recursiveSeat('R', 'L')(code.substr(7).toList, 0, 7)
+  )
+
+
+@annotation.tailrec
+def recursiveSeat(upChar: Char, downChar: Char)(code: List[Char], low: Int, high: Int): Int =
+  code match {
+    case c :: rest =>
+      val (nLow, nHigh) = if c == upChar then up(low, high) else down(low, high)
+      recursiveSeat(upChar, downChar)(rest, nLow, nHigh)
+    case Nil => low
+  }
+
+inline def up(low: Int, high: Int) = (high - (high - low) / 2, high)
+inline def down(low: Int, high: Int) = (low, high - (high - low) / 2 - 1)
+inline def seatId(loc: (Int, Int)) = loc._1 * 8 + loc._2
+
+def findOwn(ids: List[Int]): Option[Int] =
+  ids.sorted.sliding(2).find{l => l(1) - l(0) == 2}.map(_(0)+1)

src/main/scala/Main.scala

@@ -5,5 +5,6 @@
     case 2 => input()(day2)
     case 3 => input(boolChar('#'))(day3)
     case 4 => input()(day4)
+    case 5 => input()(day5)
   }
   println(out)

src/main/scala/lib/extensions.scala

@@ -12,3 +12,8 @@ extension (s: String)
       case Array(a, b) => Some((a.asInstanceOf[String], b.asInstanceOf[String]))
       case _ => None
     }
+
+  def substr(from: Int, to: Int): String =
+    s.substring(from, to).asInstanceOf[String]
+  def substr(from: Int): String =
+    s.substring(from).asInstanceOf[String]