/*

 * Copyright (c) 2014, 2016, 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

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package jdk.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;

    }

}