Solve 2024 day 20 part 2

Author: sim642

src/main/scala/eu/sim642/adventofcode2024/Day20.scala

@@ -25,39 +25,97 @@ object Day20 {
     }
   }
 
-  def findCheats(grid: Grid[Char]): Set[Cheat] = {
-    val forwardSearch = gridGraphSearch(grid, 'S', 'E')
-    val forwardResult = BFS.search(forwardSearch)
-    val backwardSearch = gridGraphSearch(grid, 'E', 'S')
-    val backwardResult = BFS.search(backwardSearch)
-
-    val noCheatDistance = forwardResult.target.get._2
-
-    (for {
-      (row, y) <- grid.view.zipWithIndex
-      (cell, x) <- row.view.zipWithIndex
-      if cell == '#'
-      pos = Pos(x, y)
-      startOffset <- Pos.axisOffsets
-      start = pos + startOffset
-      if grid.containsPos(start) && grid(start) != '#'
-      endOffset <- Pos.axisOffsets
-      if startOffset != endOffset
-      end = pos + endOffset
-      if grid.containsPos(end) && grid(end) != '#'
-      cheatDistance = forwardResult.distances(start) + 2 + backwardResult.distances(end)
-      //if cheatDistance <= noCheatDistance
-      save = noCheatDistance - cheatDistance
-    } yield Cheat(start, end, save)).toSet
+  trait Part {
+    def findCheats(grid: Grid[Char]): Set[Cheat]
+
+    def countGoodCheats(grid: Grid[Char]): Int = findCheats(grid).count(_.save >= 100)
+  }
+
+  object Part1 extends Part {
+    override def findCheats(grid: Grid[Char]): Set[Cheat] = {
+      val forwardSearch = gridGraphSearch(grid, 'S', 'E')
+      val forwardResult = BFS.search(forwardSearch)
+      val backwardSearch = gridGraphSearch(grid, 'E', 'S')
+      val backwardResult = BFS.search(backwardSearch)
+
+      val noCheatDistance = forwardResult.target.get._2
+
+      (for {
+        (row, y) <- grid.view.zipWithIndex
+        (cell, x) <- row.view.zipWithIndex
+        if cell == '#'
+        pos = Pos(x, y)
+        startOffset <- Pos.axisOffsets
+        start = pos + startOffset
+        if grid.containsPos(start) && grid(start) != '#'
+        endOffset <- Pos.axisOffsets
+        if startOffset != endOffset
+        end = pos + endOffset
+        if grid.containsPos(end) && grid(end) != '#'
+        cheatDistance = forwardResult.distances(start) + 2 + backwardResult.distances(end)
+        //if cheatDistance <= noCheatDistance
+        save = noCheatDistance - cheatDistance
+      } yield Cheat(start, end, save)).toSet
+    }
   }
 
-  def countGoodCheats(grid: Grid[Char]): Int = findCheats(grid).count(_.save >= 100)
+  object Part2 extends Part {
+    override def findCheats(grid: Grid[Char]): Set[Cheat] = {
+      val forwardSearch = gridGraphSearch(grid, 'S', 'E')
+      val forwardResult = BFS.search(forwardSearch)
+      val backwardSearch = gridGraphSearch(grid, 'E', 'S')
+      val backwardResult = BFS.search(backwardSearch)
+
+      val noCheatDistance = forwardResult.target.get._2
+
+      // TODO: optimize
+
+      /*(for {
+        (row, y) <- grid.view.zipWithIndex
+        (cell, x) <- row.view.zipWithIndex
+        if cell == '.'
+        start = Pos(x, y)
+        startOffset <- Pos.axisOffsets
+        startCheat <- 0 to 20
+        pos = start + startCheat *: startOffset
+        if grid.containsPos(pos)
+        endOffset <- Pos.axisOffsets
+        if startOffset != endOffset && startOffset != -endOffset
+        endCheat <- 0 to (20 - startCheat)
+        end = pos + endCheat *: endOffset
+        if grid.containsPos(end) && grid(end) != '#'
+        cheatDistance = forwardResult.distances(start) + (startCheat + endCheat) + backwardResult.distances(end)
+        //if cheatDistance <= noCheatDistance
+        save = noCheatDistance - cheatDistance
+      } yield Cheat(start, end, save)).toSet*/
+
+      (for {
+        (row, y) <- grid.view.zipWithIndex
+        (cell, x) <- row.view.zipWithIndex
+        if cell != '#'
+        start = Pos(x, y)
+        xOffset <- -20 to 20
+        pos = start + Pos(xOffset, 0)
+        if grid.containsPos(pos)
+        startCheat = xOffset.abs
+        maxEndCheat = 20 - startCheat
+        yOffset <- (-maxEndCheat) to maxEndCheat
+        end = pos + Pos(0, yOffset)
+        if grid.containsPos(end) && grid(end) != '#'
+        endCheat = yOffset.abs
+        cheatDistance = forwardResult.distances(start) + (startCheat + endCheat) + backwardResult.distances(end)
+        //if cheatDistance <= noCheatDistance
+        save = noCheatDistance - cheatDistance
+      } yield Cheat(start, end, save)).toSet
+    }
+  }
 
   def parseGrid(input: String): Grid[Char] = input.linesIterator.map(_.toVector).toVector
 
   lazy val input: String = scala.io.Source.fromInputStream(getClass.getResourceAsStream("day20.txt")).mkString.trim
 
   def main(args: Array[String]): Unit = {
-    println(countGoodCheats(parseGrid(input)))
+    println(Part1.countGoodCheats(parseGrid(input)))
+    println(Part2.countGoodCheats(parseGrid(input)))
   }
 }

src/test/scala/eu/sim642/adventofcode2024/Day20Test.scala

@@ -24,7 +24,7 @@ class Day20Test extends AnyFunSuite {
       |###############""".stripMargin
 
   test("Part 1 examples") {
-    val cheats = findCheats(parseGrid(exampleInput)).groupCount(_.save)
+    val cheats = Part1.findCheats(parseGrid(exampleInput)).groupCount(_.save)
     assert(cheats(2) == 14)
     assert(cheats(4) == 14)
     assert(cheats(6) == 2)
@@ -39,6 +39,28 @@ class Day20Test extends AnyFunSuite {
   }
 
   test("Part 1 input answer") {
-    assert(countGoodCheats(parseGrid(input)) == 1490)
+    assert(Part1.countGoodCheats(parseGrid(input)) == 1490)
+  }
+
+  test("Part 2 examples") {
+    val cheats = Part2.findCheats(parseGrid(exampleInput)).groupCount(_.save)
+    assert(cheats(50) == 32)
+    assert(cheats(52) == 31)
+    assert(cheats(54) == 29)
+    assert(cheats(56) == 39)
+    assert(cheats(58) == 25)
+    assert(cheats(60) == 23)
+    assert(cheats(62) == 20)
+    assert(cheats(64) == 19)
+    assert(cheats(66) == 12)
+    assert(cheats(68) == 14)
+    assert(cheats(70) == 12)
+    assert(cheats(72) == 22)
+    assert(cheats(74) == 4)
+    assert(cheats(76) == 3)
+  }
+
+  ignore("Part 2 input answer") { // TODO: optimize (~4.3s)
+    assert(Part2.countGoodCheats(parseGrid(input)) == 1011325)
   }
 }