蛙蛙推荐：蛙蛙牌XML压缩算法

摘要：

在用XML传输数据的时候，XML本身的元素名称，属性名称可能比有效的信息量占的地方还要大，本文示例一种简单实用的算法来进行XML压缩，主要思路是把XML标签和属性用整数来表示以便降低传输量。

单元测试代码

class Program {
   public static string XML = @"<?xml version=""1.0"" encoding=""utf-16""?>
    <Customer>
<CustomerID>ALFKI</CustomerID>
<PO>9572658</PO>
<Address AddressType=""work"">
    <Street>One Main Street</Street>
    <City>Anywhere</City>
    <State>NJ</State>
    <Zip>08080</Zip>
</Address>
<Order>
    <OrderID>10966</OrderID >
    <LineItem>
        <ProductID>37</ProductID>
        <UnitPrice>26.50 </UnitPrice>
        <Quantity>8</Quantity>
        <Description>Gravad lax </Description>            　
    </LineItem>
    <LineItem>
        <ProductID>56 </ProductID>
        <UnitPrice>38.00</UnitPrice>
        <Quantity>12</Quantity>
        <Description>Gnocchi di nonna Alice</Description>            　
    </LineItem>
</Order>    
</Customer>";
static void Main(string[] args) {
    XmlZip zip = new XmlZip();

    byte[] bs = Encoding.UTF8.GetBytes(XML);
    Console.WriteLine("原始文件长度:{0}", bs.Length);
    MemoryStream ms = new MemoryStream();
    DeflateStream compressedzipStream = new DeflateStream(ms, CompressionMode.Compress, true);
    compressedzipStream.Write(bs, 0, bs.Length);
    compressedzipStream.Close();
    Console.WriteLine("Deflate压缩后长度: {0}", ms.Length);

    zip.Init(XML);
    bs = zip.XmlToBytes(XML);
    Console.WriteLine("XML压缩后长度:{0}", bs.Length);
    string str = zip.BytesToXml(bs);
    Console.WriteLine("还原后长度:{0}", Encoding.UTF8.GetByteCount(str));
    Console.WriteLine(str);


    ms = new MemoryStream();
    compressedzipStream = new DeflateStream(ms, CompressionMode.Compress, true);
    compressedzipStream.Write(bs, 0, bs.Length);
    compressedzipStream.Close();
    Console.WriteLine("先XML压缩，再Deflate压缩后的长度：{0}", ms.Length);
    Console.ReadKey();

}
}

测试输出

原始文件长度:740
Deflate压缩后长度: 438
XML压缩后长度:295
还原后长度:727
<?xml version="1.0" encoding="utf-16"?>
<Customer>
  <CustomerID>ALFKI</CustomerID>
  <PO>9572658</PO>
  <Address AddressType="work">
    <Street>One Main Street</Street>
    <City>Anywhere</City>
    <State>NJ</State>
    <Zip>08080</Zip>
  </Address>
  <Order>
    <OrderID>10966</OrderID>
    <LineItem>
      <ProductID>37</ProductID>
      <UnitPrice>26.50 </UnitPrice>
      <Quantity>8</Quantity>
      <Description>Gravad lax </Description>            　
    </LineItem>
    <LineItem>
      <ProductID>56 </ProductID>
      <UnitPrice>38.00</UnitPrice>
      <Quantity>12</Quantity>
      <Description>Gnocchi di nonna Alice</Description>            　
    </LineItem>
  </Order>
</Customer>
先XML压缩，再Deflate压缩后的长度：357

可以看到，压缩后的数据约是原来数据的3分之一，可能没有其它专有的压缩算法的压缩率高，但效果还算是满意吧，而且我的算法是比较通用的，只要通信双方知道了XML的Schema，甚至双方只需要有一段完整的示例代码，就可以进行压缩通信，只做了功能测试，没做性能测试，大家可以先借鉴下思路。

完整代码

大致原理，就是通信双方各持有一个XML文档节点名称，属性名称的一个字典，然后发送方传输的时候用ushort代替原有的XML标签和属性名，接收方通过字典把ushort再转换成原始的元素名和属性名，这样大量不必要的重复的标签等就省去了。

代码只做本文的示例，写的比较随意，没有什么防御性和健壮性。

internal enum ItemType {
    Element,
    Attritube
}
internal class XmlNodeItem {
    public string Xpath { get; set; }
    public string Text { get; set; }
    public ItemType ItemType { get; set; }
    public override string ToString() {
        return Xpath;
    }
}
internal class MyXpath {
    LinkedList<string> _node = new LinkedList<string>();
    public void AddElement(string name) {
        _node.AddLast(string.Format("/{0}", name));
    }
    public void AddAttribute(string name) {
        _node.AddLast(string.Format("/@{0}", name));
    }
    public void RemoveLastElement() {
        _node.RemoveLast();
    }
    public override string ToString() {
        StringBuilder sb = new StringBuilder();
        LinkedListNode<string> node = _node.First;
        sb.Append(node.Value);
        while ((node = node.Next) != null) {
            sb.Append(node.Value);
        }
        return sb.ToString();
    }
}
class XmlZip {
    Dictionary<ushort, XmlNodeItem> _map = new Dictionary<ushort, XmlNodeItem>();
    Dictionary<string, ushort> _map2 = new Dictionary<string, ushort>();
    MyXpath _path = new MyXpath();

    public void Init(string xmlInput) {
        StringReader sr = new StringReader(xmlInput);
        XmlReader reader = XmlReader.Create(sr);
        MemoryStream ms = new MemoryStream();
        ushort i = 1;
        while (reader.Read()) {
            switch (reader.NodeType) {
                case XmlNodeType.Element:
                    _path.AddElement(reader.Name);
                    _map[i++] = new XmlNodeItem() {
                        Xpath = _path.ToString(),
                        Text = reader.Name,
                        ItemType = ItemType.Element
                    };
                    if (reader.HasAttributes) {
                        reader.MoveToFirstAttribute();
                        _path.AddAttribute(reader.Name);
                        _map[i++] = new XmlNodeItem() {
                            Xpath = _path.ToString(),
                            Text = reader.Name,
                            ItemType = ItemType.Attritube
                        };
                        _path.RemoveLastElement();
                        while (reader.MoveToNextAttribute()) {
                            _path.AddAttribute(reader.Name);
                            _map[i++] = new XmlNodeItem() {
                                Xpath = _path.ToString(),
                                Text = reader.Name,
                                ItemType = ItemType.Attritube
                            };
                            _path.RemoveLastElement();
                        }
                        reader.MoveToElement();
                    }
                    if (reader.IsEmptyElement) _path.RemoveLastElement();
                    break;
                case XmlNodeType.EndElement:
                    _path.RemoveLastElement();
                    break;
                default:
                    break;
            }
        }
        foreach (KeyValuePair<ushort, XmlNodeItem> pair in _map) {
            _map2[pair.Value.Xpath] = pair.Key;
        }
    }

    public byte[] XmlToBytes(string xmlInput) {
        StringReader sr = new StringReader(xmlInput);
        XmlReader reader = XmlReader.Create(sr);
        MemoryStream ms = new MemoryStream();
        BinaryWriter bw = new BinaryWriter(ms);
        while (reader.Read()) {
            ushort index;
            byte[] bs;
            switch (reader.NodeType) {
                case XmlNodeType.Element:
                    _path.AddElement(reader.Name);
                    if (_map2.TryGetValue(_path.ToString(), out index)) {
                        bw.Write(index);
                    }
                    if (reader.HasAttributes) {
                        reader.MoveToFirstAttribute();
                        _path.AddAttribute(reader.Name);
                        if (_map2.TryGetValue(_path.ToString(), out index)) {
                            _path.RemoveLastElement();
                            bw.Write(index);
                            bs = Encoding.UTF8.GetBytes(reader.Value);
                            bw.Write((ushort)bs.Length);
                            bw.Write(bs);
                        }
                        while (reader.MoveToNextAttribute()) {
                            _path.AddAttribute(reader.Name);
                            if (_map2.TryGetValue(_path.ToString(), out index)) {
                                _path.RemoveLastElement();
                                bw.Write(index);
                                bs = Encoding.UTF8.GetBytes(reader.Value);
                                bw.Write((ushort)bs.Length);
                                bw.Write(bs);
                            }
                        }
                        reader.MoveToElement();
                    }
                    if (reader.IsEmptyElement) {
                        _path.RemoveLastElement();
                        bw.Write(ushort.MaxValue);
                    }
                    break;
                case XmlNodeType.EndElement:
                    _path.RemoveLastElement();
                    bw.Write(ushort.MaxValue);
                    break;
                case XmlNodeType.Text:
                    bw.Write((ushort)0);
                    bs = Encoding.UTF8.GetBytes(reader.Value);
                    bw.Write((ushort)bs.Length);
                    bw.Write(bs);
                    break;
                default:
                    break;
            }
        }
        bw.Close();
        ms.Close();
        reader.Close();
        return ms.ToArray();
    }

    public string BytesToXml(byte[] bytes) {
        MemoryStream ms = new MemoryStream(bytes);
        BinaryReader br = new BinaryReader(ms);
        StringBuilder sb = new StringBuilder();
        StringWriter sw = new StringWriter(sb);
        XmlWriterSettings settings = new XmlWriterSettings();
        settings.Indent = true;
        XmlWriter writer = XmlWriter.Create(sw, settings);

        XmlNodeItem item;
        while (br.PeekChar() != -1) {
            ushort readFlag = br.ReadUInt16();
            int len;
            byte[] bs;
            string str;
            if (_map.TryGetValue(readFlag, out item)) {
                if (item.ItemType == ItemType.Element)
                    writer.WriteStartElement(item.Text);
                else if (item.ItemType == ItemType.Attritube) {
                    len = br.ReadUInt16();
                    bs = br.ReadBytes(len);
                    str = Encoding.UTF8.GetString(bs);
                    writer.WriteAttributeString(item.Text, str);
                }
            }
            else if (readFlag == 0) {
                len = br.ReadUInt16();
                bs = br.ReadBytes(len);
                str = Encoding.UTF8.GetString(bs);
                writer.WriteString(str);
            }
            else if (readFlag == ushort.MaxValue) {
                writer.WriteEndElement();
            }
        }
        writer.Flush();
        writer.Close();
        sw.Close();
        br.Close();
        return sb.ToString();
    }
}

参考链接

XML压缩和传输性能的改善

http://blog.csdn.net/BruceWayen/archive/2006/03/13/623483.aspx

XQzip：XML压缩技术（1）--介绍

http://qiyanfeng.blog.51cto.com/503144/105203

XQzip：可查询MXL压缩算法分析(1)

http://qiyanfeng.blog.51cto.com/503144/105578

WAP Binary XML Content Format

http://www.w3.org/TR/wbxml/