C# HttpWebRequest 从google服务器获取google的PageRank PR值

首先，本文的实现参考了这篇：

http://www.codeproject.com/KB/aspnet/Google_Pagerank.aspx

简述一下原理：

获取PR值，通过向google服务器发送一个http请求来实现。

http://toolbarqueries.google.com.hk/search?client=navclient-auto&hl=en&ch=6771535612&ie=UTF-8&oe=UTF-8&features=Rank&q=info:http%3A%2F%2Fwww.codeproject.com%2F

在浏览器中输入上面的链接会google服务器会返回一个字符串

上面的链接返回的是：Rank_1:1:6

要实现对任意链接的PR值的查询，关键要正确构造请求的URL，ch=6771535612这一项很重要，它是请求网址的hash值，这个hash值得算法在上面的那篇博文中就有详细的说明。

这里将主要的代码转贴在下面：

        private const UInt32 GOOGLE_MAGIC = 0xE6359A60;

        private static void _mix(ref UInt32 a, ref UInt32 b, ref UInt32 c)
        {
            a -= b; a -= c; a ^= c >> 13;
            b -= c; b -= a; b ^= a << 8;
            c -= a; c -= b; c ^= b >> 13;
            a -= b; a -= c; a ^= c >> 12;
            b -= c; b -= a; b ^= a << 16;
            c -= a; c -= b; c ^= b >> 5;
            a -= b; a -= c; a ^= c >> 3;
            b -= c; b -= a; b ^= a << 10;
            c -= a; c -= b; c ^= b >> 15;
        }

        public static string GoogleCH(string url)
        {
            url = string.Format("info:{0}", url);

            int length = url.Length;

            UInt32 a, b;
            UInt32 c = GOOGLE_MAGIC;

            int k = 0;
            int len = length;

            a = b = 0x9E3779B9;

            while (len >= 12)
            {
                a += (UInt32)(url[k + 0] + (url[k + 1] << 8) + (url[k + 2] << 16) + (url[k + 3] << 24));
                b += (UInt32)(url[k + 4] + (url[k + 5] << 8) + (url[k + 6] << 16) + (url[k + 7] << 24));
                c += (UInt32)(url[k + 8] + (url[k + 9] << 8) + (url[k + 10] << 16) + (url[k + 11] << 24));
                _mix(ref a, ref b, ref c);
                k += 12;
                len -= 12;
            }
            c += (UInt32)length;
            switch (len)  /* all the case statements fall through */
            {
                case 11:
                    c += (UInt32)(url[k + 10] << 24);
                    goto case 10;
                case 10:
                    c += (UInt32)(url[k + 9] << 16);
                    goto case 9;
                case 9:
                    c += (UInt32)(url[k + 8] << 8);
                    goto case 8;
                /* the first byte of c is reserved for the length */
                case 8:
                    b += (UInt32)(url[k + 7] << 24);
                    goto case 7;
                case 7:
                    b += (UInt32)(url[k + 6] << 16);
                    goto case 6;
                case 6:
                    b += (UInt32)(url[k + 5] << 8);
                    goto case 5;
                case 5:
                    b += (UInt32)(url[k + 4]);
                    goto case 4;
                case 4:
                    a += (UInt32)(url[k + 3] << 24);
                    goto case 3;
                case 3:
                    a += (UInt32)(url[k + 2] << 16);
                    goto case 2;
                case 2:
                    a += (UInt32)(url[k + 1] << 8);
                    goto case 1;
                case 1:
                    a += (UInt32)(url[k + 0]);
                    break;
                default:
                    break;
                /* case 0: nothing left to add */
            }

            _mix(ref a, ref b, ref c);

            return string.Format("6{0}", c);
        }

　　

再给出一个调用的用例供参考：

            try
            {
                //构造请求的URL
                string checksum = GoogleCH(txtUrl.Text);
                string query = string.Format(@"http://toolbarqueries.google.com/search?client=navclient-auto&ch={0}&features=Rank&q=info:{1}", checksum, txtUrl.Text);

                //请求并获得响应
                request = (HttpWebRequest)HttpWebRequest.Create(query);
                response = (HttpWebResponse)request.GetResponse();
                
                if(response==null)
                {
                    txtResponse.Text = "response==NULL";
                    return;
                }

                Stream stream = response.GetResponseStream();
                txtResponse.Text = "";

                byte []buf=new byte[1024];

                int readlen;

                while ((readlen = stream.Read(buf, 0, 1024)) > 0)
                {
                    //注意这里解析显示PR值
                    txtPR.Text = int.Parse(Regex.Match(Encoding.UTF8.GetString(buf, 0, readlen), "Rank_1:[0-9]:([0-9]+)").Groups[1].Value).ToString();
                }
            }
            catch (System.UriFormatException)
            {
                txtResponse.Text = "无效的URL";
            }