StringPool class (ArrayPool<T>-like to cache strings)

[Sergio0694]

PR Type

What kind of change does this PR introduce?

• Feature

What is the new behavior?

This PR introduces a new StringPool type, inspired by the ArrayPool<T> type from the BCL, which can be used to cache and reuse string instances. It is a similar concept to string interning, with the additional benefit of working with just a ReadOnlySpan<char> as input (so that the string allocation can be skipped entirely if an equal instance exists already), and offers more customizaton compared to string interning. It also allows users to reset the cache if needed, as well as creating multiple, configurable cache instances for different purposes, if necessary. This is the new API surface:

namespace Microsoft.Toolkit.HighPerformance.Buffers
{
    public sealed class StringPool
    {
        StringPool();
        StringPool(int minimumSize);

        static StringPool Shared { get; }

        int Size { get; }

        void Add(string value);
        string GetOrAdd(string value);
        string GetOrAdd(ReadOnlySpan<char> span);
        string GetOrAdd(ReadOnlySpan<byte> span, Encoding encoding);
        bool TryGet(ReadOnlySpan<char> span, out string? value);
        void Reset();
    }
}

Example usage

This sample shows how to parse a domain string from a given URL, using StringPool:

public static string GetHost(string url)
{
    // We assume the input might start either with eg. https:// (or other prefix),
    // or directly with the host name. Furthermore, we also assume that the input
    // URL will always have a '/' character right after the host name.
    // For instance: "https://docs.microsoft.com/dotnet/api/system.string.intern".
    int
        prefixOffset = url.AsSpan().IndexOf(stackalloc char[] { ':', '/', '/' }),
        startIndex = prefixOffset == -1 ? 0 : prefixOffset + 3,
        endIndex = url.AsSpan(startIndex).IndexOf('/');
    // In this example, it would be "docs.microsoft.com"
    ReadOnlySpan<char> span = url.AsSpan(startIndex, endIndex);
    return StringPool.Shared.GetOrAdd(span);
}

Benchmarks

These are some benchmark results from here, when parsing a ~25 MB .csv file in UTF8 format with many duplicates. The difference between StackallocGetOrAdd and EmbeddedGetOrAdd is whether the UTF16 conversion is done externally or using the implicit StringPool API. In both cases, the memory usage goes down considerably, and performance is still pretty fast 🚀

Method	Mean	Error	StdDev	Ratio	Gen 0	Gen 1	Gen 2	Allocated
Default	377.2 ms	0.68 ms	0.45 ms	1.00	11000.0000	4000.0000	-	67108800 B
StackallocGetOrAdd	334.6 ms	6.30 ms	4.17 ms	0.89	-	-	-	224 B
EmbeddedGetOrAdd	416.0 ms	4.51 ms	2.98 ms	1.10	-	-	-	272 B

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link: #366
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

[ghost]

Thanks Sergio0694 for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌

[ghost]

This PR has been marked as "needs attention 👋" and awaiting a response from the team.

[Sergio0694]

This PR should be at 100% code coverage now, I've added some more tests in the last commit 😊

NOTE: that 1% left is just a line with a closing bracket being incorrectly counted for some reason.

[Sergio0694]

@sonnemaf have you tested this out from the CI build as well yet, would love any feedback you have since you indicated this would be useful to you.

Hey @sonnemaf - just a small ping in case you missed the previous message from @michael-hawker here. Did you have a chance to test the final (current) implementation of StringPool from the CI? Any other feedbacks on this PR? Thanks! 😊

[michael-hawker]

If @azchohfi or @sonnemaf can sign-off on this, I'm good. 🙂

[sonnemaf]

I have tested it with my small test projects and found no issues. I think it is good to go. Signing it off.

Can't wait to use it in my production code.

[Sergio0694]

Hey @sonnemaf that's great to hear, awesome! 😄

Looks like the CI failed due to the Azure outage from yesterday (perfect timing!), but I don't have the option to re-trigger the pipeline myself, @azchohfi would you mind just bumping that so we can check that everything looks good here? Thanks! 😊

[azchohfi]

Done.

[Sergio0694]

[azchohfi]

Absolutely, young padawan!

[Sergio0694]

Oof was looking in the wrong place, yeah I see that now! Thank you! 😊

[Sergio0694]

@michael-hawker Small ping in case you missed the notification, Fons approved this PR in his comment above (here), let me know if this is alright to merge now or if you'd like to have another look at it first. Excited to have this be part of the Preview 3 😄🚀

[ghost]

Hello @michael-hawker!

Because this pull request has the auto merge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

p.s. you can customize the way I help with merging this pull request, such as holding this pull request until a specific person approves. Simply @mention me (@msftbot) and give me an instruction to get started! Learn more here.

[Sergio0694]

Awesome! 🎉🎉🎉

Thank you @sonnemaf for the review and feedbacks, and @michael-hawker for the approval and merge! 😄

[sonnemaf]

I just saw that the StringPool is not mentioned in the description of the preview 3 NuGet package.

[Sergio0694]

@sonnemaf Nice catch! Fixed that in aef54c6 😄