RecordHeader.java

/*******************************************************************************
 * Copyright 2010 Cees De Groot, Alex Boisvert, Jan Kotek
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 ******************************************************************************/


package org.apache.jdbm;

/**
 * The data that comes at the start of a record of data. It stores
 * both the current size and the avaliable size for the record - the latter
 * can be bigger than the former, which allows the record to grow without
 * needing to be moved and which allows the system to put small records
 * in larger free spots.
 * <p/>
 * In JDBM 1.0 both values were stored as four-byte integers. This was very wastefull.
 * Now available size is stored in two bytes, it is compressed, so maximal value is up to 120 MB (not sure with exact number)
 * Current size is stored as two-byte-unsigned-short difference from Available Size.
 */
final class RecordHeader {
    // offsets
    private static final short O_CURRENTSIZE = 0; // int currentSize
    private static final short O_AVAILABLESIZE = Magic.SZ_BYTE; // int availableSize
    static final int MAX_RECORD_SIZE = 8355839;
    static final int SIZE = O_AVAILABLESIZE + Magic.SZ_SHORT;
    /**
     * Maximal difference between current and available size,
     * Maximal value is reserved for currentSize 0, so use -1
     */
    static final int MAX_SIZE_SPACE = 255 - 1;


    /**
     * Returns the current size
     */
    static int getCurrentSize(final PageIo page, final short pos) {
        int s = page.readByte(pos + O_CURRENTSIZE) & 0xFF;
        if (s == MAX_SIZE_SPACE + 1)
            return 0;
        return getAvailableSize(page, pos) - s;
    }

    /**
     * Sets the current size
     */
    static void setCurrentSize(final PageIo page, final short pos, int value) {
        if (value == 0) {
            page.writeByte(pos + O_CURRENTSIZE, (byte) (MAX_SIZE_SPACE + 1));
            return;
        }
        int availSize = getAvailableSize(page, pos);
        if (value < (availSize - MAX_SIZE_SPACE) || value > availSize)
            throw new IllegalArgumentException("currentSize out of bounds, need to realocate " + value + " - " + availSize);
        page.writeByte(pos + O_CURRENTSIZE, (byte) (availSize - value));
    }

    /**
     * Returns the available size
     */
    static int getAvailableSize(final PageIo page, final short pos) {
        return deconvertAvailSize(page.readShort(pos + O_AVAILABLESIZE));
    }

    /**
     * Sets the available size
     */
    static void setAvailableSize(final PageIo page, final short pos, int value) {
        if (value != roundAvailableSize(value))
            throw new IllegalArgumentException("value is not rounded");
        int oldCurrSize = getCurrentSize(page, pos);

        page.writeShort(pos + O_AVAILABLESIZE, convertAvailSize(value));
        setCurrentSize(page, pos, oldCurrSize);
    }


    static short convertAvailSize(final int recordSize) {
        if (recordSize <= Short.MAX_VALUE)
            return (short) recordSize;
        else {
            int shift = recordSize - Short.MAX_VALUE;
            if (shift % MAX_SIZE_SPACE == 0)
                shift = shift / MAX_SIZE_SPACE;
            else
                shift = 1 + shift / MAX_SIZE_SPACE;
            shift = -shift;
            return (short) (shift);
        }

    }

    static int deconvertAvailSize(final short converted) {
        if (converted >= 0)
            return converted;
        else {
            int shifted = -converted;
            shifted = shifted * MAX_SIZE_SPACE;
            return Short.MAX_VALUE + shifted;
        }

    }



    static int roundAvailableSize(int value) {
        if (value > MAX_RECORD_SIZE)
            new InternalError("Maximal record size (" + MAX_RECORD_SIZE + ") exceeded: " + value);
        return deconvertAvailSize(convertAvailSize(value));
    }


}