aoc2024

commit ef3482d10654fca6868840aaa425b9ca1a02e1b5
parent ddaa88f98f745cf6df86ceca1ccc03511b5856f5
Author: Martin Ashby <martin@ashbysoft.com>
Date:   Sat, 14 Dec 2024 09:30:08 +0000

Day6 pt2

Diffstat:
M
src/main/scala/Day6.scala
 | 
56
++++++++++++++++++++++----------------------------------
M
src/main/scala/Main.scala
 | 
4
++--

2 files changed, 24 insertions(+), 36 deletions(-)
diff --git a/src/main/scala/Day6.scala b/src/main/scala/Day6.scala
@@ -3,15 +3,16 @@ def day6_pt1(input: Iterator[String]): Int =
     val lst = input.toList
     val pos = lst.zipWithIndex.find((s, y) => s.contains("^")).map((s, y) => (s.indexOf('^'), y)).get
     val grid = updated(lst, pos, '.')
-    do_follow(grid, pos, '^')
+    val grid2 = do_follow(grid, pos, '^')
+    grid2.map(line => line.count(ch => ch == 'X')).sum
 
 @tailrec()
-def do_follow(grid: List[String], pos: (Int,Int), dir: Char): Int =
+def do_follow(grid: List[String], pos: (Int,Int), dir: Char): List[String] =
     val grid2 = updated(grid, pos, 'X')
     val pos2 = next_pos(pos, dir)
     val (x2, y2) = pos2
     if (oob(grid2, pos2)) {
-        return grid2.map(line => line.count(ch => ch == 'X')).sum
+        return grid2
     }
     if (grid_at(grid2, pos2) == '#') {
         val dir2 = next_dir(dir)
@@ -24,44 +25,31 @@ def day6_pt2(input: Iterator[String]): Int =
     val lst = input.toList
     val pos = lst.zipWithIndex.find((s, y) => s.contains("^")).map((s, y) => (s.indexOf('^'), y)).get
     val grid = updated(lst, pos, '.')
-    do_follow2(grid, pos, '^', List(), Set())
+    val grid2 = do_follow(grid, pos, '^')
+    List.range(0, grid.size).flatMap(y => List.range(0, grid(0).size).map(x => (x, y)))
+        .filter(block => block != pos) // Can't add a block where the guard currenly is... 
+        .filter(block => grid_at(grid2, block) == 'X') // only bother testing locations the guard actually visited; otherwise it's an ineffective block
+        .filter(block => {
+            val res = do_follow3(updated(grid, block, '#'), pos, '^', Set())
+            println(s"Testing new block at $block: $res")
+            res
+        })
+        .size
 
 @tailrec()
-def do_follow2(grid: List[String], pos: (Int,Int), dir: Char, hist: List[((Int,Int), Char)], cand: Set[(Int,Int)]): Int =
-    val hist2 = hist.appended((pos, dir))
+def do_follow3(grid: List[String], pos: (Int,Int), dir: Char, hist: Set[((Int,Int), Char)]): Boolean =
+    if (hist.contains(pos, dir)) {
+        return true
+    }
     val pos2 = next_pos(pos, dir)
     if (oob(grid, pos2)) {
-        println(cand)
-        return cand.size
+        return false
     } 
-    if (grid_at(grid, pos2) == '#') {
-        do_follow2(grid, pos, next_dir(dir), hist2, cand)
-    } else {
-        // Test if there _was_ a block at pos2; would it result in a cycle?
-        val dir_spec = next_dir(dir)
-        val pos_spec = next_pos(pos2, dir_spec)
-        val cand2 = if (!oob(grid, pos_spec) && grid_at(grid, pos_spec) != '#' && do_follow3(grid, pos_spec, dir_spec, hist2)) {
-            cand + pos2
-        } else {
-            cand
-        }
-        do_follow2(grid, pos2, dir, hist2, cand2)
-    }
 
-@tailrec()
-def do_follow3(grid: List[String], pos: (Int,Int), dir: Char, hist: List[((Int,Int), Char)]): Boolean =
-    val pos2 = next_pos(pos, dir)
-    if (oob(grid, pos2)) {
-        false
-    } else if (hist.contains(pos2, dir)) {
-        true
+    if (grid_at(grid, pos2) == '#') {
+        do_follow3(grid, pos, next_dir(dir), hist + ((pos, dir)))
     } else {
-        val hist2 = hist.appended((pos2, dir))
-        if (grid_at(grid, pos2) == '#') {
-            do_follow3(grid, pos, next_dir(dir), hist2)
-        } else {
-            do_follow3(grid, pos2, dir, hist2)
-        }
+        do_follow3(grid, pos2, dir, hist + ((pos, dir)))
     }
 
 // def print(grid: List[String]): Unit =
diff --git a/src/main/scala/Main.scala b/src/main/scala/Main.scala
@@ -1,2 +1,2 @@
 @main def run(): Unit = 
-  println(day6_pt2(scala.io.Source.fromFile("day6.txt").getLines()))
-\ No newline at end of file
+  println(day6_pt1(scala.io.Source.fromFile("day6.txt").getLines()))
+\ No newline at end of file