Part 2

AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day16.fs

@@ -6,8 +6,6 @@ namespace AdventOfCode2023
 #endif
 
 open System
-open System.Collections.Generic
-open System.Globalization
 
 [<RequireQualifiedAccess>]
 module Day16 =
@@ -58,36 +56,9 @@ module Day16 =
             printfn ""
         printfn ""
 
-    let part1 (s : string) =
-        let numLines = s.AsSpan().Count '\n'
-        let numCols = s.IndexOf '\n'
-        let arr = Array.zeroCreate (numLines * numCols)
-#if DEBUG
-        let arr : Arr2D<bool> =
-            {
-                Elements = arr
-                Width = numCols
-            }
-#else
-        use ptr = fixed arr
-        let arr : Arr2D<bool> =
-            {
-                Elements = arr
-                Width = numCols
-                Length = arr.Count
-            }
-#endif
-        let going = ResizeArray ()
-        going.Add (storeDirectionAndPos numCols LanguagePrimitives.GenericZero LanguagePrimitives.GenericZero Direction.Right)
-        let seen = Array.zeroCreate (int (maxEncoded numCols numLines))
-
-        while going.Count > 0 do
-            let nextUp = going.[going.Count - 1]
-            match seen.[int nextUp] with
-            | true ->
-                going.RemoveAt (going.Count - 1)
-            | false ->
-            seen.[int nextUp] <- true
+    let advance (arr : Arr2D<_>) (going : ResizeArray<_>) (s : string) (nextUp : uint16) =
+        let numCols = arr.Width
+        let numLines = arr.Height
         let col = getCol numCols nextUp
         let row = getRow numCols nextUp
         let dir = getDirection nextUp
@@ -198,8 +169,127 @@ module Day16 =
             | c ->
                 failwith $"Unrecognised char: %c{c}"
 
+    let part1 (s : string) =
+        let numLines = s.AsSpan().Count '\n'
+        let numCols = s.IndexOf '\n'
+        let arr = Array.zeroCreate (numLines * numCols)
+#if DEBUG
+        let arr : Arr2D<bool> =
+            {
+                Elements = arr
+                Width = numCols
+            }
+#else
+        use ptr = fixed arr
+        let arr : Arr2D<bool> =
+            {
+                Elements = arr
+                Width = numCols
+                Length = arr.Count
+            }
+#endif
+        let going = ResizeArray ()
+        going.Add (storeDirectionAndPos numCols LanguagePrimitives.GenericZero LanguagePrimitives.GenericZero Direction.Right)
+        let seen = Array.zeroCreate (int (maxEncoded numCols numLines) + 1)
+
+        while going.Count > 0 do
+            let nextUp = going.[going.Count - 1]
+            match seen.[int nextUp] with
+            | true ->
+                going.RemoveAt (going.Count - 1)
+            | false ->
+                seen.[int nextUp] <- true
+                advance arr going s nextUp
+
         arr.Elements.AsSpan().Count true
 
     let part2 (s : string) =
-        use lines = StringSplitEnumerator.make '\n' s
-        0
+        let numLines = s.AsSpan().Count '\n'
+        let numCols = s.IndexOf '\n'
+        let arr = Array.zeroCreate (numLines * numCols)
+#if DEBUG
+        let arr : Arr2D<bool> =
+            {
+                Elements = arr
+                Width = numCols
+            }
+#else
+        use ptr = fixed arr
+        let arr : Arr2D<bool> =
+            {
+                Elements = arr
+                Width = numCols
+                Length = arr.Count
+            }
+#endif
+        let going = ResizeArray ()
+        let seen = Array.zeroCreate (int (maxEncoded numCols numLines) + 1)
+        let mutable best = 0
+
+        for start = 0 to numCols - 1 do
+            going.Clear ()
+            Array.Clear seen
+            Array.Clear arr.Elements
+            going.Add (storeDirectionAndPos numCols start LanguagePrimitives.GenericZero Direction.Down)
+
+            while going.Count > 0 do
+                let nextUp = going.[going.Count - 1]
+                match seen.[int nextUp] with
+                | true ->
+                    going.RemoveAt (going.Count - 1)
+                | false ->
+                    seen.[int nextUp] <- true
+                    advance arr going s nextUp
+            let lit = arr.Elements.AsSpan().Count true
+            best <- max best lit
+
+            going.Clear ()
+            Array.Clear seen
+            Array.Clear arr.Elements
+            going.Add (storeDirectionAndPos numCols start (numLines - 1) Direction.Up)
+
+            while going.Count > 0 do
+                let nextUp = going.[going.Count - 1]
+                match seen.[int nextUp] with
+                | true ->
+                    going.RemoveAt (going.Count - 1)
+                | false ->
+                    seen.[int nextUp] <- true
+                    advance arr going s nextUp
+            let lit = arr.Elements.AsSpan().Count true
+            best <- max best lit
+
+        for start = 0 to numLines - 1 do
+            going.Clear ()
+            Array.Clear seen
+            Array.Clear arr.Elements
+            going.Add (storeDirectionAndPos numCols LanguagePrimitives.GenericZero start Direction.Right)
+
+            while going.Count > 0 do
+                let nextUp = going.[going.Count - 1]
+                match seen.[int nextUp] with
+                | true ->
+                    going.RemoveAt (going.Count - 1)
+                | false ->
+                    seen.[int nextUp] <- true
+                    advance arr going s nextUp
+            let lit = arr.Elements.AsSpan().Count true
+            best <- max best lit
+
+            going.Clear ()
+            Array.Clear seen
+            Array.Clear arr.Elements
+            going.Add (storeDirectionAndPos numCols (numCols - 1) start Direction.Left)
+
+            while going.Count > 0 do
+                let nextUp = going.[going.Count - 1]
+                match seen.[int nextUp] with
+                | true ->
+                    going.RemoveAt (going.Count - 1)
+                | false ->
+                    seen.[int nextUp] <- true
+                    advance arr going s nextUp
+            let lit = arr.Elements.AsSpan().Count true
+            best <- max best lit
+
+        best

AdventOfCode2023.FSharp/Test/TestDay16.fs

@@ -43,4 +43,4 @@ module TestDay16 =
                 Assert.Inconclusive ()
                 failwith "unreachable"
 
-        Day16.part2 s |> shouldEqual 0
+        Day16.part2 s |> shouldEqual 8314