Make identifier cache key creation deterministic

We shouldn't rely on deterministic behavior when calling `.map`  on a Set.

It seems possible we shouldn't be including langauge strings when creating identifiers for the reference cache but we can look at that in a future PR.

Author: ethan-kusters

Sources/SwiftDocC/Model/Identifier.swift

@@ -126,7 +126,7 @@ public struct ResolvedTopicReference: Hashable, Codable, Equatable, CustomString
 
     /// The reference cache key
     var cacheKey: String {
-        return "\(path):\(fragment ?? ""):\(sourceLanguages.map(\.id))"
+        return Self.cacheKey(path: path, fragment: fragment, sourceLanguages: sourceLanguages)
     }
 
     /// - Note: The `path` parameter is escaped to a path readable string.
@@ -137,7 +137,7 @@ public struct ResolvedTopicReference: Hashable, Codable, Equatable, CustomString
     public init(bundleIdentifier: String, path: String, fragment: String? = nil, sourceLanguages: Set<SourceLanguage>) {
         precondition(!sourceLanguages.isEmpty, "ResolvedTopicReference.sourceLanguages cannot be empty")
         // Check for a cached instance of the reference
-        let key = "\(path):\(fragment ?? ""):\(sourceLanguages.map(\.id))"
+        let key = Self.cacheKey(path: path, fragment: fragment, sourceLanguages: sourceLanguages)
         let cached = Self.sharedPool.sync { $0[bundleIdentifier]?[key] }
         if let resolved = cached {
             self = resolved
@@ -155,6 +155,20 @@ public struct ResolvedTopicReference: Hashable, Codable, Equatable, CustomString
         Self.sharedPool.sync { $0[bundleIdentifier, default: [:]][cacheKey] = self }
     }
 
+    private static func cacheKey(
+        path: String,
+        fragment: String?,
+        sourceLanguages: Set<SourceLanguage>
+    ) -> String {
+        let sourceLanguagesString = sourceLanguages.map(\.id).sorted().joined(separator: "-")
+        
+        if let fragment = fragment {
+            return "\(path):\(fragment):\(sourceLanguagesString)"
+        } else {
+            return "\(path):\(sourceLanguagesString)"
+        }
+    }
+    
     /// The topic URL as you would write in a link.
     private (set) public var url: URL! = nil
 

Tests/SwiftDocCTests/Model/IdentifierTests.swift

@@ -121,4 +121,20 @@ class IdentifierTests: XCTestCase {
         ref1 = ref1.removingLastPathComponent()
         XCTAssertEqual(ref1.absoluteString, "doc://bundle/MyClass")
     }
+    
+    func testMixedLanguageCacheKeyCreationIsDeterministic() {
+        for _ in 0..<20 {
+            let firstReference = ResolvedTopicReference(
+                bundleIdentifier: "bundle",
+                path: "/MyClass",
+                sourceLanguages: [.swift, .objectiveC]
+            )
+            
+            XCTAssertEqual(
+                firstReference.cacheKey,
+                "/MyClass:occ-swift",
+                "The reference's cache key is not deterministic."
+            )
+        }
+    }
 }