/**
*    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];
    }
};
}