Day8.kt

object Day8 : AdventDay() {
  override fun solve() {
    val positions = reads<String>()?.map { it.toDigitsEntry() } ?: return

    positions.sumOf { it.outputs.count(Digit::isEasy) }.printIt()
    positions.sumOf { it.decode() }.printIt()
  }
}

private fun String.toDigitsEntry() = split(" | ").map { part ->
  part.split(" ").map { Digit(it.toSet()) }
}.let { (input, output) -> DigitsEntry(input, output) }

private data class Digit(val segments: Set<Char>) {
  val isEasy = segments.size in setOf(2, 3, 4, 7)
  operator fun minus(o: Digit) = Digit(segments - o.segments)
}

private data class DigitsEntry(val inputs: List<Digit>, val outputs: List<Digit>) {
  fun decode(): Int = deduce().let { enc ->
    outputs.fold(0) { acc, dig -> 10 * acc + enc[dig]!! }
  }

  private fun deduce(): Map<Digit, Int> {
    val seg = inputs.toSet().groupBy { it.segments.size }
    val one = seg[2]!!.single()
    val four = seg[4]!!.single()
    val seven = seg[3]!!.single()
    val eight = seg[7]!!.single()

    fun MutableSet<Digit>.extract(by: Digit, diff: Int) =
      single { (it - by).segments.size == diff }.also { this -= it }

    val fiveSeg = seg[5]!!.toMutableSet()
    val three = fiveSeg.extract(one, 3)
    val two = fiveSeg.extract(four, 3)
    val five = fiveSeg.single()

    val sixSeg = seg[6]!!.toMutableSet()
    val nine = sixSeg.extract(four, 2)
    val six = sixSeg.extract(one, 5)
    val zero = sixSeg.single()

    return listOf(zero, one, two, three, four, five, six, seven, eight, nine)
      .zip(0..9).toMap()
  }
}