改进Javascript Base64编码传送

    今天更新博客时，顺便翻看了一下以前写的的东西，发现08年写的一篇用Javascript进行Base64编码传送数据的文章。说实在的，当初写这篇东西的时候，自己几乎是不懂Javascript的，只是看到公司的产品在应对各种Location, Language以及各种页面编码设置，存在大量的bug时，有心尝试一下用Base64解决的可能性，而最终并没有在产品中采用。

    

    而如今（我指的是三年后的今天），我们的确在最新的产品中，成功使用了这一方案，并彻底摆脱了各种编码传送的问题。算法方面还是和以前一模一样，只不过，性能上做了局部优化，并在封装形式上有些变化，一如我提倡的Javascript用Java like的风格来写出Java程序员可以维护的代码一样，封装出一个js.util.Base64类：

$package("js.util");

js.util.Base64 = new function (){
    var base64chars = "ABCDEFGHIJKLMNOP" +
        "QRSTUVWXYZabcdef"+
        "ghijklmnopqrstuv"+
        "wxyz0123456789@*"+
        "-";

    /**
     * Encode a string to a Base64 string follow Bse64 regular.
     * @param s, a normal string
     * @return a Base64 string
     */
    this.encode = function(s){
        if(!s || s.length == 0) return s;

        var d = [], b = ucs2_utf8(s), len = b.length,
        b0, b1, b2, b3, i = 0, tmp;

        while(i < len){
            tmp = b[i++];
            b0 = (tmp & 0xfc) >> 2;
            b1 = (tmp & 0x03) << 4;
            if(i < len){
                tmp = b[i++];
                b1 |= (tmp & 0xf0) >> 4;
                b2 = (tmp & 0x0f) << 2;
                if(i< len){
                    tmp = b[i++];
                    b2 |= (tmp & 0xc0) >> 6;
                    b3 = tmp & 0x3f;
                }else{
                    b3 = 64; // 1 byte "-" is supplement

                }
            }else{
                b2 = b3 = 64; // 2 bytes "-" are supplement

            }

            d.push(base64chars.charAt(b0));
            d.push(base64chars.charAt(b1));
            d.push(base64chars.charAt(b2));
            d.push(base64chars.charAt(b3));
        }

        return d.join("");
    };

    /**
     * Decode a Base64 string to a string follow Base64 regular.
     * @param s, a Base64 string
     * @return a normal string
     */
    this.decode = function(s){
        if(!s) return null;

        var len = s.length;
        if(len%4 != 0)
            throw new Error(s+" is not a valid Base64 string.");

        var b = [], i=0, j=0, e=0, c, tmp;
        while(i < len){
            c = base64chars.indexOf(s.charAt(i++));
            tmp = c << 18;
            c = base64chars.indexOf(s.charAt(i++));
            tmp |= c << 12;
            c = base64chars.indexOf(s.charAt(i++));
            if(c < 64){
                tmp |= c << 6;
                c = base64chars.indexOf(s.charAt(i++));
                if(c < 64){
                    tmp |= c;
                }else{
                    e = 1;
                }
            }else{
                e = 2;
                i++;
            }

            b[j+2] = tmp & 0xff;
            tmp >>= 8;
            b[j+1] = tmp & 0xff;
            tmp >>= 8;
            b[j+0] = tmp & 0xff;
            j += 3;

        }

        b.splice(b.length-e, e);

        return utf8_ucs2(b);

    };

    /**
     * Encodes a utf8 integer array to a ucs2 string.
     * @param s, an integer array
     * @return a string
     */
    var utf8_ucs2 = function(s){
        if(!s) return null;
        var len = s.length;
        if(len == 0) return "";

        var d = [], c = 0, i = 0, tmp = 0;
        while(i < len){
            c = s[i++];
            if((c & 0xe0) == 0xe0){
                // 3 bytes

                tmp = (c & 0x0f) << 12;
                c = s[i++];
                tmp |= ((c & 0x3f) << 6);
                c = s[i++];
                tmp |= (c & 0x3f);
            }else
                if((c & 0xc0) == 0xc0){
                    // 2 bytes

                    tmp = (c & 0x1f) << 6;
                    c = s[i++];
                    tmp |= (c & 0x3f);
                }else{
                    // 1 byte

                    tmp = c;
                }

            d.push(String.fromCharCode(tmp));
        }

        return d.join("");
    };

    /**
     * Encodes a ucs2 string to a utf8 integer array.
     * @param s, a string
     * @return an integer array
     */
    var ucs2_utf8 = function(s){
        if (!s) return null;
        var d = [];
        if (s == "") return d;

        var c = 0, i = 0, j = 0, len = s.length;
        while(i < len){
            c = s.charCodeAt(i++);
            if(c <= 0x7f){
                // 1 byte

                d[j++] = c;
            }else
                if((c >= 0x80) && (c <= 0x7ff)){
                    // 2 bytes

                    d[j++] = ((c >> 6) & 0x1f) | 0xc0;
                    d[j++] = (c & 0x3f) | 0x80;
                }else{
                    // 3 bytes

                    d[j++] = (c >> 12) | 0xe0;
                    d[j++] = ((c >> 6) & 0x3f) | 0x80;
                    d[j++] = (c & 0x3f) | 0x80;
                }
        }

        return d;
    };

}();