Improve OpenAPI parse + codegen throughput by removing avoidable allocations and repeated regex work

[Copilot]

Description:

Refitter’s parse/generation pipeline accumulated avoidable per-operation and per-document overhead. This PR reduces hot-path CPU/allocation cost in Refitter.Core without changing generated API shape.

• Core generation hot paths

  • Cached IsApiResponseType regexes (static readonly Regex) and reused the computed result per operation.
  • Replaced Accept-header content-type dedup (GroupBy/SelectMany/Distinct) with a single HashSet<string> pass.
  • Updated multi-interface generators to use the optimized header path.

• Contract sanitization + filtering

  • Reworked SanitizeGeneratedContracts from split/per-line regex matching to a single precompiled multiline regex replace.
  • Reduced path-filter matching overhead by avoiding delegate-heavy TrueForAll flow in favor of a tight loop.

• Schema cleanup

  • Compiled keepSchemaPatterns once in SchemaCleaner constructor and reused compiled regexes during traversal.

• OpenAPI load/merge path

  • Reused a static decompression-enabled HttpClient instead of creating one per request.
  • Optimized merge loops (for over Skip(1)) and tag dedup (HashSet instead of repeated All scans).
  • Made HTTP/YAML detection case-insensitive (OrdinalIgnoreCase) to avoid extension/protocol casing misses.

• Behavior safety coverage

  • Added regression assertions for uppercase YAML extensions (.YAML, .YML).
  • Extended inline-json-converter test to assert removal of fully-qualified JsonStringEnumConverter attributes as well.

• Implementation excerpt

private static readonly Regex HttpResponseMessageTypeRegex =
    new("(Task|IObservable)<HttpResponseMessage>", RegexOptions.Compiled, TimeSpan.FromSeconds(1));

private static readonly Regex ApiResponseTypeRegex =
    new("(Task|IObservable)<(I)?ApiResponse(<[\\w<>]+>)?>", RegexOptions.Compiled, TimeSpan.FromSeconds(1));

protected static bool IsApiResponseType(string typeName) =>
    HttpResponseMessageTypeRegex.IsMatch(typeName) || ApiResponseTypeRegex.IsMatch(typeName);

Example OpenAPI Specifications:

openapi: 3.0.0
info:
  title: Perf sample
  version: 1.0.0
paths:
  /users/{id}:
    get:
      operationId: GetUser
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
      responses:
        "200":
          description: ok
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/User'
components:
  schemas:
    User:
      type: object
      properties:
        id:
          type: string

Example generated Refit interface

public partial interface IPerfSample
{
    [Get("/users/{id}")]
    Task<User> GetUser(string id);
}

Warning

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• collector.exceptionless.io
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• petstore.swagger.io
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  • Triggering command: /home/REDACTED/work/refitter/refitter/src/Refitter.Tests/bin/Release/net10.0/Refitter.Tests /home/REDACTED/work/refitter/refitter/src/Refitter.Tests/bin/Release/net10.0/Refitter.Tests --server dotnettestcli --dotnet-test-pipe /tmp/5559119fb74b4ddb8878b6555f6685d8 /var/lib/dpkg/info/libpam-modules-bin.list /var/lib/dpkg/info/libthai-data.list /var/lib/dpkg/info/libzip4t64:amd64.list /var/lib/dpkg/info/libtirpc-common.list /var/lib/dpkg/info/crun.list /var/lib/dpkg/info/lld-18.list /var/lib/dpkg/info/libacl1:amd64.list (dns block)
• petstore3.swagger.io
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[Copilot]

Pull request overview

This PR optimizes Refitter’s OpenAPI load/merge and C# code-generation hot paths by reducing per-operation allocations and repeated regex work, while aiming to keep the generated API surface unchanged.

Changes:

• Reduced overhead in interface generation (cached ApiResponse-type checks; optimized Accept header dedup; updated multi-interface generators to use the optimized path).
• Reduced regex/iteration overhead in contract sanitization and path filtering.
• Optimized OpenAPI loading/merging (static decompression-enabled HttpClient, faster merge loops, case-insensitive HTTP/YAML detection) and added targeted regressions.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Refitter.Tests/OpenApiDocumentFactoryTests.cs	Adds regression coverage for uppercase .YAML/.YML format detection.
src/Refitter.Tests/Examples/InlineJsonConvertersTests.cs	Extends assertions to ensure fully-qualified JsonStringEnumConverter attributes are removed when disabled.
src/Refitter.Core/SchemaCleaner.cs	Compiles keep-schema regex patterns once in the constructor and reuses them during traversal.
src/Refitter.Core/RefitMultipleInterfaceGenerator.cs	Updates header generation call to the optimized signature/path.
src/Refitter.Core/RefitMultipleInterfaceByTagGenerator.cs	Updates header generation call to the optimized signature/path.
src/Refitter.Core/RefitInterfaceGenerator.cs	Adds static cached regexes for ApiResponse detection and replaces Accept header dedup with a single-pass set.
src/Refitter.Core/RefitGenerator.cs	Uses a precompiled multiline regex for contract sanitization and streamlines path-filter matching.
src/Refitter.Core/OpenApiDocumentFactory.cs	Introduces a shared decompressing HttpClient, optimizes merge loops/tag dedup, and makes HTTP/YAML detection case-insensitive.

Comments suppressed due to low confidence (2)

src/Refitter.Core/RefitInterfaceGenerator.cs:271

• uniqueContentTypes is a HashSet, and its enumeration order isn’t guaranteed. Since this is fed directly into string.Join for the generated Accept header, the emitted header value order can become nondeterministic across runs/frameworks, which is undesirable for code generation and can cause noisy diffs. Consider preserving insertion order (e.g., track a List<string> plus a membership HashSet) or sorting the set before joining.

            var uniqueContentTypes = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
            foreach (var response in operations.Value.Responses.Values)
            {
                foreach (var contentType in response.Content.Keys)
                {
                    uniqueContentTypes.Add(contentType);
                }
            }

            if (uniqueContentTypes.Any())
            {
                headers.Add($"\"Accept: {string.Join(", ", uniqueContentTypes)}\"");
            }

src/Refitter.Core/OpenApiDocumentFactory.cs:18

• The private static field is named HttpClient, which shadows the HttpClient type name within this class and makes the code harder to read/maintain (e.g., future additions can’t reference the type unqualified). Consider renaming it to something like sharedHttpClient / decompressingHttpClient to avoid the name collision.

    private static readonly HttpClient HttpClient = new(
        new HttpClientHandler
        {
            AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate
        });

---

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[codecov]

Codecov Report

❌ Patch coverage is 96.61017% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.41%. Comparing base (87e227d) to head (c97f994).
⚠️ Report is 365 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Refitter.Core/OpenApiDocumentFactory.cs	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #937   +/-   ##
=======================================
  Coverage   89.41%   89.41%           
=======================================
  Files          26       26           
  Lines        1511     1521   +10     
=======================================
+ Hits         1351     1360    +9     
  Misses         85       85           
- Partials       75       76    +1     

Flag	Coverage Δ
unittests	89.41% <96.61%> (+<0.01%)	⬆️

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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