calculate file downloaded correctly

lib/file.js

@@ -36,18 +36,30 @@ class File extends EventEmitter {
 
   get downloaded () {
     if (!this._torrent.bitfield) return 0
-    let downloaded = 0
-    for (let index = this._startPiece; index <= this._endPiece; ++index) {
-      if (this._torrent.bitfield.get(index)) {
+
+    const { pieces, bitfield, pieceLength } = this._torrent
+    const { _startPiece: start, _endPiece: end } = this
+    const piece = pieces[start]
+
+    // Calculate first piece diffrently, it sometimes have a offset
+    let downloaded = bitfield.get(start)
+      ? pieceLength - this.offset
+      : Math.max(piece.length - piece.missing - this.offset, 0)
+
+    for (let index = start + 1; index <= end; ++index) {
+      if (bitfield.get(index)) {
         // verified data
-        downloaded += (index === this._endPiece) ? this._torrent.lastPieceLength : this._torrent.pieceLength
+        downloaded += pieceLength
       } else {
         // "in progress" data
-        const piece = this._torrent.pieces[index]
+        const piece = pieces[index]
         downloaded += piece.length - piece.missing
       }
     }
-    return downloaded
+
+    // We don't have a end-offset and one small file can fith in the middle
+    // of one chunk, so return this.length if it's oversized
+    return Math.min(downloaded, this.length)
   }
 
   get progress () {

test/node/basic.js

@@ -112,7 +112,7 @@ test('client.seed: filesystem path to folder with one file, string', function (t
 })
 
 test('client.seed: filesystem path to folder with multiple files, string', function (t) {
-  t.plan(6)
+  t.plan(7)
 
   var client = new WebTorrent({ dht: false, tracker: false })
 
@@ -124,6 +124,17 @@ test('client.seed: filesystem path to folder with multiple files, string', funct
     t.equal(torrent.infoHash, fixtures.numbers.parsedTorrent.infoHash)
     t.equal(torrent.magnetURI, fixtures.numbers.magnetURI)
 
+    const downloaded = torrent.files.map(file => ({
+      length: file.length,
+      downloaded: file.downloaded
+    }))
+
+    t.deepEqual(downloaded, [
+      { length: 1, downloaded: 1 },
+      { length: 2, downloaded: 2 },
+      { length: 3, downloaded: 3 }
+    ], 'expected downloaded to be calculated correctly')
+
     client.remove(torrent, function (err) { t.error(err, 'torrent destroyed') })
     t.equal(client.torrents.length, 0)