Fix file progress sometimes < 0 and > 1

Code reviewed by @jhiesey

1 files changed, 28 insertions, 15 deletions

diff --git a/lib/file.js b/lib/file.js
index abd4f5d..caa395d 100644
--- a/lib/file.js
+++ b/lib/file.js
@@ -38,30 +38,43 @@ class File extends EventEmitter {
   get downloaded () {
     if (!this._torrent.bitfield) return 0
 
-    const { pieces, bitfield, pieceLength } = this._torrent
+    const { pieces, bitfield, pieceLength, lastPieceLength } = this._torrent
     const { _startPiece: start, _endPiece: end } = this
-    const piece = pieces[start]
 
-    // First piece may have an offset, e.g. irrelevant bytes from the end of
-    // the previous file
-    const irrelevantFirstPieceBytes = this.offset % pieceLength
-    let downloaded = bitfield.get(start)
-      ? pieceLength - irrelevantFirstPieceBytes
-      : Math.max(pieceLength - irrelevantFirstPieceBytes - piece.missing, 0)
+    const getPieceLength = (pieceIndex) => (
+      pieceIndex === pieces.length - 1 ? lastPieceLength : pieceLength
+    )
 
-    for (let index = start + 1; index <= end; ++index) {
-      if (bitfield.get(index)) {
+    const getPieceDownloaded = (pieceIndex) => {
+      const len = pieceIndex === pieces.length - 1 ? lastPieceLength : pieceLength
+      if (bitfield.get(pieceIndex)) {
         // verified data
-        downloaded += pieceLength
+        return len
       } else {
         // "in progress" data
-        const piece = pieces[index]
-        downloaded += pieceLength - piece.missing
+        return len - pieces[pieceIndex].missing
       }
     }
 
-    const irrelevantLastPieceBytes = pieceLength - ((this.offset + this.length) % pieceLength)
-    downloaded -= irrelevantLastPieceBytes
+    let downloaded = 0
+    for (let index = start; index <= end; index += 1) {
+      const pieceDownloaded = getPieceDownloaded(index)
+      downloaded += pieceDownloaded
+
+      if (index === start) {
+        // First piece may have an offset, e.g. irrelevant bytes from the end of
+        // the previous file
+        const irrelevantFirstPieceBytes = this.offset % pieceLength
+        downloaded -= Math.min(irrelevantFirstPieceBytes, pieceDownloaded)
+      }
+
+      if (index === end) {
+        // Last piece may have an offset, e.g. irrelevant bytes from the start
+        // of the next file
+        const irrelevantLastPieceBytes = getPieceLength(end) - (this.offset + this.length) % pieceLength
+        downloaded -= Math.min(irrelevantLastPieceBytes, pieceDownloaded)
+      }
+    }
 
     return downloaded
   }