Remove unnecessary genericism and generic constraints.

Motivation:

Early on in the design of Structured Headers I made a conscious effort
to try to avoid transforming types in the low-level API. This design was
built on the assumption that strings, tokens, and some other data types
could be built entirely as slices of the backing data.

Unfortunately, this isn't true: because of the need to escape chars in
Strings, it became necessary to start using more complex types in Item.
Eventually, we ended up in a place where the only field in Item that
actually stored backing data was `.undecodedByteSequence`.

This field forced almost the entire set of types in the
StructuredHeaders module to be generic, just for the sake of carrying
this one slice type around. This prevented powerful optimisations,
bloated code size, and forced additional unnecessary heap allocations.
It also forced us to constrain the types used in the parser to those
where the subsequence type was Hashable, which excluded some really
important types (like `String.UTF8View`).

Fundamentally, this genericism was hurting more than it helped at this
point, so I bit the bullet and replaced the slice type with `String` for
`undecodedByteSequence`. This led to a huge cascading removal of generic
type parameters for a wide range of types, as well as the removal of the
"must have hashable subsequence" constraints on the parser data type.
This lifts an enormous amount of complexity out of the module at the
extremely minor cost of forcing an allocation whenever we have a lot of
undecoded binary data: not a huge cost, in my view, and well worth what
we got back in return.

Modifications:

- Remove generic type parameter in BaseItem.
- Remove generic types everywhere else.
- Remove hashable constraint.

Result:

Better, faster, simpler code.

Author: Lukasa

Sources/CodableStructuredHeaders/Decoder/BareInnerListDecoder.swift

@@ -14,14 +14,14 @@
 import Foundation
 import StructuredHeaders
 
-struct BareInnerListDecoder<BaseData: RandomAccessCollection> where BaseData.Element == UInt8, BaseData.SubSequence: Hashable {
-    private var list: BareInnerList<BaseData.SubSequence>
+struct BareInnerListDecoder<BaseData: RandomAccessCollection> where BaseData.Element == UInt8 {
+    private var list: BareInnerList
 
     private var currentOffset: Int
 
     private var decoder: _StructuredFieldDecoder<BaseData>
 
-    init(_ list: BareInnerList<BaseData.SubSequence>, decoder: _StructuredFieldDecoder<BaseData>) {
+    init(_ list: BareInnerList, decoder: _StructuredFieldDecoder<BaseData>) {
         self.list = list
         self.currentOffset = 0
         self.decoder = decoder

Sources/CodableStructuredHeaders/Decoder/BareItemDecoder.swift

@@ -14,12 +14,12 @@
 import Foundation
 import StructuredHeaders
 
-struct BareItemDecoder<BaseData: RandomAccessCollection> where BaseData.Element == UInt8, BaseData.SubSequence == BaseData, BaseData: Hashable {
-    private var item: BareItem<BaseData>
+struct BareItemDecoder {
+    private var item: BareItem
 
     private var _codingPath: [_StructuredHeaderCodingKey]
 
-    init(_ item: BareItem<BaseData>, codingPath: [_StructuredHeaderCodingKey]) {
+    init(_ item: BareItem, codingPath: [_StructuredHeaderCodingKey]) {
         self.item = item
         self._codingPath = codingPath
     }
@@ -102,7 +102,7 @@ extension BareItemDecoder: SingleValueDecodingContainer {
             throw StructuredHeaderError.invalidTypeForItem
         }
 
-        guard let decoded = Data(base64Encoded: Data(data)) else {
+        guard let decoded = Data(base64Encoded: data) else {
             throw StructuredHeaderError.invalidByteSequence
         }
 

Sources/CodableStructuredHeaders/Decoder/DictionaryKeyedContainer.swift

@@ -14,12 +14,12 @@
 import Foundation
 import StructuredHeaders
 
-struct DictionaryKeyedContainer<Key: CodingKey, BaseData: RandomAccessCollection> where BaseData.Element == UInt8, BaseData.SubSequence: Hashable {
-    private var dictionary: OrderedMap<String, ItemOrInnerList<BaseData.SubSequence>>
+struct DictionaryKeyedContainer<Key: CodingKey, BaseData: RandomAccessCollection> where BaseData.Element == UInt8 {
+    private var dictionary: OrderedMap<String, ItemOrInnerList>
 
     private var decoder: _StructuredFieldDecoder<BaseData>
 
-    init(_ dictionary: OrderedMap<String, ItemOrInnerList<BaseData.SubSequence>>, decoder: _StructuredFieldDecoder<BaseData>) {
+    init(_ dictionary: OrderedMap<String, ItemOrInnerList>, decoder: _StructuredFieldDecoder<BaseData>) {
         self.dictionary = dictionary
         self.decoder = decoder
     }

Sources/CodableStructuredHeaders/Decoder/KeyedInnerListDecoder.swift

@@ -19,12 +19,12 @@ private let keyedInnerListDecoderSupportedKeys = ["items", "parameters"]
 /// Used when someone has requested a keyed decoder for a property of inner list type.
 ///
 /// There are only two valid keys for this: "items" and "parameters".
-struct KeyedInnerListDecoder<Key: CodingKey, BaseData: RandomAccessCollection> where BaseData.Element == UInt8, BaseData.SubSequence: Hashable {
-    private var innerList: InnerList<BaseData.SubSequence>
+struct KeyedInnerListDecoder<Key: CodingKey, BaseData: RandomAccessCollection> where BaseData.Element == UInt8 {
+    private var innerList: InnerList
 
     private var decoder: _StructuredFieldDecoder<BaseData>
 
-    init(_ innerList: InnerList<BaseData.SubSequence>, decoder: _StructuredFieldDecoder<BaseData>) {
+    init(_ innerList: InnerList, decoder: _StructuredFieldDecoder<BaseData>) {
         self.innerList = innerList
         self.decoder = decoder
     }

Sources/CodableStructuredHeaders/Decoder/KeyedItemDecoder.swift

@@ -19,12 +19,12 @@ private let keyedItemDecoderSupportedKeys = ["item", "parameters"]
 /// Used when someone has requested a keyed decoder for a property of item type.
 ///
 /// There are only two valid keys for this: "item" and "parameters".
-struct KeyedItemDecoder<Key: CodingKey, BaseData: RandomAccessCollection> where BaseData.Element == UInt8, BaseData.SubSequence: Hashable {
-    private var item: Item<BaseData.SubSequence>
+struct KeyedItemDecoder<Key: CodingKey, BaseData: RandomAccessCollection> where BaseData.Element == UInt8 {
+    private var item: Item
 
     private var decoder: _StructuredFieldDecoder<BaseData>
 
-    init(_ item: Item<BaseData.SubSequence>, decoder: _StructuredFieldDecoder<BaseData>) {
+    init(_ item: Item, decoder: _StructuredFieldDecoder<BaseData>) {
         self.item = item
         self.decoder = decoder
     }

Sources/CodableStructuredHeaders/Decoder/KeyedTopLevelListDecoder.swift

@@ -19,12 +19,12 @@ private let keyedTopLevelListDecoderSupportedKeys = ["items"]
 /// Used when someone has requested a keyed decoder for a property of list type.
 ///
 /// There is only one valid key for this: "items".
-struct KeyedTopLevelListDecoder<Key: CodingKey, BaseData: RandomAccessCollection> where BaseData.Element == UInt8, BaseData.SubSequence: Hashable {
-    private var list: [ItemOrInnerList<BaseData.SubSequence>]
+struct KeyedTopLevelListDecoder<Key: CodingKey, BaseData: RandomAccessCollection> where BaseData.Element == UInt8 {
+    private var list: [ItemOrInnerList]
 
     private var decoder: _StructuredFieldDecoder<BaseData>
-
-    init(_ list: [ItemOrInnerList<BaseData.SubSequence>], decoder: _StructuredFieldDecoder<BaseData>) {
+    
+    init(_ list: [ItemOrInnerList], decoder: _StructuredFieldDecoder<BaseData>) {
         self.list = list
         self.decoder = decoder
     }

Sources/CodableStructuredHeaders/Decoder/ParametersDecoder.swift

@@ -14,12 +14,12 @@
 import Foundation
 import StructuredHeaders
 
-struct ParametersDecoder<Key: CodingKey, BaseData: RandomAccessCollection> where BaseData.Element == UInt8, BaseData.SubSequence: Hashable {
-    private var parameters: OrderedMap<String, BareItem<BaseData.SubSequence>>
+struct ParametersDecoder<Key: CodingKey, BaseData: RandomAccessCollection> where BaseData.Element == UInt8 {
+    private var parameters: OrderedMap<String, BareItem>
 
     private var decoder: _StructuredFieldDecoder<BaseData>
 
-    init(_ parameters: OrderedMap<String, BareItem<BaseData.SubSequence>>, decoder: _StructuredFieldDecoder<BaseData>) {
+    init(_ parameters: OrderedMap<String, BareItem>, decoder: _StructuredFieldDecoder<BaseData>) {
         self.parameters = parameters
         self.decoder = decoder
     }

Sources/CodableStructuredHeaders/Decoder/StructuredFieldDecoder.swift

@@ -46,7 +46,7 @@ extension StructuredFieldDecoder {
     ///     - data: The bytes of the structured header field.
     /// - throws: If the header field could not be parsed, or could not be decoded.
     /// - returns: An object of type `StructuredField`.
-    public func decode<StructuredField: StructuredHeaderField, BaseData: RandomAccessCollection>(_ type: StructuredField.Type = StructuredField.self, from data: BaseData) throws -> StructuredField where BaseData.Element == UInt8, BaseData.SubSequence: Hashable {
+    public func decode<StructuredField: StructuredHeaderField, BaseData: RandomAccessCollection>(_ type: StructuredField.Type = StructuredField.self, from data: BaseData) throws -> StructuredField where BaseData.Element == UInt8 {
         switch StructuredField.structuredFieldType {
         case .item:
             return try self.decodeItemField(from: data)
@@ -64,7 +64,7 @@ extension StructuredFieldDecoder {
     ///     - data: The bytes of the structured header field.
     /// - throws: If the header field could not be parsed, or could not be decoded.
     /// - returns: An object of type `StructuredField`.
-    private func decodeDictionaryField<StructuredField: Decodable, BaseData: RandomAccessCollection>(_ type: StructuredField.Type = StructuredField.self, from data: BaseData) throws -> StructuredField where BaseData.Element == UInt8, BaseData.SubSequence: Hashable {
+    private func decodeDictionaryField<StructuredField: Decodable, BaseData: RandomAccessCollection>(_ type: StructuredField.Type = StructuredField.self, from data: BaseData) throws -> StructuredField where BaseData.Element == UInt8 {
         let parser = StructuredFieldParser(data)
         let decoder = _StructuredFieldDecoder(parser, keyDecodingStrategy: self.keyDecodingStrategy)
         try decoder.parseDictionaryField()
@@ -78,7 +78,7 @@ extension StructuredFieldDecoder {
     ///     - data: The bytes of the structured header field.
     /// - throws: If the header field could not be parsed, or could not be decoded.
     /// - returns: An object of type `StructuredField`.
-    private func decodeListField<StructuredField: Decodable, BaseData: RandomAccessCollection>(_ type: StructuredField.Type = StructuredField.self, from data: BaseData) throws -> StructuredField where BaseData.Element == UInt8, BaseData.SubSequence: Hashable {
+    private func decodeListField<StructuredField: Decodable, BaseData: RandomAccessCollection>(_ type: StructuredField.Type = StructuredField.self, from data: BaseData) throws -> StructuredField where BaseData.Element == UInt8 {
         let parser = StructuredFieldParser(data)
         let decoder = _StructuredFieldDecoder(parser, keyDecodingStrategy: self.keyDecodingStrategy)
         try decoder.parseListField()
@@ -92,7 +92,7 @@ extension StructuredFieldDecoder {
     ///     - data: The bytes of the structured header field.
     /// - throws: If the header field could not be parsed, or could not be decoded.
     /// - returns: An object of type `StructuredField`.
-    private func decodeItemField<StructuredField: Decodable, BaseData: RandomAccessCollection>(_ type: StructuredField.Type = StructuredField.self, from data: BaseData) throws -> StructuredField where BaseData.Element == UInt8, BaseData.SubSequence: Hashable {
+    private func decodeItemField<StructuredField: Decodable, BaseData: RandomAccessCollection>(_ type: StructuredField.Type = StructuredField.self, from data: BaseData) throws -> StructuredField where BaseData.Element == UInt8 {
         let parser = StructuredFieldParser(data)
         let decoder = _StructuredFieldDecoder(parser, keyDecodingStrategy: self.keyDecodingStrategy)
         try decoder.parseItemField()
@@ -111,7 +111,7 @@ extension StructuredFieldDecoder {
     }
 }
 
-class _StructuredFieldDecoder<BaseData: RandomAccessCollection> where BaseData.Element == UInt8, BaseData.SubSequence: Hashable {
+class _StructuredFieldDecoder<BaseData: RandomAccessCollection> where BaseData.Element == UInt8 {
     private var parser: StructuredFieldParser<BaseData>
 
     // For now we use a stack here because the CoW operations on Array would suck. Ideally I'd just have us decode
@@ -220,13 +220,13 @@ extension _StructuredFieldDecoder: Decoder {
 extension _StructuredFieldDecoder {
     /// The basic elements that make up a Structured Header
     fileprivate enum Element {
-        case dictionary(OrderedMap<String, ItemOrInnerList<BaseData.SubSequence>>)
-        case list([ItemOrInnerList<BaseData.SubSequence>])
-        case item(Item<BaseData.SubSequence>)
-        case innerList(InnerList<BaseData.SubSequence>)
-        case bareItem(BareItem<BaseData.SubSequence>)
-        case bareInnerList(BareInnerList<BaseData.SubSequence>)
-        case parameters(OrderedMap<String, BareItem<BaseData.SubSequence>>)
+        case dictionary(OrderedMap<String, ItemOrInnerList>)
+        case list([ItemOrInnerList])
+        case item(Item)
+        case innerList(InnerList)
+        case bareItem(BareItem)
+        case bareInnerList(BareInnerList)
+        case parameters(OrderedMap<String, BareItem>)
 
         func innerElement(for key: _StructuredHeaderCodingKey) throws -> Element {
             switch self {

Sources/CodableStructuredHeaders/Decoder/TopLevelListDecoder.swift

@@ -14,14 +14,14 @@
 import Foundation
 import StructuredHeaders
 
-struct TopLevelListDecoder<BaseData: RandomAccessCollection> where BaseData.Element == UInt8, BaseData.SubSequence: Hashable {
-    private var list: [ItemOrInnerList<BaseData.SubSequence>]
+struct TopLevelListDecoder<BaseData: RandomAccessCollection> where BaseData.Element == UInt8 {
+    private var list: [ItemOrInnerList]
 
     private var currentOffset: Int
 
     private var decoder: _StructuredFieldDecoder<BaseData>
 
-    init(_ list: [ItemOrInnerList<BaseData.SubSequence>], decoder: _StructuredFieldDecoder<BaseData>) {
+    init(_ list: [ItemOrInnerList], decoder: _StructuredFieldDecoder<BaseData>) {
         self.list = list
         self.currentOffset = 0
         self.decoder = decoder

Sources/CodableStructuredHeaders/Encoder/StructuredFieldEncoder.swift

@@ -276,7 +276,7 @@ extension _StructuredFieldEncoder: SingleValueEncodingContainer {
     }
 
     func encode(_ data: Data) throws {
-        let encoded = data.base64EncodedData()
+        let encoded = data.base64EncodedString()
         try self.currentStackEntry.storage.insertBareItem(.undecodedByteSequence(encoded))
     }
 
@@ -434,7 +434,7 @@ extension _StructuredFieldEncoder {
     }
 
     func append(_ value: Data) throws {
-        try self.currentStackEntry.storage.appendBareItem(.undecodedByteSequence(value.base64EncodedData()))
+        try self.currentStackEntry.storage.appendBareItem(.undecodedByteSequence(value.base64EncodedString()))
     }
 
     func append(_ value: Decimal) throws {
@@ -601,7 +601,7 @@ extension _StructuredFieldEncoder {
 
     func encode(_ value: Data, forKey key: String) throws {
         let key = self.sanitizeKey(key)
-        try self.currentStackEntry.storage.insertBareItem(.undecodedByteSequence(value.base64EncodedData()), atKey: key)
+        try self.currentStackEntry.storage.insertBareItem(.undecodedByteSequence(value.base64EncodedString()), atKey: key)
     }
 
     func encode(_ value: Decimal, forKey key: String) throws {
@@ -758,24 +758,22 @@ extension _StructuredFieldEncoder {
     /// Note that we never have a bare item here. This is deliberate: bare items
     /// are not a container for anything else, and so can never appear.
     internal enum NodeType {
-        typealias DataType = Data
-
         case dictionaryHeader
         case listHeader
         case itemHeader
-        case dictionary(OrderedMap<String, ItemOrInnerList<DataType>>)
-        case list([ItemOrInnerList<DataType>])
-        case innerList(InnerList<DataType>)
+        case dictionary(OrderedMap<String, ItemOrInnerList>)
+        case list([ItemOrInnerList])
+        case innerList(InnerList)
         case item(PartialItem)
-        case bareInnerList(BareInnerList<DataType>)
-        case parameters(OrderedMap<String, BareItem<DataType>>)
-        case itemOrInnerList(OrderedMap<String, BareItem<DataType>>)
+        case bareInnerList(BareInnerList)
+        case parameters(OrderedMap<String, BareItem>)
+        case itemOrInnerList(OrderedMap<String, BareItem>)
 
         /// A helper struct used to tolerate the fact that we need partial items,
         /// but our `Item` struct doesn't like that much.
         struct PartialItem {
-            var bareItem: BareItem<DataType>?
-            var parameters: OrderedMap<String, BareItem<DataType>>
+            var bareItem: BareItem?
+            var parameters: OrderedMap<String, BareItem>
         }
 
         /// This is called when a complete object has been built.
@@ -848,7 +846,7 @@ extension _StructuredFieldEncoder {
         ///
         /// If the key is missing we will require the type to be `item`, in which case
         /// this will be for the "item" key.
-        mutating func insertBareItem(_ bareItem: BareItem<DataType>, atKey key: String? = nil) throws {
+        mutating func insertBareItem(_ bareItem: BareItem, atKey key: String? = nil) throws {
             switch self {
             case .itemHeader:
                 guard key == nil || key == "item" else {
@@ -875,7 +873,7 @@ extension _StructuredFieldEncoder {
 
             case .dictionaryHeader:
                 // Ok cool, this is a dictionary.
-                var map = OrderedMap<String, ItemOrInnerList<DataType>>()
+                var map = OrderedMap<String, ItemOrInnerList>()
 
                 // Bare item here means item, no parameters.
                 map[key!] = .item(.init(bareItem: bareItem, parameters: [:]))
@@ -897,7 +895,7 @@ extension _StructuredFieldEncoder {
 
         /// Appends a bare item to the given container. This must be a list-type
         /// container that stores either bare items, or items.
-        mutating func appendBareItem(_ bareItem: BareItem<DataType>) throws {
+        mutating func appendBareItem(_ bareItem: BareItem) throws {
             switch self {
             case .listHeader:
                 self = .list([.item(Item(bareItem: bareItem, parameters: [:]))])
@@ -926,7 +924,7 @@ extension _StructuredFieldEncoder {
     }
 }
 
-extension Item where BaseData == _StructuredFieldEncoder.NodeType.DataType {
+extension Item {
     fileprivate init(_ partialItem: _StructuredFieldEncoder.NodeType.PartialItem) {
         self.init(bareItem: partialItem.bareItem!, parameters: partialItem.parameters)
     }