SizeClasses.java — netty Source File

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/*
 * Copyright 2020 The Netty Project
 *
 * The Netty Project licenses this file to you 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:
 *
 *   https://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 io.netty.buffer;

import static io.netty.buffer.PoolThreadCache.*;

/**
 * SizeClasses requires {@code pageShifts} to be defined prior to inclusion,
 * and it in turn defines:
 * <p>
 *   LOG2_SIZE_CLASS_GROUP: Log of size class count for each size doubling.
 *   LOG2_MAX_LOOKUP_SIZE: Log of max size class in the lookup table.
 *   sizeClasses: Complete table of [index, log2Group, log2Delta, nDelta, isMultiPageSize,
 *                 isSubPage, log2DeltaLookup] tuples.
 *     index: Size class index.
 *     log2Group: Log of group base size (no deltas added).
 *     log2Delta: Log of delta to previous size class.
 *     nDelta: Delta multiplier.
 *     isMultiPageSize: 'yes' if a multiple of the page size, 'no' otherwise.
 *     isSubPage: 'yes' if a subpage size class, 'no' otherwise.
 *     log2DeltaLookup: Same as log2Delta if a lookup table size class, 'no'
 *                      otherwise.
 * <p>
 *   nSubpages: Number of subpages size classes.
 *   nSizes: Number of size classes.
 *   nPSizes: Number of size classes that are multiples of pageSize.
 *
 *   smallMaxSizeIdx: Maximum small size class index.
 *
 *   lookupMaxClass: Maximum size class included in lookup table.
 *   log2NormalMinClass: Log of minimum normal size class.
 * <p>
 *   The first size class and spacing are 1 << LOG2_QUANTUM.
 *   Each group has 1 << LOG2_SIZE_CLASS_GROUP of size classes.
 *
 *   size = 1 << log2Group + nDelta * (1 << log2Delta)
 *
 *   The first size class has an unusual encoding, because the size has to be
 *   split between group and delta*nDelta.
 *
 *   If pageShift = 13, sizeClasses looks like this:
 *
 *   (index, log2Group, log2Delta, nDelta, isMultiPageSize, isSubPage, log2DeltaLookup)
 * <p>
 *   ( 0,     4,        4,         0,       no,             yes,        4)
 *   ( 1,     4,        4,         1,       no,             yes,        4)
 *   ( 2,     4,        4,         2,       no,             yes,        4)
// ... (354 more lines)