rdar://139080443 (Fix incorrect unicode escape sequence encodings) (#1036)

Author: kperryua

Sources/FoundationEssentials/JSON/JSONWriter.swift

@@ -140,11 +140,11 @@ internal struct JSONWriter {
                     appendAccumulatedBytes(from: mark, to: cursor, followedByContentsOf: [._backslash, UInt8(ascii: "t")])
                 case 0x0...0xf:
                     appendAccumulatedBytes(from: mark, to: cursor, followedByContentsOf: [._backslash, UInt8(ascii: "u"), UInt8(ascii: "0"), UInt8(ascii: "0"), UInt8(ascii: "0")])
-                    writer(ascii: valueToASCII(cursor.pointee / 16))
+                    writer(ascii: valueToASCII(cursor.pointee))
                 case 0x10...0x1f:
                     appendAccumulatedBytes(from: mark, to: cursor, followedByContentsOf: [._backslash, UInt8(ascii: "u"), UInt8(ascii: "0"), UInt8(ascii: "0")])
-                    writer(ascii: valueToASCII(cursor.pointee % 16))
                     writer(ascii: valueToASCII(cursor.pointee / 16))
+                    writer(ascii: valueToASCII(cursor.pointee % 16))
                 default:
                     // Accumulate this byte
                     cursor += 1

Tests/FoundationEssentialsTests/JSONEncoderTests.swift

@@ -1159,12 +1159,22 @@ final class JSONEncoderTests : XCTestCase {
             "\"\\u00E9\"" : "é",
 
             // G-clef (UTF16 surrogate pair) 0x1D11E
-            "\"\\uD834\\uDD1E\"" : "𝄞"
+            "\"\\uD834\\uDD1E\"" : "𝄞",
         ]
         for (input, expectedOutput) in testCases {
             _test(JSONString: input, to: expectedOutput)
         }
     }
+    
+    func test_encodingJSONHexUnicodeEscapes() throws {
+        let testCases = [
+            "\u{0001}\u{0002}\u{0003}": "\"\\u0001\\u0002\\u0003\"",
+            "\u{0010}\u{0018}\u{001f}": "\"\\u0010\\u0018\\u001f\"",
+        ]
+        for (string, json) in testCases {
+            _testRoundTrip(of: string, expectedJSON: Data(json.utf8))
+        }
+    }
 
     func test_JSONBadUnicodeEscapes() {
         let badCases = ["\\uD834", "\\uD834hello", "hello\\uD834", "\\uD834\\u1221", "\\uD8", "\\uD834x\\uDD1E"]