qetag.js

/*
 计算文件的eTag，参数为buffer或者readableStream或者文件路径
 参考地址：
 https://github.com/qiniu/qetag/blob/master/qetag.js
*/

function getEtag(buffer,callback){

    // 判断传入的参数是buffer还是stream还是filepath
    var mode = 'buffer';

    if(typeof buffer === 'string'){
        buffer = require('fs').createReadStream(buffer);
        mode='stream';
    }else if(buffer instanceof require('stream')){
        mode='stream';
    }

    // sha1算法
    var sha1 = function(content){
        var crypto = require('crypto');
        var sha1 = crypto.createHash('sha1');
        sha1.update(content);
        return sha1.digest();
    };

    // 以4M为单位分割
    var blockSize = 4*1024*1024;
    var sha1String = [];
    var prefix = 0x16;
    var blockCount = 0;

    switch(mode){
        case 'buffer':
            var bufferSize = buffer.length;
            blockCount = Math.ceil(bufferSize / blockSize);

            for(var i=0;i<blockCount;i++){
                sha1String.push(sha1(buffer.slice(i*blockSize,(i+1)*blockSize)));
            }
            process.nextTick(function(){
                callback(calcEtag());
            });
            break;
        case 'stream':
            var stream = buffer;
            stream.on('readable', function() {
                var chunk;
                while (chunk = stream.read(blockSize)) {
                    sha1String.push(sha1(chunk));
                    blockCount++;
                }
            });
            stream.on('end',function(){
                callback(calcEtag());
            });
            break;
    }

    function calcEtag(){
        if(!sha1String.length){
            return 'Fto5o-5ea0sNMlW_75VgGJCv2AcJ';
        }
        var sha1Buffer = Buffer.concat(sha1String,blockCount * 20);

        // 如果大于4M，则对各个块的sha1结果再次sha1
        if(blockCount > 1){
            prefix = 0x96;
            sha1Buffer = sha1(sha1Buffer);
        }

        sha1Buffer = Buffer.concat(
            [new Buffer([prefix]),sha1Buffer],
            sha1Buffer.length + 1
        );

        return sha1Buffer.toString('base64')
            .replace(/\//g,'_').replace(/\+/g,'-');

    }

}

module.exports = getEtag;