NativeBuferring，一种零分配的数据类型[下篇]

上文说到Unmanaged、BufferedBinary和BufferedString是NativeBuffering支持的三个基本数据类型，其实我们也可以说NativeBuffering只支持Unmanaged和IReadOnlyBufferedObject两种类型，BufferedString、NativeBuffering和通过Source Generator生成的BufferedMessage类型，以及下面介绍的几种集合和字典类型，都实现了IReadOnlyBufferedObject接口。

一、IReadOnlyBufferedObject
二、集合
三、字典
四、为什么不直接返回接口?

一、IReadOnlyBufferedObject

顾名思义，IReadOnlyBufferedObject表示一个针对缓冲字节序列创建的只读数据类型。如下面的代码片段所示，该接口只定义了一个名为Parse的静态方法，意味着对于任何一个实现了该接口的类型，对应的实例都可以利用一个代表缓冲字节序列的NativeBuffer的对象进行创建。

public interface IReadOnlyBufferedObject where T: IReadOnlyBufferedObject
{
    static abstract T Parse(NativeBuffer buffer);
}

public unsafe readonly struct NativeBuffer
{
    public byte[] Bytes { get; }
    public void* Start { get; }

    public NativeBuffer(byte[] bytes, void* start)
    {
        Bytes = bytes ?? throw new ArgumentNullException(nameof(bytes));
        Start = start;
    }

    public NativeBuffer(byte[] bytes, int index = 0)
    {
        Bytes = bytes ?? throw new ArgumentNullException(nameof(bytes));
        Start = Unsafe.AsPointer(ref bytes[index]);
    }
}

由于IReadOnlyBufferedObject是NativeBuffering支持的基础类型，而生成的BufferedMessage类型也实现了这个接口。通过这种“无限嵌套”的形式，我们可以定义一个具有任意结构的数据类型。比如我们具有如下这个表示联系人的Contact类型，我们需要利用它作为“源类型”生成对应BufferedMessage类型。

[BufferedMessageSource]
public partial class Contact
{
    public Contact(string id, string name, Address address)
    {
        Id = id;
        Name = name;
        ShipAddress = address;
    }

    public string Id { get; }
    public string Name { get; }
    public Address ShipAddress { get; }
}

[BufferedMessageSource]
public partial class Address
{
    public string Province { get; }
    public string City { get; }
    public string District { get; }
    public string Street { get; }
    public Address(string province, string city, string district, string street)
    {
        Province = province ?? throw new ArgumentNullException(nameof(province));
        City = city ?? throw new ArgumentNullException(nameof(city));
        District = district ?? throw new ArgumentNullException(nameof(district));
        Street = street ?? throw new ArgumentNullException(nameof(street));
    }
}

Contact具有Id、Name和ShipAddress 三个数据成员，ShipAddress 对应的Address又是一个复合类型，具有四个表示省、市、区和介绍的字符串类型成员。现在我们为Contact和Address这两个类型生成对应的ContactBufferedMessage和AddressBufferedMessage。

public unsafe readonly struct ContactBufferedMessage : IReadOnlyBufferedObject
{
    public NativeBuffer Buffer { get; }
    public ContactBufferedMessage(NativeBuffer buffer) => Buffer = buffer;
    public static ContactBufferedMessage Parse(NativeBuffer buffer) => new ContactBufferedMessage(buffer);
    public BufferedString Id => Buffer.ReadBufferedObjectField(0);
    public BufferedString Name => Buffer.ReadBufferedObjectField(1);
    public AddressBufferedMessage ShipAddress => Buffer.ReadBufferedObjectField(2);
}

public unsafe readonly struct AddressBufferedMessage : IReadOnlyBufferedObject
{
    public NativeBuffer Buffer { get; }
    public AddressBufferedMessage(NativeBuffer buffer) => Buffer = buffer;
    public static AddressBufferedMessage Parse(NativeBuffer buffer) => new AddressBufferedMessage(buffer);
    public BufferedString Province => Buffer.ReadBufferedObjectField(0);
    public BufferedString City => Buffer.ReadBufferedObjectField(1);
    public BufferedString District => Buffer.ReadBufferedObjectField(2);
    public BufferedString Street => Buffer.ReadBufferedObjectField(3);
}

如下的程序演示了如何将一个Contact对象转换成字节数组，然后利用这这段字节序列生成一个ContactBufferedMessage对象。给出的调试断言验证了Contact和ContactBufferedMessage对象承载了一样的数据，fixed关键字是为了将字节数组“固定住”。（源代码从这里下载）

using NativeBuffering;
using System.Diagnostics;

var address = new Address("Jiangsu", "Suzhou", "Industory Park", "#328, Xinghu St");
var contact = new Contact("123456789", "John Doe", address);
var size = contact.CalculateSize();
var bytes = new byte[size];
var context = new BufferedObjectWriteContext(bytes);
contact.Write(context);

unsafe
{
    fixed (byte* _ = bytes)
    {
        var contactMessage = ContactBufferedMessage.Parse(new NativeBuffer(bytes));
        Debug.Assert(contactMessage.Id == "123456789");
        Debug.Assert(contactMessage.Name == "John Doe");
        Debug.Assert(contactMessage.ShipAddress.Province == "Jiangsu");
        Debug.Assert(contactMessage.ShipAddress.City == "Suzhou");
        Debug.Assert(contactMessage.ShipAddress.District == "Industory Park");
        Debug.Assert(contactMessage.ShipAddress.Street == "#328, Xinghu St");
    }
}

二、集合

NativeBuffering同样支持集合。由于Unmanaged和IReadOnlyBufferedObject是两种基本的数据类型，它们的根据区别在于：前者的长度有类型本身决定，是固定长度类型，后者则是可变长度类型。元素类型为Unmanaged和IReadOnlyBufferedObject的集合分别通过ReadOnlyFixedLengthTypedList和ReadOnlyVaraibleLengthTypedList类型（结构体）表示，它们同样实现了IReadOnlyBufferedObject接口。ReadOnlyFixedLengthTypedList采用如下的字节布局：集合元素数量（4字节整数）+所有元素的字节内容（下图-上）。对于ReadOnlyVaraibleLengthTypedList类型，我们会在前面为每个元素添加一个索引（4字节的整数），该索引指向目标元素在整个缓冲区的偏移量（下图-下）。

以如下所示的Entity为例，它具有两个数组类型的属性成员Collection1和Collection2，数组元素类型分别为Foobar和double，它们分别代表了上述的两种集合类型。

[BufferedMessageSource]
public partial class Entity
{
    public Foobar[] Collection1 { get; }
    public double[] Collection2 { get; }
    public Entity(Foobar[] collection1, double[] collection2)
    {
        Collection1 = collection1;
        Collection2 = collection2;
    }
}

[BufferedMessageSource]
public partial class Foobar
{
    public int Foo { get; }
    public string Bar { get; }
    public Foobar(int foo, string bar)
    {
        Foo = foo;
        Bar = bar;
    }
}

NativeBuffering.Generator会将作为“源类型”的Entity和Foobar类型的生成对应的BufferedMessage类型（EntityBufferredMessage和FoobarBufferedMessage）。从EntityBufferredMessage类型的定义可以看出，两个集合属性的分别是ReadOnlyVariableLengthTypeList和ReadOnlyFixedLengthTypedList。

public unsafe readonly struct EntityBufferedMessage : IReadOnlyBufferedObject
{
    public NativeBuffer Buffer { get; }
    public EntityBufferedMessage(NativeBuffer buffer) => Buffer = buffer;
    public static EntityBufferedMessage Parse(NativeBuffer buffer) => new EntityBufferedMessage(buffer);
    public ReadOnlyVariableLengthTypeList Collection1 => Buffer.ReadBufferedObjectCollectionField(0);
    public ReadOnlyFixedLengthTypedList Collection2 => Buffer.ReadUnmanagedCollectionField(1);
}

public unsafe readonly struct FoobarBufferedMessage : IReadOnlyBufferedObject
{
    public NativeBuffer Buffer { get; }
    public FoobarBufferedMessage(NativeBuffer buffer) => Buffer = buffer;
    public static FoobarBufferedMessage Parse(NativeBuffer buffer) => new FoobarBufferedMessage(buffer);
    public System.Int32 Foo => Buffer.ReadUnmanagedField(0);
    public BufferedString Bar => Buffer.ReadBufferedObjectField(1);
}

两个集合类型都实现了IEnumerable接口，还提供了索引。下面的代码演示了以索引的形式提取集合元素（源代码从这里下载）。

using NativeBuffering;
using System.Diagnostics;

var entity = new Entity(
    collection1: new Foobar[] { new Foobar(1, "foo"), new Foobar(2, "bar") },
    collection2: new double[] { 1.1, 2.2 });
var bytes = new byte[entity.CalculateSize()];
var context = new BufferedObjectWriteContext(bytes);
entity.Write(context);

unsafe
{
    fixed (byte* p = bytes)
    {
        var entityMessage = EntityBufferedMessage.Parse(new NativeBuffer(bytes));
        var foobar = entityMessage.Collection1[0];
        Debug.Assert(foobar.Foo == 1);
        Debug.Assert(foobar.Bar == "foo");

        foobar = entityMessage.Collection1[1];
        Debug.Assert(foobar.Foo == 2);
        Debug.Assert(foobar.Bar == "bar");

        Debug.Assert(entityMessage.Collection2[0] == 1.1);
        Debug.Assert(entityMessage.Collection2[1] == 2.2);
    }
}

三、字典

从数据的存储来看，字典就是键值对的集合，所以我们采用与集合一致的存储形式。NativeBuffering对集合的Key作了限制，要求其类型只能是Unmanaged和字符串（String/BufferredString）。按照Key和Value的类型组合，我们一共定义了四种类型的字典类型，它们分别是：

• ReadOnlyUnmanagedUnmanagedDictionary：Key=Unmanaged; Value = Unmanaged
• ReadOnlyUnmanagedBufferedObjectDictionary：Key=Unmanaged; Value = IReadOnlyBufferedObject
• ReadOnlyStringUnmanagedDictionary：Key=String/BufferredString; Value = Unmanaged
• ReadOnlyStringBufferedObjectDictionary：Key=String/BufferredString; Value = IReadOnlyBufferedObject

如果Key和Value的类型都是Unmanaged，键值对就是定长类型，所以我们会采用类似于ReadOnlyFixedLengthTypedList的字节布局方式（下图-上），至于其他三种字典类型，则采用类似于ReadOnlyVaraibleLengthTypedList的字节布局形式（下图-下）。

但是这仅仅解决了字段数据存储的问题，字典基于哈希检索定位的功能是没有办法实现的。这里我们不得不作出妥协，四种字典的索引均不能提供时间复杂度O(1)的哈希检索方式。为了在现有的数据结构上使针对Key的查找尽可能高效，在生成字节内容之前，我们会按照Key对键值对进行排序，这样我们至少可以采用二分法的形式进行检索，所以四种类型的字典的索引在根据指定的Key查找对应Value，对应的时间复杂度为Log(N)。如果字典包含的元素比较多，这样的查找方式不能满足我们的需求，我们可以I将它们转换成普通的Dictionary类型，但是这就没法避免内存分配了。

我们照例编写一个简答的程序来演示针对字典的使用。我们定义了如下这个Entity作为“源类型”，它的四个属性对应的字典类型刚好对应上述四种键值对的组合。从生成的EntityBufferedMessage类型可以看出，四个成员的类型正好对应上述的四种字典类型。

[BufferedMessageSource]
public partial class Entity
{
    public Dictionary<int, long> Dictionary1 { get; set; }
    public Dictionary<int, string> Dictionary2 { get; set; }
    public Dictionary<string, long> Dictionary3 { get; set; }
    public Dictionary<string, string> Dictionary4 { get; set; }
}

public unsafe readonly struct EntityBufferedMessage : IReadOnlyBufferedObject
{
    public NativeBuffer Buffer { get; }
    public EntityBufferedMessage(NativeBuffer buffer) => Buffer = buffer;
    public static EntityBufferedMessage Parse(NativeBuffer buffer) => new EntityBufferedMessage(buffer);
    public ReadOnlyUnmanagedUnmanagedDictionary Dictionary1 => Buffer.ReadUnmanagedUnmanagedDictionaryField(0);
    public ReadOnlyUnmanagedBufferedObjectDictionary Dictionary2 => Buffer.ReadUnmanagedBufferedObjectDictionaryField(1);
    public ReadOnlyStringUnmanagedDictionary Dictionary3 => Buffer.ReadStringUnmanagedDictionaryField(2);
    public ReadOnlyStringBufferedObjectDictionary Dictionary4 => Buffer.ReadStringBufferedObjectDictionaryField(3);
}

如下的代码演示了基于四种字典类型基于“索引”的检索方式（源代码从这里下载）。

using NativeBuffering;
using System.Diagnostics;

var entity = new Entity
{
    Dictionary1 = new Dictionary<int, long> { { 1, 1 }, { 2, 2 }, { 3, 3 } },
    Dictionary2 = new Dictionary<int, string> { { 1, "foo" }, { 2, "bar" }, { 3, "baz" } },
    Dictionary3 = new Dictionary<string, long> { { "foo", 1 }, { "bar", 2 }, { "baz", 3 } },
    Dictionary4 = new Dictionary<string, string> { { "a", "foo" }, { "b", "bar" }, { "c", "baz" } }
};

var bytes = new byte[entity.CalculateSize()];
var context = new BufferedObjectWriteContext(bytes);
entity.Write(context);
unsafe
{
    fixed (void* _ = bytes)
    {
        var bufferedMessage = EntityBufferedMessage.Parse(new NativeBuffer(bytes));

        ref var value1 = ref bufferedMessage.Dictionary1.AsRef(1);
        Debug.Assert(value1 == 1);
        ref var value2 = ref bufferedMessage.Dictionary3.AsRef("baz");
        Debug.Assert(value2 == 3);

        var dictionary1 = bufferedMessage.Dictionary1;
        Debug.Assert(dictionary1[1] == 1);
        Debug.Assert(dictionary1[2] == 2);
        Debug.Assert(dictionary1[3] == 3);

        var dictionary2 = bufferedMessage.Dictionary2;
        Debug.Assert(dictionary2[1] == "foo");
        Debug.Assert(dictionary2[2] == "bar");
        Debug.Assert(dictionary2[3] == "baz");

        var dictionary3 = bufferedMessage.Dictionary3;
        Debug.Assert(dictionary3["foo"] == 1);
        Debug.Assert(dictionary3["bar"] == 2);
        Debug.Assert(dictionary3["baz"] == 3);

        var dictionary4 = bufferedMessage.Dictionary4;
        Debug.Assert(dictionary4["a"] == "foo");
        Debug.Assert(dictionary4["b"] == "bar");
        Debug.Assert(dictionary4["c"] == "baz");
    }
}

四、为什么不直接返回接口

针对集合，NativeBuffering提供了两种类型；针对字典，更是定义了四种类型，为什么不直接返回IList/IDictionary（或者IReadOnlyList/IReadOnlyDictionary）接口呢？这主要有两个原因，第一：为了尽可能地减少内存占用，我们将四种字典类型都定义成了结构体，如果使用接口的话会导致装箱；第二，四种字典类型的提供的API是有差异的，比如ReadOnlyFixedLengthTypedList 和ReadOnlyUnmanagedUnmanagedDictionary都提供了一个额外的AsRef方法，它直接返回值的引用（只读）。如果这个值被定义成一个成员较多的结构体，传引用的方式可以避免较多的拷贝。

public readonly unsafe struct ReadOnlyFixedLengthTypedList : IReadOnlyList, IReadOnlyBufferedObject>
    where T: unmanaged
{
    public readonly ref T AsRef(int index);
    ...
}

public unsafe readonly struct ReadOnlyUnmanagedUnmanagedDictionary : IReadOnlyDictionary, IReadOnlyBufferedObject>
    where TKey : unmanaged, IComparable
    where TValue : unmanaged
{
    public readonly ref TValue AsRef(TKey index) ;
    ...
}