adventofcode 2025 Day 9

GitOrigin-RevId: 11c34a287af6c42cde0986e863d639deaa5c9754

Author: will-molloy

advent-of-code/src/main/kotlin/com/willmolloy/adventofcode/_2025/Day9.kt

@@ -0,0 +1,121 @@
+package com.willmolloy.adventofcode._2025
+
+import com.willmolloy.adventofcode.common.Day
+import com.willmolloy.adventofcode.common.Input
+import com.willmolloy.adventofcode.common.extensions.debug
+import com.willmolloy.adventofcode.common.extensions.size
+import com.willmolloy.adventofcode.common.grid.Point
+import kotlin.collections.lastIndex
+import kotlin.math.abs
+import kotlin.math.max
+import kotlin.math.min
+
+/** https://adventofcode.com/2025/day/9 */
+object Day9 : Day(2025, 9) {
+
+  override fun part1(input: Input): Any {
+    val redTiles = getRedTiles(input)
+
+    var max = 0L
+
+    for ((i, p1) in redTiles.withIndex()) {
+      for (p2 in redTiles.drop(i + 1)) {
+        val area = (abs(p2.y - p1.y) + 1) * (abs(p2.x - p1.x) + 1)
+        max = max(max, area)
+      }
+    }
+
+    return max
+  }
+
+  private fun getRedTiles(input: Input): List<Point> =
+    input.readLines().map {
+      val split = it.split(",")
+      Point(split[0], split[1])
+    }
+
+  private fun getLoopOfGreenTiles(redTiles: List<Point>): List<Point> {
+    val greenTiles = mutableListOf<Point>()
+
+    for (i in 0 until redTiles.size) {
+      val point = redTiles[i]
+      val adjPoint = if (i < redTiles.lastIndex) redTiles[i + 1] else redTiles[0]
+      // "Tiles that are adjacent will always be on either the same row or the same column."
+      if (point.x == adjPoint.x) {
+        // same col
+        for (y in min(point.y, adjPoint.y)..max(point.y, adjPoint.y)) {
+          greenTiles.add(Point(point.x, y))
+        }
+      } else {
+        // same row
+        for (x in min(point.x, adjPoint.x)..max(point.x, adjPoint.x)) {
+          greenTiles.add(Point(x, point.y))
+        }
+      }
+    }
+
+    return greenTiles
+  }
+
+  override fun part2(input: Input): Any {
+    val redTiles = getRedTiles(input)
+    val greenTiles = getLoopOfGreenTiles(redTiles)
+    val redGreenTiles = redTiles.union(greenTiles)
+
+    // "In addition, all the tiles inside this loop of red and green tiles are also green."
+    val rowStart = redGreenTiles.minOf { it.y }
+    val rowEnd = redGreenTiles.maxOf { it.y }
+    println("ROWS = $rowStart..$rowEnd")
+
+    // ROW -> COL RANGE
+    val tiledRanges = mutableMapOf<Long, LongRange>()
+
+    for (row in rowStart..rowEnd) {
+      val tilesInRow = redGreenTiles.filter { it.y == row }
+      if (tilesInRow.isEmpty()) {
+        continue
+      }
+
+      val colStart = tilesInRow.minOf { it.x }
+      val colEnd = tilesInRow.maxOf { it.x }
+
+      tiledRanges[row] = colStart..colEnd
+    }
+
+    tiledRanges.debug()
+
+    fun area(p1: Point, p2: Point): Long {
+      // no need to filter for topLeft/bottomRight points because all the points in the rectangle
+      // are checked that they're tiled - so just force topLeft/bottomRight:
+      val cols = min(p1.x, p2.x)..max(p1.x, p2.x)
+      val rows = min(p1.y, p2.y)..max(p1.y, p2.y)
+
+      for (row in rows) {
+        val tiledRange = tiledRanges[row] ?: return 0L
+        if (tiledRange.start > cols.start) {
+          return 0L
+        }
+        if (tiledRange.endInclusive < cols.endInclusive) {
+          return 0L
+        }
+      }
+
+      return cols.size * rows.size
+    }
+
+    // "The rectangle you choose still must have red tiles in opposite corners"
+    var max = 0L
+
+    for ((i, p1) in redTiles.withIndex()) {
+      for (p2 in redTiles.drop(i + 1)) {
+        val area = area(p1, p2)
+        if (area > max) {
+          max = area
+          println("NEW MAX: $p1 and $p2 = $area")
+        }
+      }
+    }
+
+    return max
+  }
+}

advent-of-code/src/main/kotlin/com/willmolloy/adventofcode/common/datastructures/DisjointSet.kt

@@ -5,7 +5,7 @@ package com.willmolloy.adventofcode.common.datastructures
  *
  * @author <a href=https://willmolloy.com>Will Molloy</a>
  */
-class DisjointSet(private val parent: IntArray, private val size: IntArray) {
+class DisjointSet private constructor(private val parent: IntArray, private val size: IntArray) {
 
   constructor(numNodes: Int) : this(IntArray(numNodes) { it }, IntArray(numNodes) { 1 })
 

advent-of-code/src/main/resources/input/2025/example_input/day9.txt

@@ -0,0 +1,8 @@
+7,1
+11,1
+11,7
+9,7
+9,5
+2,5
+2,3
+7,3

advent-of-code/src/test/kotlin/com/willmolloy/adventofcode/_2025/Day9Test.kt

@@ -0,0 +1,10 @@
+package com.willmolloy.adventofcode._2025
+
+import com.willmolloy.adventofcode.common.DayTest
+
+object Day9Test : DayTest(Day9) {
+
+  override fun part1() = Answer(50, 4774877510)
+
+  override fun part2() = Answer(24, 1560475800)
+}