Retain operation level parameters over path level ones (#183)

### Motivation

Fixes #168.

### Modifications

Previously, an operation's parameters were returned as a concatenation
of the parameters from the path item level with those at operation
level.

However, this is incorrect according to
https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.3.md#fixed-fields-7.
> A list of parameters that are applicable for all the operations
described under this path. These parameters can be overridden at the
operation level, but cannot be removed there. The list MUST NOT include
duplicated parameters.

With this change, we are merging the two arrays of parameters using a
unique identifier of the location + name of parameters. If duplicate
parameters exist, only the parameters from the operation level are
preserved.

### Result

After this change, an operation's parameters won't contain any
duplicate.

### Test Plan

Add tests for `OperationDescription.allParameters` at
`Tests/OpenAPIGeneratorCoreTests/Translator/Operations/Test_OperationDescription.swift`.

---------

Co-authored-by: Honza Dvorsky <[email protected]>
Co-authored-by: Honza Dvorsky <[email protected]>

Sources/_OpenAPIGeneratorCore/Translator/Operations/OperationDescription.swift

@@ -144,10 +144,31 @@ extension OperationDescription {
         )
     }
 
-    /// Returns parameters from both the path item level
-    /// and the operation level.
+    /// Merged parameters from both the path item level and the operation level.
+    /// If duplicate parameters exist, only the parameters from the operation level are preserved.
+    ///
+    /// - Returns: An array of merged path item and operation level parameters without duplicates.
+    /// - Throws: When an invalid JSON reference is found.
     var allParameters: [UnresolvedParameter] {
-        pathParameters + operation.parameters
+        get throws {
+            var mergedParameters: [UnresolvedParameter] = []
+            var uniqueIdentifiers: Set<String> = []
+
+            let allParameters = pathParameters + operation.parameters
+            for parameter in allParameters.reversed() {
+                let resolvedParameter = try parameter.resolve(in: components)
+                let identifier = resolvedParameter.location.rawValue + ":" + resolvedParameter.name
+
+                guard !uniqueIdentifiers.contains(identifier) else {
+                    continue
+                }
+
+                mergedParameters.append(parameter)
+                uniqueIdentifiers.insert(identifier)
+            }
+
+            return mergedParameters.reversed()
+        }
     }
 
     /// Returns all parameters by resolving any parameter references first.

Tests/OpenAPIGeneratorCoreTests/Translator/Operations/Test_OperationDescription.swift

@@ -0,0 +1,133 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+import OpenAPIKit30
+import XCTest
+@testable import _OpenAPIGeneratorCore
+
+final class Test_OperationDescription: Test_Core {
+
+    func testAllParameters_duplicates_retainOnlyOperationParameters() throws {
+        let pathLevelParameter = UnresolvedParameter.b(
+            OpenAPI.Parameter(
+                name: "test",
+                context: .query(required: false),
+                schema: .integer
+            )
+        )
+        let operationLevelParameter = UnresolvedParameter.b(
+            OpenAPI.Parameter(
+                name: "test",
+                context: .query(required: false),
+                schema: .string
+            )
+        )
+
+        let pathItem = OpenAPI.PathItem(
+            parameters: [pathLevelParameter],
+            get: .init(
+                parameters: [operationLevelParameter],
+                requestBody: .b(.init(content: [:])),
+                responses: [:]
+            ),
+            vendorExtensions: [:]
+        )
+        let allParameters = try _test(pathItem)
+
+        XCTAssertEqual(allParameters, [operationLevelParameter])
+    }
+
+    func testAllParameters_duplicates_keepsDuplicatesFromDifferentLocation() throws {
+        let pathLevelParameter = UnresolvedParameter.b(
+            OpenAPI.Parameter(
+                name: "test",
+                context: .query(required: false),
+                schema: .integer
+            )
+        )
+        let operationLevelParameter = UnresolvedParameter.b(
+            OpenAPI.Parameter(
+                name: "test",
+                context: .path,
+                schema: .string
+            )
+        )
+
+        let pathItem = OpenAPI.PathItem(
+            parameters: [pathLevelParameter],
+            get: .init(
+                parameters: [operationLevelParameter],
+                requestBody: .b(.init(content: [:])),
+                responses: [:]
+            ),
+            vendorExtensions: [:]
+        )
+        let allParameters = try _test(pathItem)
+
+        XCTAssertEqual(allParameters, [pathLevelParameter, operationLevelParameter])
+    }
+
+    func testAllParameters_duplicates_ordering() throws {
+        let pathLevelParameter = UnresolvedParameter.b(
+            OpenAPI.Parameter(
+                name: "test1",
+                context: .query(required: false),
+                schema: .integer
+            )
+        )
+        let duplicatedParameter = UnresolvedParameter.b(
+            OpenAPI.Parameter(
+                name: "test2",
+                context: .query(required: false),
+                schema: .integer
+            )
+        )
+        let operationLevelParameter = UnresolvedParameter.b(
+            OpenAPI.Parameter(
+                name: "test3",
+                context: .query(required: false),
+                schema: .string
+            )
+        )
+
+        let pathItem = OpenAPI.PathItem(
+            parameters: [pathLevelParameter, duplicatedParameter],
+            get: .init(
+                parameters: [duplicatedParameter, operationLevelParameter],
+                requestBody: .b(.init(content: [:])),
+                responses: [:]
+            ),
+            vendorExtensions: [:]
+        )
+        let allParameters = try _test(pathItem)
+
+        XCTAssertEqual(allParameters, [pathLevelParameter, duplicatedParameter, operationLevelParameter])
+    }
+
+    private func _test(_ pathItem: OpenAPI.PathItem) throws -> [UnresolvedParameter] {
+        guard let endpoint = pathItem.endpoints.first else {
+            XCTFail("Unable to retrieve the path item first endpoint.")
+            return []
+        }
+
+        let operationDescription = OperationDescription(
+            path: .init(["/test"]),
+            endpoint: endpoint,
+            pathParameters: pathItem.parameters,
+            components: .init(),
+            asSwiftSafeName: { $0 }
+        )
+
+        return try operationDescription.allParameters
+    }
+}