// 计算文件的eTag，参数为buffer或者readableStream或者文件路径
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(null, 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(null, 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;