Add support for sorted keys.

Sources/URLEncodedForm/Codable/URLEncodedFormEncoder.swift

@@ -11,6 +11,22 @@
 /// See [Mozilla's](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/POST) docs for more information about
 /// url-encoded forms.
 public final class URLEncodedFormEncoder: DataEncoder {
+    /// The formatting of the output data.
+    public struct OutputFormatting : OptionSet {
+        /// The format's default value.
+        public let rawValue: UInt
+
+        /// Creates an OutputFormatting value with the given raw value.
+        public init(rawValue: UInt) {
+            self.rawValue = rawValue
+        }
+
+        /// Produce output with dictionary keys sorted in lexicographic order.
+        public static let sortedKeys = OutputFormatting(rawValue: 1 << 1)
+    }
+
+    public var outputFormatting: OutputFormatting = []
+
     /// Create a new `URLEncodedFormEncoder`.
     public init() {}
 
@@ -28,7 +44,7 @@ public final class URLEncodedFormEncoder: DataEncoder {
         let context = URLEncodedFormDataContext(.dict([:]))
         let encoder = _URLEncodedFormEncoder(context: context, codingPath: [])
         try encodable.encode(to: encoder)
-        let serializer = URLEncodedFormSerializer()
+        let serializer = URLEncodedFormSerializer(sortedKeys: self.outputFormatting.contains(.sortedKeys))
         guard case .dict(let dict) = context.data else {
             throw URLEncodedFormError(
                 identifier: "invalidTopLevel",

Sources/URLEncodedForm/Data/URLEncodedFormSerializer.swift

@@ -3,19 +3,37 @@ import Bits
 /// Converts `[String: URLEncodedFormData]` structs to `Data`.
 final class URLEncodedFormSerializer {
     /// Default form url encoded serializer.
-    static let `default` = URLEncodedFormSerializer()
+    static let `default` = URLEncodedFormSerializer(sortedKeys: false)
+
+    private var sortedKeys: Bool
 
     /// Create a new form-urlencoded data serializer.
-    init() {}
+    init(sortedKeys: Bool) {
+        self.sortedKeys = sortedKeys
+    }
 
     /// Serializes the data.
     func serialize(_ URLEncodedFormEncoded: [String: URLEncodedFormData]) throws -> Data {
         var data: [Data] = []
-        for (key, val) in URLEncodedFormEncoded {
-            let key = try key.urlEncodedFormEncoded()
-            let subdata = try serialize(val, forKey: key)
-            data.append(subdata)
+
+        if self.sortedKeys {
+            let elements = URLEncodedFormEncoded.sorted { (left, right) -> Bool in
+                return left.key.compare(right.key, options: [.numeric, .caseInsensitive, .forcedOrdering]) == .orderedAscending
+            }
+
+            for (key, val) in elements {
+                let key = try key.urlEncodedFormEncoded()
+                let subdata = try serialize(val, forKey: key)
+                data.append(subdata)
+            }
+        } else {
+            for (key, val) in URLEncodedFormEncoded {
+                let key = try key.urlEncodedFormEncoded()
+                let subdata = try serialize(val, forKey: key)
+                data.append(subdata)
+            }
         }
+
         return data.joinedWithAmpersands()
     }