FT: move the MD5Sum from S3

Move the MD5Sum class from scality/S3#97

index.js

@@ -1,4 +1,5 @@
 module.exports = {
+    MD5Sum: require('./lib/MD5Sum.js'),
     auth: require('./lib/auth/auth'),
     constants: require('./lib/constants'),
     db: require('./lib/db'),

lib/MD5Sum.js

@@ -0,0 +1,50 @@
+'use strict'; // eslint-disable-line strict
+
+const Transform = require('stream').Transform;
+const crypto = require('crypto');
+
+/**
+ * This class is design to compute md5 hash at the same time as sending
+ * data through a stream
+ */
+class MD5Sum extends Transform {
+
+    /**
+     * @constructor
+     * @param {function} done - Callback(hash) - This callback is called when
+     * the hash computation is finished, this function need to be synchronous
+     * and being call before the end of the stream object
+     */
+    constructor(done) {
+        super({});
+        this.hash = crypto.createHash('md5');
+        this.done = done;
+    }
+
+    /**
+     * This function will update the current md5 hash with the next chunk
+     *
+     * @param {Buffer|string} chunk - Chunk to compute
+     * @param {string} encoding - Data encoding
+     * @param {function} callback - Callback(err, chunk, encoding)
+     * @return {undefined}
+     */
+    _transform(chunk, encoding, callback) {
+        this.hash.update(chunk, encoding);
+        callback(null, chunk, encoding);
+    }
+
+    /**
+     * This function will end the hash computation
+     *
+     * @param {function} callback(err)
+     * @return {undefined}
+     */
+    _flush(callback) {
+        this.done(this.hash.digest('hex'));
+        callback(null);
+    }
+
+}
+
+module.exports = { MD5Sum };