JSONSerialization: some gardening for parseUnicodeSequence(_:)

Author: broadwaylamb

Foundation/JSONSerialization.swift

@@ -741,23 +741,40 @@ private struct JSONReader {
         let isLeadSurrogate = UTF16.isLeadSurrogate(codeUnit)
         let isTrailSurrogate = UTF16.isTrailSurrogate(codeUnit)
 
-        if !isLeadSurrogate && !isTrailSurrogate {
+        guard isLeadSurrogate || isTrailSurrogate else {
+            // The code units that are neither lead surrogates nor trail surrogates
+            // form valid unicode scalars.
             return (String(UnicodeScalar(codeUnit)!), index)
         }
 
-        guard !isTrailSurrogate else {
-            throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
-                "NSDebugDescription" : "Unable to convert unicode escape sequence (no high-surrogate code point) to UTF8-encoded character at position \(source.distanceFromStart(input))"
-            ])
+        // Surrogates must always come in pairs.
+
+        guard isLeadSurrogate else {
+            // Trail surrogate must supercede lead surrogate
+            throw NSError(domain: NSCocoaErrorDomain,
+                          code: CocoaError.propertyListReadCorrupt.rawValue,
+                          userInfo: [
+                            "NSDebugDescription" : """
+                             Unable to convert unicode escape sequence (no high-surrogate code point) \
+                             to UTF8-encoded character at position \(source.distanceFromStart(input))
+                             """
+                          ])
         }
 
-        guard let (trailCodeUnit, finalIndex) = try consumeASCIISequence("\\u", input: index).flatMap(parseCodeUnit), UTF16.isTrailSurrogate(trailCodeUnit) else {
-            throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
-                "NSDebugDescription" : "Unable to convert unicode escape sequence (no low-surrogate code point) to UTF8-encoded character at position \(source.distanceFromStart(input))"
-            ])
+        guard let (trailCodeUnit, finalIndex) = try consumeASCIISequence("\\u", input: index).flatMap(parseCodeUnit),
+              UTF16.isTrailSurrogate(trailCodeUnit) else {
+
+            throw NSError(domain: NSCocoaErrorDomain,
+                          code: CocoaError.propertyListReadCorrupt.rawValue,
+                          userInfo: [
+                            "NSDebugDescription" : """
+                             Unable to convert unicode escape sequence (no low-surrogate code point) \
+                             to UTF8-encoded character at position \(source.distanceFromStart(input))
+                             """
+                          ])
         }
 
-        let highValue = (UInt32(codeUnit  - 0xD800) << 10)
+        let highValue = (UInt32(codeUnit - 0xD800) << 10)
         let lowValue  =  UInt32(trailCodeUnit - 0xDC00)
         return (String(UnicodeScalar(highValue + lowValue + 0x10000)!), finalIndex)
     }