Add support for filtering OpenAPI document (#319)

### Motivation

When generating client code, Swift OpenAPI Generator generates code for
the entire OpenAPI document, even if the user only makes use of a subset
of its types and operations.

Generating code that is unused constitutes overhead for the adopter:
- The overhead of generating code for unused types and operations
- The overhead of compiling the generated code
- The overhead of unused code in the users codebase (AOT generation)

This is particularly noticeable when working with a small subset of a
large API, which can result in O(100k) lines of unused code and long
generation and compile times.

For a more detailed motivation and design, see the proposal in #303.

### Modifications

- Add document filter to the generator config.
- Run filter as a post-transition hook in the generator pipeline after
parsing the document.
- Provide a CLI command that outputs the filtered document to stdout.

### Result

Users can filter a document before code-generation. For large APIs, this
can result in >90% speedup (see proposal).

### Test Plan

- Unit tests.

---------

Signed-off-by: Si Beaumont <beaumont@apple.com>
Co-authored-by: Honza Dvorsky <honza@apple.com>

Sources/_OpenAPIGeneratorCore/Config.swift

@@ -26,21 +26,27 @@ public struct Config: Sendable {
     /// Additional imports to add to each generated file.
     public var additionalImports: [String]
 
+    /// Filter to apply to the OpenAPI document before generation.
+    public var filter: DocumentFilter?
+
     /// Additional pre-release features to enable.
     public var featureFlags: FeatureFlags
 
     /// Creates a configuration with the specified generator mode and imports.
     /// - Parameters:
     ///   - mode: The mode to use for generation.
     ///   - additionalImports: Additional imports to add to each generated file.
+    ///   - filter: Filter to apply to the OpenAPI document before generation.
     ///   - featureFlags: Additional pre-release features to enable.
     public init(
         mode: GeneratorMode,
         additionalImports: [String] = [],
+        filter: DocumentFilter? = nil,
         featureFlags: FeatureFlags = []
     ) {
         self.mode = mode
         self.additionalImports = additionalImports
+        self.filter = filter
         self.featureFlags = featureFlags
     }
 }

Sources/_OpenAPIGeneratorCore/GeneratorPipeline.swift

@@ -126,13 +126,19 @@ func makeGeneratorPipeline(
                 )
             },
             postTransitionHooks: [
+                { document in
+                    guard let documentFilter = config.filter else {
+                        return document
+                    }
+                    return try documentFilter.filter(document)
+                },
                 { doc in
                     let validationDiagnostics = try validator(doc, config)
                     for diagnostic in validationDiagnostics {
                         diagnostics.emit(diagnostic)
                     }
                     return doc
-                }
+                },
             ]
         ),
         translateOpenAPIToStructuredSwiftStage: .init(

Sources/_OpenAPIGeneratorCore/Hooks/DocumentFilter.swift

@@ -0,0 +1,76 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+@preconcurrency import OpenAPIKit
+
+/// Rules used to filter an OpenAPI document.
+public struct DocumentFilter: Codable, Sendable {
+
+    /// Operations with these operation IDs will be included in the filter.
+    public var operations: [String]?
+
+    /// Operations tagged with these tags will be included in the filter.
+    public var tags: [String]?
+
+    /// These paths will be included in the filter.
+    public var paths: [OpenAPI.Path]?
+
+    /// These (additional) schemas will be included in the filter.
+    ///
+    /// These schemas are included in  addition to the transitive closure of schema dependencies of
+    /// the paths included in the filter.
+    public var schemas: [String]?
+
+    /// Create a new DocumentFilter.
+    ///
+    /// - Parameters:
+    ///   - operations: Operations with these IDs will be included in the filter.
+    ///   - tags: Operations tagged with these tags will be included in the filter.
+    ///   - paths: These paths will be included in the filter.
+    ///   - schemas: These (additional) schemas will be included in the filter.
+    public init(
+        operations: [String] = [],
+        tags: [String] = [],
+        paths: [OpenAPI.Path] = [],
+        schemas: [String] = []
+    ) {
+        self.operations = operations
+        self.tags = tags
+        self.paths = paths
+        self.schemas = schemas
+    }
+
+    /// Filter an OpenAPI document.
+    ///
+    /// - Parameter document: The OpenAPI document to filter.
+    /// - Returns: The filtered document.
+    /// - Throws: If any requested document components do not exist in the original document.
+    /// - Throws: If any dependencies of the requested document components cannot be resolved.
+    public func filter(_ document: OpenAPI.Document) throws -> OpenAPI.Document {
+        var builder = FilteredDocumentBuilder(document: document)
+        for tag in tags ?? [] {
+            try builder.includeOperations(tagged: tag)
+        }
+        for operationID in operations ?? [] {
+            try builder.includeOperation(operationID: operationID)
+        }
+        for path in paths ?? [] {
+            try builder.includePath(path)
+        }
+        for schema in schemas ?? [] {
+            try builder.includeSchema(schema)
+        }
+        return try builder.filter()
+    }
+}