c# - 如何在 .NET 中获取 xml 元素的流位置

Question

如何以合理有效的方式获取 .NET 4 中 XElement 的流位置？

          1         2         3         4         5         6         7         8
01234567890123456789012345678901234567890123456789012345678901234567890123456789012
<root><group id="0" combiner="or"><filter id="1" /><filter id="2" /></group></root>

我想从上面创建一个到段的映射

{ { "/root",                  Segment(0 , 82) },
  { "/root/group-0",          Segment(6 , 75) },
  { "/root/group-0/filter-1", Segment(34, 50) },
  { "/root/group-0/filter-2", Segment(51, 67) } }

注意事项

• 段的第二个字段可以是长度而不是结束索引
• 方法可以更通用/扩展到其他字节表示

关于我的答案的博文和内存分析屏幕截图

http://corsis.posterous.com/xml-keyvalue-cache-optimizations

奖金

• 使用一种压缩形式，允许O(1) 次访问 元素但 只需要整体的一个副本没有任何子元素在内存中重复的文档。

奖励示例

store["/root"].Decompress()         **O(1)**
store["/root/group-0"].Decompress() **O(1)**

Answer 1

这是我最初的尝试:

using System;
using System.Collections.Generic;
using System.Collections.Concurrent;
using System.Linq;
using System.IO;
using System.Xml;
using System.Xml.Linq;
using System.Text;

namespace XMLTest
{
    public struct Segment
    {
        public Segment(long index, long length)
        {
            Index = index;
            Length = length;
        }

        public long Index;
        public long Length;

        public override string ToString()
        {
            return string.Format("Segment({0}, {1})", Index, Length);
        }
    }

    public static class GeneralSerializationExtensions
    {
        public static string Segment(this string buffer, Segment segment)
        {
            return buffer.Substring((int)segment.Index, (int)segment.Length);
        }

        public static byte[] Bytes(this Stream stream, int startIndex = 0, bool setBack = false)
        {
            var bytes = new byte[stream.Length];
            if (stream.CanSeek && stream.CanRead)
            {
                var position = stream.Position;
                stream.Seek(startIndex, SeekOrigin.Begin);
                stream.Read(bytes, 0, (int)stream.Length);
                if (setBack)
                    stream.Position = position;
            }
            return bytes;
        }        
    }

    class Program
    {
        static void Main(string[] args)
        {
            var stream = new MemoryStream();
            var element = XElement.Parse(@"<root><group id=""0"" combiner=""or""><filter id=""1"" /><filter id=""2"" /></group></root>");            
            //var element = XElement.Parse("<a>i<b id='1' o='2' p=''/><b id='2'><c /></b><b id='3' /><b id='4' o='u'>2</b></a>");

            var pie = new PathIndexedXElement(element);

            foreach (var path in pie.Paths.OrderBy(p => p))
            {
                var s = pie.store[path];
                var t = pie[path];
                Console.WriteLine("> {2,-30} {0,-20} {1}", s, t, path);
            }
        }
    }

    public class PathIndexedXElement
    {
        internal string buffer;
        internal ConcurrentDictionary<string, Segment> store;

        public PathIndexedXElement(XElement element)
        {
            buffer = XmlPathSegmenter.StringBuffer(element);
            store = element.PathSegments();
        }

        public IEnumerable<string> Paths
        {
            get { return store.Keys; }
        }

        public string this[string path]
        {
            get { return buffer.Segment(store[path]); }
        }

        public bool TryGetValue(string path, out string xelement)
        {
            Segment segment;
            if (store.TryGetValue(path, out segment))
            {
                xelement = buffer.Segment(segment);
                return true;
            }
            xelement = null;
            return false;
        }
    }

    public static class XmlPathSegmenter
    {
        public static XmlWriter CreateWriter(Stream stream)
        {
            var settings = new XmlWriterSettings() { Encoding = Encoding.UTF8, Indent = false, OmitXmlDeclaration = true, NewLineHandling = NewLineHandling.None };

            return XmlWriter.Create(stream, settings);
        }

        public static MemoryStream MemoryBuffer(XElement element)
        {
            var stream = new MemoryStream();
            var writer = CreateWriter(stream);
            element.Save(writer);
            writer.Flush();
            stream.Position = 0;
            return stream;
        }

        public static string StringBuffer(XElement element)
        {
            return Encoding.UTF8.GetString(MemoryBuffer(element).Bytes()).Substring(1);
        }

        public static ConcurrentDictionary<string, Segment> PathSegments(string xmlElement, ConcurrentDictionary<string, Segment> store = null)
        {
            return PathSegments(XElement.Parse(xmlElement), store);
        }

        public static ConcurrentDictionary<string, Segment> PathSegments(this XElement element, ConcurrentDictionary<string, Segment> store = null)
        {
            var stream = new MemoryStream();
            var writer = CreateWriter(stream);
            element.Save(writer);
            writer.Flush();
            stream.Position = 0;

            return PathSegments(stream, store);
        }

        public static ConcurrentDictionary<string, Segment> PathSegments(Stream stream, ConcurrentDictionary<string, Segment> store = null)
        {
            if (store == null)
                store = new ConcurrentDictionary<string, Segment>();

            var stack = new ConcurrentStack<KeyValuePair<string, int>>();
            PathSegments(stream, stack, store);

            return store;
        }

        //
        static void PathSegments(Stream stream, ConcurrentStack<KeyValuePair<string, int>> stack, ConcurrentDictionary<string, Segment> store)
        {
            var reader = XmlReader.Create(stream, new XmlReaderSettings() { });
            var line = reader as IXmlLineInfo;

            while (reader.Read())
            {
                KeyValuePair<string, int> ep;
            ok:
                if (reader.IsStartElement())
                {
                    stack.TryPeek(out ep);
                    stack.Push(new KeyValuePair<string, int>(ep.Key + Path(reader), line.LinePosition - 2));
                }

                if (reader.IsEmptyElement)
                {
                    var name = reader.LocalName;
                    var d = reader.Depth;
                    reader.Read();
                    if (stack.TryPop(out ep))
                    {
                        var length = line.LinePosition - 2 - ep.Value - (d > reader.Depth ? 1 : 0);
                        Console.WriteLine("/{3}|{0} : {1} -> {2}", name, ep.Value, length, line.LineNumber);

                        store.TryAdd(ep.Key, new Segment(ep.Value, length));
                    }
                    goto ok;
                }

                if (reader.NodeType == XmlNodeType.EndElement)
                {
                    if (stack.TryPop(out ep))
                    {
                        var length = line.LinePosition + reader.LocalName.Length - ep.Value;
                        Console.WriteLine("|{3}|{0} : {1} -> {2}", reader.LocalName, ep.Value, length, line.LineNumber);

                        store.TryAdd(ep.Key, new Segment(ep.Value, length));
                    }
                }

            }
        }
        //

        public static string Path(XmlReader element)
        {
            if (!(element.IsStartElement() || element.IsEmptyElement))
                return null;

            if (!element.HasAttributes)
                return "/" + element.LocalName;
            var id = element.GetAttribute("id");
            return string.Format(id == null ? "/{0}" : "/{0}-{1}", element.LocalName, id);
        }
    }
}

输出:

/1|filter : 34 -> 17
/1|filter : 51 -> 17
|1|group : 6 -> 70
|1|root : 0 -> 83
> /root                          Segment(0, 83)       <root><group id="0" combiner="or"><filter id="1" /><filter id="2" /></group></root>
> /root/group-0                  Segment(6, 70)       <group id="0" combiner="or"><filter id="1" /><filter id="2" /></group>
> /root/group-0/filter-1         Segment(34, 17)      <filter id="1" />
> /root/group-0/filter-2         Segment(51, 17)      <filter id="2" />

插入者正在发现 IXmlLineInfo由 XmlReader 类显式实现的接口(interface)，这是一条很难找到的信息。

注意事项

在我收到关于 this question 的所有评论之后，现在有点预防 :) :

• 集合的并发版本在此示例中不起作用。我知道并乐于使用它们 :)
• 路径方案可以很容易地推广，但这涵盖了我的所有需求。
• 我知道 id 通常用作文档范围内的唯一标识符，我很高兴在这个特定的上下文中使用它们。
• 可以很容易地扩展段，使另一个长度属性指向开始标记的结束 > 符号，从而可以仅提取文档树中任何给定元素的属性，以上下文重建任何其他元素目标元素。对于浅树，这应该提供一个很好的常量因子来访问目标元素以及上下文信息。
• 我完全清楚所有这些可能值得也可能不值得的尝试:我还没有我的场景的任何数字。只是想开发一种方法并与人们分享。