_verifyPiecesUsingHash (pieces, cb) {
    parallelLimit(pieces.map((piece, index) => cb => {
      const target = Number.isInteger(piece) ? piece : index
      this._verifyPiece(target, (err, isVerified) => {
        if (err) return cb(err)
        if (isVerified) {
          this._debug('piece verified %s', target)
          this._markVerified(target)
        } else {
          this._markUnverified(target)
          this._debug('piece invalid %s', target)
        }
        cb(null)
      })
    }), FILESYSTEM_CONCURRENCY, cb)
  }
Verifies the pieces of the torrent using hashing ( it can be very slow )
@param {Array<number>} pieces
@param {callbackWithError} cb
@private