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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/>.
*/
#pragma once
#include "mongo/pch.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;
const unsigned long long *p = (const unsigned long long *) buf;
unsigned long long a = 0;
for( unsigned i = 0; i < n; i++ ) {
a += (*p ^ i);
p++;
}
unsigned long long b = 0;
for( unsigned i = 0; i < n; i++ ) {
b += (*p ^ i);
p++;
}
unsigned long long c = 0;
for( unsigned i = n * 2 * 8; i < len; i++ ) { // 0-7 bytes left
c = (c << 8) | ((const 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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