124 lines
4.3 KiB
Java
124 lines
4.3 KiB
Java
/*******************************************************************************
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* Copyright 2010 Cees De Groot, Alex Boisvert, Jan Kotek
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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******************************************************************************/
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package org.apache.jdbm;
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/**
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* The data that comes at the start of a record of data. It stores
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* both the current size and the avaliable size for the record - the latter
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* can be bigger than the former, which allows the record to grow without
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* needing to be moved and which allows the system to put small records
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* in larger free spots.
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* <p/>
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* In JDBM 1.0 both values were stored as four-byte integers. This was very wastefull.
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* Now available size is stored in two bytes, it is compressed, so maximal value is up to 120 MB (not sure with exact number)
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* Current size is stored as two-byte-unsigned-short difference from Available Size.
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*/
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final class RecordHeader {
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// offsets
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private static final short O_CURRENTSIZE = 0; // int currentSize
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private static final short O_AVAILABLESIZE = Magic.SZ_BYTE; // int availableSize
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static final int MAX_RECORD_SIZE = 8355839;
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static final int SIZE = O_AVAILABLESIZE + Magic.SZ_SHORT;
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/**
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* Maximal difference between current and available size,
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* Maximal value is reserved for currentSize 0, so use -1
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*/
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static final int MAX_SIZE_SPACE = 255 - 1;
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/**
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* Returns the current size
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*/
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static int getCurrentSize(final PageIo page, final short pos) {
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int s = page.readByte(pos + O_CURRENTSIZE) & 0xFF;
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if (s == MAX_SIZE_SPACE + 1)
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return 0;
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return getAvailableSize(page, pos) - s;
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}
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/**
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* Sets the current size
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*/
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static void setCurrentSize(final PageIo page, final short pos, int value) {
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if (value == 0) {
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page.writeByte(pos + O_CURRENTSIZE, (byte) (MAX_SIZE_SPACE + 1));
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return;
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}
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int availSize = getAvailableSize(page, pos);
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if (value < (availSize - MAX_SIZE_SPACE) || value > availSize)
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throw new IllegalArgumentException("currentSize out of bounds, need to realocate " + value + " - " + availSize);
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page.writeByte(pos + O_CURRENTSIZE, (byte) (availSize - value));
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}
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/**
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* Returns the available size
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*/
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static int getAvailableSize(final PageIo page, final short pos) {
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return deconvertAvailSize(page.readShort(pos + O_AVAILABLESIZE));
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}
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/**
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* Sets the available size
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*/
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static void setAvailableSize(final PageIo page, final short pos, int value) {
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if (value != roundAvailableSize(value))
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throw new IllegalArgumentException("value is not rounded");
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int oldCurrSize = getCurrentSize(page, pos);
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page.writeShort(pos + O_AVAILABLESIZE, convertAvailSize(value));
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setCurrentSize(page, pos, oldCurrSize);
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}
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static short convertAvailSize(final int recordSize) {
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if (recordSize <= Short.MAX_VALUE)
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return (short) recordSize;
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else {
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int shift = recordSize - Short.MAX_VALUE;
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if (shift % MAX_SIZE_SPACE == 0)
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shift = shift / MAX_SIZE_SPACE;
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else
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shift = 1 + shift / MAX_SIZE_SPACE;
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shift = -shift;
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return (short) (shift);
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}
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}
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static int deconvertAvailSize(final short converted) {
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if (converted >= 0)
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return converted;
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else {
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int shifted = -converted;
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shifted = shifted * MAX_SIZE_SPACE;
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return Short.MAX_VALUE + shifted;
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}
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}
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static int roundAvailableSize(int value) {
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if (value > MAX_RECORD_SIZE)
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new InternalError("Maximal record size (" + MAX_RECORD_SIZE + ") exceeded: " + value);
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return deconvertAvailSize(convertAvailSize(value));
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}
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}
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