content: /*
   * Copyright (c) 2022, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code 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 General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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  package javac.internal.jimage.decompressor;
  
  import java.io.DataInputStream;
  import java.io.IOException;
  import java.nio.ByteBuffer;
  import java.util.ArrayList;
  import java.util.List;
  
  /**
   *
   * Index compressor. Use the minimal amount of bytes required to store
   * an integer.
   *
   * @implNote This class needs to maintain JDK 8 source compatibility.
   *
   * It is used internally in the JDK to implement jimage/jrtfs access,
   * but also compiled and delivered as part of the jrtfs.jar to support access
   * to the jimage file provided by the shipped JDK by tools running on JDK 8.
   */
  public class CompressIndexes {
      private static final int COMPRESSED_FLAG = 1 << (Byte.SIZE - 1);
      private static final int HEADER_WIDTH = 3;
      private static final int HEADER_SHIFT = Byte.SIZE - HEADER_WIDTH;
  
      public static List<Integer> decompressFlow(byte[] values) {
          List<Integer> lst = new ArrayList<>();
  
          for (int i = 0; i < values.length; i += getHeaderLength(values[i])) {
              int decompressed = decompress(values, i);
              lst.add(decompressed);
          }
  
          return lst;
      }
  
      public static int readInt(DataInputStream cr) throws IOException {
          // Get header byte.
          byte header = cr.readByte();
          // Determine size.
          int size = getHeaderLength(header);
          // Prepare result.
          int result = getHeaderValue(header);
  
          // For each value byte
          for (int i = 1; i < size; i++) {
              // Merge byte value.
              result <<= Byte.SIZE;
              result |= cr.readByte() & 0xFF;
          }
  
          return result;
      }
  
      private static boolean isCompressed(byte b) {
          return (b & COMPRESSED_FLAG) != 0;
      }
  
      private static int getHeaderLength(byte b) {
          return isCompressed(b) ? (b >> HEADER_SHIFT) & 3 : Integer.BYTES;
      }
  
      private static int getHeaderValue(byte b) {
          return isCompressed(b) ? b & (1 << HEADER_SHIFT) - 1 : b;
      }
  
      public static int decompress(byte[] value, int offset) {
          // Get header byte.
          byte header = value[offset];
          // Determine size.
          int size = getHeaderLength(header);
          // Prepare result.
          int result = getHeaderValue(header);
  
          // For each value byte
          for (int i = 1; i < size; i++) {
              // Merge byte value.
              result <<= Byte.SIZE;
              result |= value[offset + i] & 0xFF;
          }
  
          return result;
      }
  
      public static byte[] compress(int value) {
          // Only positive values are supported.
          if (value < 0) {
              throw new  IllegalArgumentException("value < 0");
          }
  
          // Determine number of significant digits.
          int width = 32 - Integer.numberOfLeadingZeros(value);
          // Determine number of byte to represent.  Allow for header if
          // compressed.
          int size = Math.min(((width + HEADER_WIDTH - 1) >> 3) + 1, Integer.BYTES);
  
          // Allocate result buffer.
          byte[] result = new byte[size];
  
          // Insert significant bytes in result.
          for (int i = 0; i < size; i++) {
              result[i] = (byte)(value >> ((size - i - 1) * Byte.SIZE));
          }
  
          // If compressed, mark and insert size.
          if (size < Integer.BYTES) {
              result[0] |= (byte)(COMPRESSED_FLAG | (size << HEADER_SHIFT));
          }
  
          return result;
      }
  
  }
  