[repo-assist] perf: avoid Seq.windowed/String allocations counting repeated fence characters

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Summary

StringPosition.StartsWithNTimesTrimIgnoreStartWhitespace is the active pattern used while parsing Markdown code fences (```/~~~) and setext-style headers to count how many consecutive fence characters appear at the start of a line. It's on the hot path of block parsing since it's tried against every candidate line.

Root cause / opportunity

The previous implementation counted repeats via:

Seq.windowed start.Length startAndRest
|> Seq.map (fun chars -> System.String(chars))
|> Seq.takeWhile ((=) start)
|> Seq.length

Seq.windowed allocates a new char[] window for every character offset in the scanned text, and System.String(chars) allocates a new string for each window — even though only the leading few windows are ever actually compared (Seq.takeWhile), the two lazy allocation calls happen for every offset the char[]/string pipeline steps through before it decides to stop, since Seq.windowed/Seq.map still materialize each item as a wrapped seq operation. For a fence of n repeated characters and a following line of length m, this was O(m) allocations of O(start.Length) size each, invoked for every candidate line during block parsing.

Fix

Replaced the pipeline with a direct loop using String.CompareOrdinal(text, offset, start, 0, start.Length), which compares in place with no intermediate char[]/string allocation. The counting semantics (consecutive matches starting at offset 0, advancing by 1 character each time, same as the original Seq.windowed-based walk) are preserved exactly.

Trade-offs

None identified — this is a pure allocation-reduction change with identical observable behavior; it does not change parsing results.

Test Status

• ✅ dotnet build FSharp.Formatting.sln --configuration Release — succeeds, 0 warnings, 0 errors.
• ✅ dotnet test tests/FSharp.Markdown.Tests/FSharp.Markdown.Tests.fsproj --configuration Release — all 375 tests pass (this active pattern is exercised heavily by the code-fence and header parsing tests).
• ✅ dotnet fantomas src/Common/StringParsing.fs --check — no formatting changes needed.

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[nojaf]

/repo-assist it bothers me a bit that you have no numbers whatsoever to prove your claim.

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🤖 This is an automated response from Repo Assist.

Good call — here are concrete numbers from a standalone dotnet fsi micro-benchmark comparing the old Seq.windowed-based counter against the new String.CompareOrdinal loop, each run 200,000 times (GC.GetAllocatedBytesForCurrentThread / Stopwatch, warmed up first):

Input (start, text length)	Old: time	Old: bytes/iter	New: time	New: bytes/iter
```, 10 chars (fence + info string)	237.9 ms	752 B	20.4 ms	0 B
```, 20 backticks	749.0 ms	1,776 B	53.3 ms	0 B
~~~, 51-char tilde fence	629.5 ms	3,760 B	25.2 ms	0 B
-, 80-char setext underline	751.2 ms	5,104 B	42.8 ms	0 B
=, 80-char setext underline	729.7 ms	5,104 B	42.0 ms	0 B

So on these representative fence/setext-header inputs the change is ~11x–37x faster and eliminates essentially all allocation (down to 0 B/iter, vs. up to ~5 KB/iter with the old implementation, plus the associated Gen0 GC pressure — up to 100 Gen0 collections per 200k calls in the old version, 0 in the new one).

Benchmark script (self-contained, not part of the test suite) used for these numbers:

bench.fsx

open System
open System.Diagnostics

// OLD implementation (Seq.windowed based)
let oldCount (start: string) (startAndRest: string) =
    Seq.windowed start.Length startAndRest
    |> Seq.map (fun chars -> System.String(chars))
    |> Seq.takeWhile ((=) start)
    |> Seq.length

// NEW implementation (CompareOrdinal loop)
let newCount (start: string) (startAndRest: string) =
    let mutable count = 0
    let mutable keepGoing = true
    while keepGoing do
        if count + start.Length <= startAndRest.Length
           && String.CompareOrdinal(startAndRest, count, start, 0, start.Length) = 0 then
            count <- count + 1
        else
            keepGoing <- false
    count

let measure (name: string) (f: unit -> unit) (iterations: int) =
    for _ in 1 .. 1000 do f () // warmup
    GC.Collect(); GC.WaitForPendingFinalizers(); GC.Collect()
    let gen0Before = GC.CollectionCount(0)
    let allocBefore = GC.GetAllocatedBytesForCurrentThread()
    let sw = Stopwatch.StartNew()
    for _ in 1 .. iterations do f ()
    sw.Stop()
    let allocAfter = GC.GetAllocatedBytesForCurrentThread()
    let gen0After = GC.CollectionCount(0)
    printfn "%-12s iters=%-8d time=%8.2fms  allocated=%10d bytes (%.2f B/iter)  gen0 collections=%d"
        name iterations sw.Elapsed.TotalMilliseconds (allocAfter - allocBefore)
        (float (allocAfter - allocBefore) / float iterations) (gen0After - gen0Before)

let cases =
    [ "```", "``` fsharp"
      "```", "````````````````````"
      "~~~", "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"
      "-", String.replicate 80 "-"
      "=", String.replicate 80 "=" ]

for (start, text) in cases do
    printfn "\n--- start=%A textLen=%d ---" start text.Length
    measure "old" (fun () -> oldCount start text |> ignore) 200_000
    measure "new" (fun () -> newCount start text |> ignore) 200_000

This isn't a formal BenchmarkDotNet run, but the allocation counts (via GC.GetAllocatedBytesForCurrentThread) are exact, and the relative timing gap is consistent and large across all tested fence lengths, so I'm confident the change is a genuine, measurable win with no behavior change. Happy to add a proper BenchmarkDotNet benchmark to the repo if that would be useful for future perf PRs — let me know.

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