/**
 *    Copyright (C) 2018-present MongoDB, Inc.
 *
 *    This program is free software: you can redistribute it and/or modify
 *    it under the terms of the Server Side Public License, version 1,
 *    as published by MongoDB, Inc.
 *
 *    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
 *    Server Side Public License for more details.
 *
 *    You should have received a copy of the Server Side Public License
 *    along with this program. If not, see
 *    <http://www.mongodb.com/licensing/server-side-public-license>.
 *
 *    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 Server Side 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) {
        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];
    }
};
}