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/**
* Copyright (C) 2012 10gen Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* 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 Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the GNU Affero General Public License in all respects
* for all of the code used other than as permitted herein. If you modify
* file(s) with this exception, 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. If you
* delete this exception statement from all source files in the program,
* then also delete it in the license file.
*/
#pragma once
#include "mongo/base/data_cursor.h"
#include "mongo/platform/basic.h"
namespace mongo {
/** a simple, rather dumb, but very fast checksum. see perftests.cpp for unit tests. */
struct Checksum {
union {
unsigned char bytes[16];
unsigned long long words[2];
};
// if you change this you must bump dur::CurrentVersion
void gen(const void* buf, unsigned len) {
wassert(((size_t)buf) % 8 == 0); // performance warning
unsigned n = len / 8 / 2;
ConstDataCursor cdc(static_cast<const char*>(buf));
uint64_t a = 0;
for (unsigned i = 0; i < n; i++) {
a += (cdc.readAndAdvance<LittleEndian<uint64_t>>() ^ i);
}
uint64_t b = 0;
for (unsigned i = 0; i < n; i++) {
b += (cdc.readAndAdvance<LittleEndian<uint64_t>>() ^ i);
}
uint64_t c = 0;
for (unsigned i = n * 2 * 8; i < len; i++) { // 0-7 bytes left
c = (c << 8) | ((const signed char*)buf)[i];
}
words[0] = a ^ len;
words[1] = b ^ c;
}
bool operator==(const Checksum& rhs) const {
return words[0] == rhs.words[0] && words[1] == rhs.words[1];
}
bool operator!=(const Checksum& rhs) const {
return words[0] != rhs.words[0] || words[1] != rhs.words[1];
}
};
}
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