用JavaScript实现Bencode算法

很久以前，在写JavaScript版六维空间流量作弊工具的时候，需要用到Bencode算法。在《Bittorrent Protocol Specification》上面找到了一个JScript的实现，然而，不幸的是，上面的那个算法是错误的，程序会陷入死循环，用不了多久电脑就死机了。当时急于求成，用迂回的的方法完成了作弊工具，却一直没有完成Bencode算法。今天参考BitTorrent 5.3的Python源码，把JavaScript版的Bencode算法完成了。

/*
 * Author: Demon
 * Website: https://demon.tw
 * Email: 380401911@qq.com
 */
function decode_int(x, f) {
    f++;
    var newf = x.indexOf('e', f);
    var n = parseInt(x.substring(f,newf));
    if (x.charAt(f) == '-' && x.charAt(f+1) == '0') {
        throw("ValueError");
    } else if (x.charAt(f) == '0' && newf != f+1) {
        throw("ValueError");
    }
    return [n, newf+1];
}

function decode_string(x, f) {
    var colon = x.indexOf(':', f);
    var n = parseInt(x.substring(f,colon));
    if (x.charAt(f) == '0' && colon != f+1) {
        throw("ValueError");
    }
    colon++;
    return [x.substring(colon,colon+n), colon+n];
}

function decode_list(x, f) {
    var r = []; f++;
    while (x.charAt(f) != 'e') {
        var a = decode_func[x.charAt(f)](x, f);
        var v = a[0]; f = a[1];
        r.push(v);
    }
    return [r, f + 1];
}

function decode_dict(x, f) {
    var r = {}; f++;
    while (x.charAt(f) != 'e') {
        var a = decode_string(x, f);
        var k = a[0]; f = a[1];
        a = decode_func[x.charAt(f)](x, f)
        r[k] = a[0]; f = a[1];
    }
    return [r, f + 1];
}

decode_func = {};
decode_func['l'] = decode_list;
decode_func['d'] = decode_dict;
decode_func['i'] = decode_int;
decode_func['0'] = decode_string;
decode_func['1'] = decode_string;
decode_func['2'] = decode_string;
decode_func['3'] = decode_string;
decode_func['4'] = decode_string;
decode_func['5'] = decode_string;
decode_func['6'] = decode_string;
decode_func['7'] = decode_string;
decode_func['8'] = decode_string;
decode_func['9'] = decode_string;

// x is a string containing bencoded data, 
// where each charCodeAt value matches the byte of data
function bdecode(x) {
    try {
        var a = decode_func[x.charAt(0)](x, 0);
        var r = a[0]; var l = a[1];
    } catch(e) {
        throw("not a valid bencoded string");
    }
    if (l != x.length) {
        throw("invalid bencoded value (data after valid prefix)");
    }
    return r;
}

/*
 * Author: Demon
 * Website: https://demon.tw
 * Email: 380401911@qq.com
 */

function encode_int(x,r) {
    r.push('i'); r.push(x+''); r.push('e');
}

function encode_string(x,r) {
    r.push(x.length+''); r.push(':'); r.push(x);
}

function encode_list(x,r) {
    r.push('l');
    for (var i in x){
        var type = typeof(x[i]);
        type = (type == 'object') ? ((x[i] instanceof Array) ? 'list' : 'dict') : type;
        encode_func[type](x[i], r)
    }
    r.push('e');
}

function encode_dict(x,r) {
    r.push('d');
    var keys = [], ilist = {};
    for (var i in x) {
        keys.push(i);
    }
    keys.sort();
    for (var j in keys) {
        ilist[keys[j]] = x[keys[j]];
    }
    for (var k in ilist) {
        r.push(k.length+''); r.push(':'); r.push(k);
        var v = ilist[k];
        var type = typeof(v);
        type = (type == 'object') ? ((v instanceof Array) ? 'list' : 'dict') : type;
        encode_func[type](v, r);
    }
    r.push('e');
}

encode_func = {};
encode_func['number']  = encode_int;
encode_func['string']  = encode_string;
encode_func['list']    = encode_list;
encode_func['dict']    = encode_dict;

function bencode(x) {
    var r = [];
    var type = typeof(x);
    type = (type == 'object') ? ((x instanceof Array) ? 'list' : 'dict') : type;
    encode_func[type](x, r);
    return r.join('');
}

JavaScript不能直接读取二进制数据，需要服务端处理后返回才行，所以这里用JScript演示一下用法：

function read(path) {
    var cp1252Chars = [/\u20AC/g,/\u201A/g,/\u0192/g,/\u201E/g,/\u2026/g,/\u2020/g,/\u2021/g,/\u02C6/g,/\u2030/g,/\u0160/g,/\u2039/g,/\u0152/g,/\u017D/g,/\u2018/g,/\u2019/g,/\u201C/g,/\u201D/g,/\u2022/g,/\u2013/g,/\u2014/g,/\u02DC/g,/\u2122/g,/\u0161/g,/\u203A/g,/\u0153/g,/\u017E/g,/\u0178/g];
    var latin1Chars = ["\u0080","\u0082","\u0083","\u0084","\u0085","\u0086","\u0087","\u0088","\u0089","\u008A","\u008B","\u008C","\u008E","\u0091","\u0092","\u0093","\u0094","\u0095","\u0096","\u0097","\u0098","\u0099","\u009A","\u009B","\u009C","\u009E","\u009F"];
    var binstream = new ActiveXObject("ADODB.Stream");
    binstream.Type = 2;
    binstream.Charset = "iso-8859-1";
    binstream.Open();
    binstream.LoadFromFile(path);
    var s = binstream.ReadText();
    for (var i = 0; i < 27; i++)
        s = s.replace(cp1252Chars[i], latin1Chars[i]);
    return s;
}

function write(buf, path) {
    var binstream = new ActiveXObject("ADODB.Stream");
    binstream.Type = 2;
    binstream.Charset = "iso-8859-1";
    binstream.Open();
    binstream.WriteText(buf);
    binstream.SaveToFile(path, 2);
}

var str = read("foo.torrent");
try {
    var dic = bdecode(str);
} catch (e) {
    WScript.Echo(e);
    WScript.Quit();
}
/* get the announce url of the tracker */
var announce = dic["announce"];
/* get the name of the torrent */
var name = dic["info"]["name"];
/* get the number of files of the torrent (assuming a multi-file torrent) */
var number = dic["info"]["files"].length;
/* get the size of the first file of the torrent (assuming a multi-file torrent) */
var number = dic["info"]["files"][0]["length"];
/* change the announce url */
dic["announce"] = "https://demon.tw";
/* and then encode it back to string */
var new_str = bencode(dic);
/* then write it back to a torrent file 
 * now the torrent's announce url has been changed to "https://demon.tw"
 */
write(new_str, "bar.torrent");

由于在Bittorrent Protocol Specification上面提交了算法实现，注释就用英文来写了，免得哪个老外通过链接进来却看不懂。

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这篇文章发布于 2011年04月22日，星期五，22:20，归类于 我的作品。 您可以跟踪这篇文章的评论通过 RSS 2.0 feed。 您可以留下评论，或者从您的站点trackback。