Add customer verifier

Add custom verifier so reading the entire torrent file(s) is not necessary on startup.

lib/torrent.js

@@ -117,6 +117,7 @@ class Torrent extends EventEmitter {
     // TODO: remove this and expose a hook instead
     // optimization: don't recheck every file if it hasn't changed
     this._fileModtimes = opts.fileModtimes
+    this._verifyPiece = opts.verifyPiece
 
     if (torrentId !== null) this._onTorrentId(torrentId)
 
@@ -566,6 +567,14 @@ class Torrent extends EventEmitter {
     parallelLimit(this.pieces.map((piece, index) => cb => {
       if (this.destroyed) return cb(new Error('torrent is destroyed'))
 
+      if (this._verifyPiece) {
+        if (this._verifyPiece(index, this._hashes[index], this.infoHash)) {
+          this._debug('piece verified %s', index)
+          this._markVerified(index)
+        }
+        return process.nextTick(cb, null)
+      }
+
       this.store.get(index, (err, buf) => {
         if (this.destroyed) return cb(new Error('torrent is destroyed'))
 
@@ -611,6 +620,7 @@ class Torrent extends EventEmitter {
     this.pieces[index] = null
     this._reservations[index] = null
     this.bitfield.set(index, true)
+    this.emit('verified', index, this._hashes[index])
   }
 
   /**
@@ -1527,6 +1537,7 @@ class Torrent extends EventEmitter {
           self.pieces[index] = null
           self._reservations[index] = null
           self.bitfield.set(index, true)
+          self.emit('verified', index, hash)
 
           self.store.put(index, buf)