Day 14: Parabolic Reflector Dish

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  • cvttsd2si@programming.dev
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    11 months ago

    Scala3

    type Grid = List[List[Char]]
    
    def tiltUp(a: Grid): Grid = 
        @tailrec def go(c: List[Char], acc: List[Char]): List[Char] =
            def shifted(c: List[Char]) = 
                val (h, t) = c.splitAt(c.count(_ == 'O'))
                h.map(_ => 'O') ++ t.map(_ => '.') ++ acc
            val d = c.indexOf('#')
            if d == -1 then shifted(c) else go(c.slice(d + 1, c.size), '#'::shifted(c.slice(0, d)))
            
        a.map(go(_, List()).reverse)
    
    def weight(a: Grid): Long = a.map(d => d.zipWithIndex.filter((c, _) => c == 'O').map(1 + _._2).sum).sum
    def rotateNeg90(a: Grid): Grid = a.reverse.transpose
    def runCycle = Seq.fill(4)(tiltUp andThen rotateNeg90).reduceLeft(_ andThen _)
    
    def stateAt(target: Long, a: Grid): Grid =
        @tailrec def go(cycle: Int, state: Grid, seen: Map[Grid, Int]): Grid =
            seen.get(state) match
                case Some(i) => if (target - cycle) % (cycle - i) == 0 then state else go(cycle + 1, runCycle(state), seen)
                case None => go(cycle + 1, runCycle(state), seen + (state -> cycle))
        
        go(0, a, Map())
    
    def toColMajorGrid(a: List[String]): Grid = rotateNeg90(a.map(_.toList))
    
    def task1(a: List[String]): Long = weight(tiltUp(toColMajorGrid(a)))
    def task2(a: List[String]): Long = weight(stateAt(1_000_000_000, toColMajorGrid(a)))