Speed up bitfields conversion

The old code took 3~5 times longer than the newer version.

src/base/bittorrent/ltqbitarray.cpp

@@ -0,0 +1,59 @@
+/*
+ * Bittorrent Client using Qt and libtorrent.
+ * Copyright (C) 2022  Mike Tzou (Chocobo1)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ *
+ * In addition, as a special exception, the copyright holders give permission to
+ * link this program with the OpenSSL project's "OpenSSL" library (or with
+ * modified versions of it that use the same license as the "OpenSSL" library),
+ * and distribute the linked executables. You must obey the GNU General Public
+ * License in all respects for all of the code used other than "OpenSSL".  If you
+ * modify file(s), you may extend this exception to your version of the file(s),
+ * but you are not obligated to do so. If you do not wish to do so, delete this
+ * exception statement from your version.
+ */
+
+#include "ltqbitarray.h"
+
+#include <memory>
+
+#include <libtorrent/bitfield.hpp>
+
+#include <QBitArray>
+
+namespace
+{
+    unsigned char reverseByte(const unsigned char byte)
+    {
+        // https://graphics.stanford.edu/~seander/bithacks.html#ReverseByteWith64Bits
+        return (((byte * 0x80200802ULL) & 0x0884422110ULL) * 0x0101010101ULL) >> 32;
+    }
+}
+
+namespace BitTorrent::LT
+{
+    QBitArray toQBitArray(const lt::bitfield &bits)
+    {
+        const char *bitsData = bits.data();
+        const int dataLength = (bits.size() + 7) / 8;
+
+        auto tmp = std::make_unique<char []>(dataLength);
+        for (int i = 0; i < dataLength; ++i)
+            tmp[i] = reverseByte(bitsData[i]);
+
+        return QBitArray::fromBits(tmp.get(), bits.size());
+    }
+}