Skip duplicate enum values (#674)

### Motivation

Some OpenAPI docs, usually through imperfect conversion from another
representation, end up with duplicate raw values in `enum` schemas.

The OpenAPI specification (by referencing the JSON Schema specification)
[says](https://datatracker.ietf.org/doc/html/draft-wright-json-schema-validation-00#section-5.20):

```
The value of this keyword MUST be an array.  This array SHOULD have at least one element.  Elements in the array SHOULD be unique.
```

So elements **should** be unique, but don't have to be. Today the
generator fails to generate documents with such duplicate values.

### Modifications

Gracefully handle duplicate values by emitting a warning diagnostic and
skipping it.

### Result

This unblocks generating OpenAPI documents with duplicate enum values,
such as the [Bluesky OpenAPI
doc](https://github.com/bluesky-social/bsky-docs/blob/main/atproto-openapi-types/spec/api.json).

### Test Plan

Added a unit test for duplicates.

Author: czechboy0

Package.swift

@@ -49,6 +49,7 @@ let package = Package(
 
         // General algorithms
         .package(url: "https://github.com/apple/swift-algorithms", from: "1.2.0"),
+        .package(url: "https://github.com/apple/swift-collections", from: "1.1.4"),
 
         // Read OpenAPI documents
         .package(url: "https://github.com/mattpolzin/OpenAPIKit", from: "3.3.0"),
@@ -72,7 +73,9 @@ let package = Package(
                 .product(name: "OpenAPIKit", package: "OpenAPIKit"),
                 .product(name: "OpenAPIKit30", package: "OpenAPIKit"),
                 .product(name: "OpenAPIKitCompat", package: "OpenAPIKit"),
-                .product(name: "Algorithms", package: "swift-algorithms"), .product(name: "Yams", package: "Yams"),
+                .product(name: "Algorithms", package: "swift-algorithms"),
+                .product(name: "OrderedCollections", package: "swift-collections"),
+                .product(name: "Yams", package: "Yams"),
             ],
             swiftSettings: swiftSettings
         ),

Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateRawEnum.swift

@@ -0,0 +1,140 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 OpenAPIKit
+import OrderedCollections
+
+/// The backing type of a raw enum.
+enum RawEnumBackingType {
+
+    /// Backed by a `String`.
+    case string
+
+    /// Backed by an `Int`.
+    case integer
+}
+
+/// The extracted enum value's identifier.
+private enum EnumCaseID: Hashable, CustomStringConvertible {
+
+    /// A string value.
+    case string(String)
+
+    /// An integer value.
+    case integer(Int)
+
+    var description: String {
+        switch self {
+        case .string(let value): return "\"\(value)\""
+        case .integer(let value): return String(value)
+        }
+    }
+}
+
+/// A wrapper for the metadata about the raw enum case.
+private struct EnumCase {
+
+    /// Used for checking uniqueness.
+    var id: EnumCaseID
+
+    /// The raw Swift-safe name for the case.
+    var caseName: String
+
+    /// The literal value of the enum case.
+    var literal: LiteralDescription
+}
+
+extension EnumCase: Equatable { static func == (lhs: EnumCase, rhs: EnumCase) -> Bool { lhs.id == rhs.id } }
+
+extension EnumCase: Hashable { func hash(into hasher: inout Hasher) { hasher.combine(id) } }
+
+extension FileTranslator {
+
+    /// Returns a declaration of the specified raw value-based enum schema.
+    /// - Parameters:
+    ///   - backingType: The backing type of the enum.
+    ///   - typeName: The name of the type to give to the declared enum.
+    ///   - userDescription: A user-specified description from the OpenAPI
+    ///   document.
+    ///   - isNullable: Whether the enum schema is nullable.
+    ///   - allowedValues: The enumerated allowed values.
+    /// - Throws: A `GenericError` if a disallowed value is encountered.
+    /// - Returns: A declaration of the specified raw value-based enum schema.
+    func translateRawEnum(
+        backingType: RawEnumBackingType,
+        typeName: TypeName,
+        userDescription: String?,
+        isNullable: Bool,
+        allowedValues: [AnyCodable]
+    ) throws -> Declaration {
+        var cases: OrderedSet<EnumCase> = []
+        func addIfUnique(id: EnumCaseID, caseName: String) throws {
+            let literal: LiteralDescription
+            switch id {
+            case .string(let string): literal = .string(string)
+            case .integer(let int): literal = .int(int)
+            }
+            guard cases.append(.init(id: id, caseName: caseName, literal: literal)).inserted else {
+                try diagnostics.emit(
+                    .warning(
+                        message: "Duplicate enum value, skipping",
+                        context: ["id": "\(id)", "foundIn": typeName.description]
+                    )
+                )
+                return
+            }
+        }
+        for anyValue in allowedValues.map(\.value) {
+            switch backingType {
+            case .string:
+                // In nullable enum schemas, empty strings are parsed as Void.
+                // This is unlikely to be fixed, so handling that case here.
+                // https://github.com/apple/swift-openapi-generator/issues/118
+                if isNullable && anyValue is Void {
+                    try addIfUnique(id: .string(""), caseName: context.asSwiftSafeName(""))
+                } else {
+                    guard let rawValue = anyValue as? String else {
+                        throw GenericError(message: "Disallowed value for a string enum '\(typeName)': \(anyValue)")
+                    }
+                    let caseName = context.asSwiftSafeName(rawValue)
+                    try addIfUnique(id: .string(rawValue), caseName: caseName)
+                }
+            case .integer:
+                let rawValue: Int
+                if let intRawValue = anyValue as? Int {
+                    rawValue = intRawValue
+                } else if let stringRawValue = anyValue as? String, let intRawValue = Int(stringRawValue) {
+                    rawValue = intRawValue
+                } else {
+                    throw GenericError(message: "Disallowed value for an integer enum '\(typeName)': \(anyValue)")
+                }
+                let caseName = rawValue < 0 ? "_n\(abs(rawValue))" : "_\(rawValue)"
+                try addIfUnique(id: .integer(rawValue), caseName: caseName)
+            }
+        }
+        let baseConformance: String
+        switch backingType {
+        case .string: baseConformance = Constants.RawEnum.baseConformanceString
+        case .integer: baseConformance = Constants.RawEnum.baseConformanceInteger
+        }
+        let conformances = [baseConformance] + Constants.RawEnum.conformances
+        return try translateRawRepresentableEnum(
+            typeName: typeName,
+            conformances: conformances,
+            userDescription: userDescription,
+            cases: cases.map { ($0.caseName, $0.literal) },
+            unknownCaseName: nil,
+            unknownCaseDescription: nil
+        )
+    }
+}

Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateStringEnum.swift

@@ -1310,6 +1310,33 @@ final class SnippetBasedReferenceTests: XCTestCase {
         )
     }
 
+    func testComponentsSchemasStringEnumWithDuplicates() throws {
+        try self.assertSchemasTranslation(
+            ignoredDiagnosticMessages: ["Duplicate enum value, skipping"],
+            """
+            schemas:
+              MyEnum:
+                type: string
+                enum:
+                  - one
+                  - two
+                  - three
+                  - two
+                  - four
+            """,
+            """
+            public enum Schemas {
+                @frozen public enum MyEnum: String, Codable, Hashable, Sendable, CaseIterable {
+                    case one = "one"
+                    case two = "two"
+                    case three = "three"
+                    case four = "four"
+                }
+            }
+            """
+        )
+    }
+
     func testComponentsSchemasIntEnum() throws {
         try self.assertSchemasTranslation(
             """