Part 1

AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day13.fs

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+namespace AdventOfCode2023
+
+open System
+
+[<RequireQualifiedAccess>]
+module Day13 =
+
+    let rowToInt (row : ReadOnlySpan<char>) : uint32 =
+        let mutable mult = 1ul
+        let mutable answer = 0ul
+
+        for c = row.Length - 1 downto 0 do
+            if row.[c] = '#' then
+                answer <- answer + mult
+
+            mult <- mult * 2ul
+
+        answer
+
+    let colToInt (grid : ReadOnlySpan<char>) (rowLength : int) (colNum : int) =
+        let mutable mult = 1ul
+        let mutable answer = 0ul
+
+        for i = grid.Count '\n' - 1 downto 0 do
+            if grid.[i * (rowLength + 1) + colNum] = '#' then
+                answer <- answer + mult
+
+            mult <- mult * 2ul
+
+        answer
+
+    let verifyHorizontalReflection (group : ResizeArray<'a>) (smaller : int) (bigger : int) : bool =
+        let midPoint = (smaller + bigger) / 2
+
+        let rec isOkWithin (curr : int) =
+            if smaller + curr > midPoint then
+                true
+            else if group.[smaller + curr] = group.[bigger - curr] then
+                isOkWithin (curr + 1)
+            else
+                false
+
+        if not (isOkWithin 0) then
+            false
+        else
+
+        smaller = 0 || bigger = group.Count - 1
+
+    /// Find reflection among rows
+    [<TailCall>]
+    let rec findRow (rows : ResizeArray<'a>) (currentLine : int) =
+        if currentLine = rows.Count - 1 then
+            None
+        else
+            let mutable answer = UInt32.MaxValue
+            let mutable i = currentLine
+
+            while i < rows.Count - 1 do
+                i <- i + 1
+
+                if currentLine % 2 <> i % 2 then
+                    if rows.[i] = rows.[currentLine] then
+                        if verifyHorizontalReflection rows currentLine i then
+                            answer <- uint32 (((currentLine + i) / 2) + 1)
+                            i <- Int32.MaxValue
+
+            if answer < UInt32.MaxValue then
+                Some answer
+            else
+                findRow rows (currentLine + 1)
+
+    let part1 (s : string) =
+        let mutable s = s.AsSpan ()
+        use lines = StringSplitEnumerator.make' '\n' s
+        let rows = ResizeArray ()
+        let cols = ResizeArray ()
+        let mutable answer = 0ul
+
+        while not s.IsEmpty do
+            rows.Clear ()
+            cols.Clear ()
+            let index = s.IndexOf "\n\n"
+
+            let group =
+                if index < 0 then
+                    // last group
+                    s
+                else
+                    s.Slice (0, index + 1)
+
+            let lineLength = s.IndexOf '\n'
+            cols.EnsureCapacity lineLength |> ignore
+
+            for col = 0 to lineLength - 1 do
+                cols.Add (colToInt group lineLength col)
+
+            for row in StringSplitEnumerator.make' '\n' group do
+                if not row.IsEmpty then
+                    rows.Add (rowToInt row)
+
+            match findRow rows 0 with
+            | Some rowIndex -> answer <- answer + 100ul * rowIndex
+            | None ->
+                let colIndex = Option.get (findRow cols 0)
+                answer <- answer + colIndex
+
+            if index < 0 then
+                s <- ReadOnlySpan<char>.Empty
+            else
+                s <- s.Slice (index + 2)
+
+        answer
+
+    let part2 (s : string) =
+        use mutable lines = StringSplitEnumerator.make '\n' s
+
+        let mutable answer = 0uL
+
+        for line in lines do
+            if not line.IsEmpty then
+                ()
+
+        answer