Day5.kt

import kotlin.math.abs

object Day5 : AdventDay() {
  override fun solve() {
    val lines = reads<String>()?.map { it.toLine() } ?: return

    Diagram().apply {
      lines.filter { it.isVertical || it.isHorizontal }.forEach { markLine(it) }
      marked.count { it.value > 1 }.printIt()
    }
    Diagram().apply {
      lines.forEach { markLine(it) }
      marked.count { it.value > 1 }.printIt()
    }
  }
}

private data class P(val x: Int, val y: Int)
private data class L(val from: P, val to: P) {
  val isHorizontal = from.x == to.x
  val isVertical = from.y == to.y
  val isDiagonal = abs(from.y - to.y) == abs(from.x - to.x)
}

private fun String.toLine() = split(" -> ").map { p ->
  p.split(",").let { (x, y) -> P(x.toInt(), y.toInt()) }
}.let { (f, t) -> L(f, t) }

private class Diagram {
  private val _m = DefaultMap<P, Int>(0)
  val marked: Map<P, Int> = _m

  fun markLine(line: L) = with(line) {
    when {
      isVertical -> (from.x directedTo to.x).map { P(it, to.y) }
        .forEach { _m[it] = _m[it] + 1 }
      isHorizontal -> (from.y directedTo to.y).map { P(to.x, it) }
        .forEach { _m[it] = _m[it] + 1 }
      isDiagonal -> (from.x directedTo to.x).zip(from.y directedTo to.y)
        .map { (x, y) -> P(x, y) }
        .forEach { _m[it] = _m[it] + 1 }
      else -> Unit
    }
  }
}