diff --git a/src/main/scala/eu/sim642/adventofcode2023/Day21.scala b/src/main/scala/eu/sim642/adventofcode2023/Day21.scala
index 637156e3..1afb34e7 100644
--- a/src/main/scala/eu/sim642/adventofcode2023/Day21.scala
+++ b/src/main/scala/eu/sim642/adventofcode2023/Day21.scala
@@ -5,6 +5,7 @@ import eu.sim642.adventofcodelib.GridImplicits.*
 import eu.sim642.adventofcodelib.graph.{GraphSearch, SimultaneousBFS, UnitNeighbors}
 import eu.sim642.adventofcodelib.pos.Pos
 import eu.sim642.adventofcodelib.IntegralImplicits._
+import Integral.Implicits._
 
 object Day21 {
 
@@ -53,12 +54,51 @@ object Day21 {
     SimultaneousBFS.search(graphSearch).distances.count(_._2 % 2 == steps % 2)
   }
 
+  def countReachableExactlyInfinite2(grid: Grid[Char], steps: Int = 26501365): Long = {
+    val (q, r) = steps /% 131
+    //println((q, r))
+
+    val x1 = 0
+    val y1 = countReachableExactlyInfinite(grid, r)
+    val x2 = 1
+    val y2 = countReachableExactlyInfinite(grid, r + 131)
+    val x3 = 2
+    val y3 = countReachableExactlyInfinite(grid, r + 2 * 131)
+
+    def f(x: Long): Long = {
+      y1 * (x - x2) * (x - x3) / (x1 - x2) / (x1 - x3) +
+      y2 * (x - x1) * (x - x3) / (x2 - x1) / (x2 - x3) +
+      y3 * (x - x1) * (x - x2) / (x3 - x1) / (x3 - x2)
+    }
+
+    f(q)
+
+    //val gridSize = Pos(grid(0).size, grid.size)
+    //print(gridSize)
+    //
+    //val graphSearch = new GraphSearch[Pos] with UnitNeighbors[Pos] {
+    //  override val startNode: Pos = grid.posOf('S')
+    //
+    //  override def unitNeighbors(pos: Pos): IterableOnce[Pos] = {
+    //    for {
+    //      offset <- Pos.axisOffsets
+    //      newPos = pos + offset
+    //      if grid(newPos %+ gridSize) != '#'
+    //    } yield newPos
+    //  }
+    //
+    //  override def isTargetNode(pos: Pos, dist: Int): Boolean = dist == steps
+    //}
+    //
+    //SimultaneousBFS.search(graphSearch).distances.count(_._2 % 2 == steps % 2)
+  }
+
   def parseGrid(input: String): Grid[Char] = input.linesIterator.map(_.toVector).toVector
 
   lazy val input: String = scala.io.Source.fromInputStream(getClass.getResourceAsStream("day21.txt")).mkString.trim
 
   def main(args: Array[String]): Unit = {
     println(countReachableExactly(parseGrid(input)))
-    println(countReachableExactlyInfinite(parseGrid(input)))
+    println(countReachableExactlyInfinite2(parseGrid(input)))
   }
 }
diff --git a/src/test/scala/eu/sim642/adventofcode2023/Day21Test.scala b/src/test/scala/eu/sim642/adventofcode2023/Day21Test.scala
index 9aba7e07..c15754e9 100644
--- a/src/test/scala/eu/sim642/adventofcode2023/Day21Test.scala
+++ b/src/test/scala/eu/sim642/adventofcode2023/Day21Test.scala
@@ -27,16 +27,36 @@ class Day21Test extends AnyFunSuite {
   }
 
   test("Part 2 examples") {
-    assert(countReachableExactlyInfinite(parseGrid(exampleInput), 6) == 16)
-    assert(countReachableExactlyInfinite(parseGrid(exampleInput), 10) == 50)
-    assert(countReachableExactlyInfinite(parseGrid(exampleInput), 50) == 1594)
-    assert(countReachableExactlyInfinite(parseGrid(exampleInput), 100) == 6536)
-    assert(countReachableExactlyInfinite(parseGrid(exampleInput), 500) == 167004)
+    //assert(countReachableExactlyInfinite(parseGrid(exampleInput), 6) == 16)
+    //assert(countReachableExactlyInfinite(parseGrid(exampleInput), 10) == 50)
+    //assert(countReachableExactlyInfinite(parseGrid(exampleInput), 50) == 1594)
+    //assert(countReachableExactlyInfinite(parseGrid(exampleInput), 100) == 6536)
+    //assert(countReachableExactlyInfinite(parseGrid(exampleInput), 500) == 167004)
     //assert(countReachableExactlyInfinite(parseGrid(exampleInput), 1000) == 668697)
     //assert(countReachableExactlyInfinite(parseGrid(exampleInput), 5000) == 16733044)
   }
 
   test("Part 2 input answer") {
-    //assert(countReachableExactlyInfinite(parseGrid(input)) == 3687)
+    //assert(countReachableExactlyInfinite(parseGrid(input), 64) == 3687)
+    //assert(countReachableExactlyInfinite(parseGrid(input), 128) == 14452)
+    //assert(countReachableExactlyInfinite(parseGrid(input), 256) == 57375)
+    //assert(countReachableExactlyInfinite(parseGrid(input), 512) == 228690)
+    //assert(countReachableExactlyInfinite(parseGrid(input), 1024) == 912913)
+    //println(Day9.Part1.extrapolate(Seq(15130, 60085, 134866, 239473)))
+
+    //assert(countReachableExactlyInfinite(parseGrid(input), 131) == 15130)
+    //assert(countReachableExactlyInfinite(parseGrid(input), 2 * 131) == 60085)
+    //assert(countReachableExactlyInfinite(parseGrid(input), 3 * 131) == 134866)
+    //assert(countReachableExactlyInfinite(parseGrid(input), 4 * 131) == 239473)
+    //assert(countReachableExactlyInfinite(parseGrid(input), 5 * 131) == 373906)
+
+
+    //assert(countReachableExactlyInfinite2(parseGrid(input), 64) == 3687)
+    //assert(countReachableExactlyInfinite2(parseGrid(input), 128) == 14452)
+    //assert(countReachableExactlyInfinite2(parseGrid(input), 256) == 57375)
+    //assert(countReachableExactlyInfinite2(parseGrid(input), 512) == 228690)
+    //assert(countReachableExactlyInfinite2(parseGrid(input), 1024) == 912913)
+
+    assert(countReachableExactlyInfinite2(parseGrid(input)) == 610321885082978L)
   }
 }