[repo-assist] perf: avoid O(n^2) re-scan when splitting Markdown pipe-table rows

RELEASE_NOTES.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## [Unreleased]
+
+### Changed
+* Avoid recomputing `List.length` on the shrinking remainder for every scanned character when splitting a Markdown pipe-table row into cells (`pipeTableFindSplits` in `MarkdownTableParser.fs`). The scan now tracks the number of consumed characters incrementally, turning an O(n²) split into an O(n) one for rows with many delimiters, with no behavior change.
+
 ## [23.0.0-alpha.2] - 2026-09-10
 
 ### Fixed

src/FSharp.Formatting.Markdown/MarkdownTableParser.fs

@@ -20,26 +20,34 @@ let rec pipeTableFindSplits (delim: char array) (line: char list) =
     let cLstToStr (x: char list) =
         x |> Array.ofList |> System.String.Concat
 
+    // Scans the line and, on success, returns the number of characters consumed up to and
+    // including the found delimiter, together with the remaining list. Tracking the consumed
+    // count incrementally (rather than recomputing `List.length` over the shrinking remainder
+    // on every character) keeps this scan linear instead of quadratic in the row length.
     let rec ptfs delim line =
         match line with
-        | DelimitedLatexDisplayMath [ '$'; '$' ] (_body, rest) -> ptfs delim rest
-        | DelimitedLatexInlineMath [ '$' ] (_body, rest) -> ptfs delim rest
-        | List.DelimitedWith [ '`'; ' ' ] [ ' '; '`' ] (_body, rest, _s, _e) -> ptfs delim rest
-        | List.DelimitedNTimes '`' (_body, rest, _s, _e) -> ptfs delim rest
-        | x :: rest when Array.exists ((=) x) delim -> Some rest
-        | '\\' :: _ :: rest
-        | _ :: rest -> ptfs delim rest
+        | DelimitedLatexDisplayMath [ '$'; '$' ] (_body, rest)
+        | DelimitedLatexInlineMath [ '$' ] (_body, rest)
+        | List.DelimitedWith [ '`'; ' ' ] [ ' '; '`' ] (_body, rest, _, _)
+        | List.DelimitedNTimes '`' (_body, rest, _, _) ->
+            let consumedHere = List.length line - List.length rest
+
+            ptfs delim rest
+            |> Option.map (fun (count, remainder) -> (count + consumedHere, remainder))
+        | x :: rest when Array.exists ((=) x) delim -> Some(1, rest)
+        | '\\' :: _ :: rest -> ptfs delim rest |> Option.map (fun (count, remainder) -> (count + 2, remainder))
+        | _ :: rest -> ptfs delim rest |> Option.map (fun (count, remainder) -> (count + 1, remainder))
         | [] -> None
 
-    let rest = ptfs delim line
+    let result = ptfs delim line
 
-    match rest with
+    match result with
     | None -> [ cLstToStr line ]
-    | Some _x when List.isEmpty line -> [ "" ]
-    | Some x ->
-        let chunkSize = List.length line - List.length x - 1
+    | Some _ when List.isEmpty line -> [ "" ]
+    | Some(count, x) ->
+        let chunkSize = count - 1
 
-        cLstToStr (Seq.take chunkSize line |> Seq.toList) :: pipeTableFindSplits delim x
+        cLstToStr (List.truncate chunkSize line) :: pipeTableFindSplits delim x
 
 
 

tests/FSharp.Markdown.Tests/Markdown.fs

@@ -818,6 +818,35 @@ let ``Transform tables with delimiters in code or math correctly`` () =
 
     Markdown.ToHtml doc |> shouldEqualNoWhiteSpace expected
 
+[<Test>]
+let ``Transform tables with escaped pipe characters correctly`` () =
+    let doc =
+        """| a | b |
+|---|---|
+| 1\|2 | 3 |
+"""
+
+    let expected =
+        """<table>
+<thead>
+<tr class="header">
+<th><p>a</p></th>
+<th><p>b</p></th>
+</tr>
+</thead>
+<tbody>
+<tr class="odd">
+<td><p>1\|2</p></td>
+<td><p>3</p></td>
+</tr>
+</tbody>
+</table>
+
+"""
+        |> properNewLines
+
+    Markdown.ToHtml doc |> shouldEqualNoWhiteSpace expected
+
 [<Test>]
 let ``Parse empty blockquote followed by content`` () =
     let doc =