[Foundation] Implement NSSet#description

Because `NSSet#description` was not implemented.

Author: rikusouda

Docs/Status.md

@@ -166,7 +166,7 @@ There is no _Complete_ status for test coverage because there are always additio
     | `NSDictionary`        | Mostly Complete | Incomplete    | `NSCoding` with non-keyed-coding archivers, `descriptionInStringsFileFormat`, `sharedKeySet(forKeys:)`, and reading/writing to files/URLs remain unimplemented      |
     | `NSMutableDictionary` | Mostly Complete | Incomplete    | `descriptionInStringsFileFormat`, `sharedKeySet(forKeys:)`, and reading/writing to files/URLs remain unimplemented                                                  |
     | `NSCFDictionary`      | N/A             | N/A           | For internal use only                                                                                                                                               |
-    | `NSSet`               | Mostly Complete | Incomplete    | `description(withLocale:)` and `customMirror` remain unimplemented                                                                                                  |
+    | `NSSet`               | Mostly Complete | Incomplete    | `customMirror` remain unimplemented                                                                                                  |
     | `NSMutableSet`        | Mostly Complete | Incomplete    | `init?(coder:)` remains unimplemented                                                                                                                               |
     | `NSCountedSet`        | Mostly Complete | Incomplete    | `init?(coder:)` remains unimplemented                                                                                                                               |
     | `NSCFSet`             | N/A             | N/A           | For internal use only                                                                                                                                               |

Foundation/NSSet.swift

@@ -107,9 +107,46 @@ open class NSSet : NSObject, NSCopying, NSMutableCopying, NSSecureCoding, NSCodi
         return true
     }
 
-    open func description(withLocale locale: Locale?) -> String { 
-      // NSUnimplemented() 
-      return description
+    override open var description: String {
+        return description(withLocale: nil)
+    }
+    
+    open func description(withLocale locale: Locale?) -> String {
+        return description(withLocale: locale, indent: 0)
+    }
+    
+    private func description(withLocale locale: Locale?, indent level: Int) -> String {
+        var descriptions = [String]()
+        
+        for obj in self._storage {
+            if let string = obj as? String {
+                descriptions.append(string)
+            } else if let array = obj as? [Any] {
+                descriptions.append(NSArray(array: array).description(withLocale: locale, indent: level + 1))
+            } else if let dict = obj as? [AnyHashable : Any] {
+                descriptions.append(dict._bridgeToObjectiveC().description(withLocale: locale, indent: level + 1))
+            } else if let set = obj as? Set<AnyHashable> {
+                descriptions.append(set._bridgeToObjectiveC().description(withLocale: locale, indent: level + 1))
+            } else {
+                descriptions.append("\(obj)")
+            }
+        }
+        var indent = ""
+        for _ in 0..<level {
+            indent += "    "
+        }
+        var result = indent + "{(\n"
+        let cnt = count
+        for idx in 0..<cnt {
+            result += indent + "    " + descriptions[idx]
+            if idx + 1 < cnt {
+                result += ",\n"
+            } else {
+                result += "\n"
+            }
+        }
+        result += indent + ")}"
+        return result
     }
 
     override open var _cfTypeID: CFTypeID {

TestFoundation/TestNSSet.swift

@@ -25,7 +25,8 @@ class TestNSSet : XCTestCase {
             ("test_CountedSetRemoveObject", test_CountedSetRemoveObject),
             ("test_CountedSetCopying", test_CountedSetCopying),
             ("test_mutablesetWithDictionary", test_mutablesetWithDictionary),
-            ("test_Subsets", test_Subsets)
+            ("test_Subsets", test_Subsets),
+            ("test_description", test_description)
         ]
     }
 
@@ -237,4 +238,24 @@ class TestNSSet : XCTestCase {
         XCTAssert(otherSet.isSubset(of: otherOtherSet))
         XCTAssertFalse(newSet.isSubset(of: otherSet as! Set<AnyHashable>))
     }
+    
+    func test_description() {
+        let array = NSArray(array: ["array_element1", "array_element2"])
+        let dictionary = NSDictionary(dictionary: ["key1": "value1", "key2": "value2"])
+        let innerSet = NSSet(array: [4444, 5555])
+        let set: NSSet = NSSet(array: [array, dictionary, innerSet, 1111, 2222, 3333])
+        
+        let description = set.description
+
+        XCTAssertTrue(String(description.prefix(2)) == "{(")
+        XCTAssertTrue(String(description.suffix(2)) == ")}")
+        XCTAssertTrue(description.contains("        (\n        array_element1,\n        array_element2\n    )"))
+        XCTAssertTrue(description.contains("        key1 = value1"))
+        XCTAssertTrue(description.contains("        key2 = value2"))
+        XCTAssertTrue(description.contains("        4444"))
+        XCTAssertTrue(description.contains("        5555"))
+        XCTAssertTrue(description.contains("    1111"))
+        XCTAssertTrue(description.contains("    2222"))
+        XCTAssertTrue(description.contains("    3333"))
+    }
 }