Threema-Android-mirror/app/src/main/java/ch/threema/app/utils/GzipOutputStream.java

/*  _____ _
 * |_   _| |_  _ _ ___ ___ _ __  __ _
 *   | | | ' \| '_/ -_) -_) '  \/ _` |_
 *   |_| |_||_|_| \___\___|_|_|_\__,_(_)
 *
 * Threema for Android
 * Copyright (c) 2018-2022 Threema GmbH
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License, version 3,
 * as published by the Free Software Foundation.
 *
 * This program 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 Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see <https://www.gnu.org/licenses/>.
 */

package ch.threema.app.utils;

import static java.util.zip.GZIPInputStream.GZIP_MAGIC;

import java.io.IOException;
import java.io.OutputStream;
import java.util.zip.CRC32;
import java.util.zip.Deflater;
import java.util.zip.DeflaterOutputStream;

/**
 * Copy of AOSP's GZIPOutputStream exposing a parameter for Deflator's compression level
 */
public class GzipOutputStream extends DeflaterOutputStream {
	protected CRC32 crc = new CRC32();
	private final static int TRAILER_SIZE = 8;

	public GzipOutputStream(OutputStream out, int level, int size, boolean syncFlush) throws IOException {
		super(out, new Deflater(level, true), size, syncFlush);
		writeHeader();
		crc.reset();
	}

	public synchronized void write(byte[] buf, int off, int len) throws IOException {
		super.write(buf, off, len);
		crc.update(buf, off, len);
	}

	public void finish() throws IOException {
		if (!def.finished()) {
			def.finish();
			while (!def.finished()) {
				int len = def.deflate(buf, 0, buf.length);
				if (def.finished() && len <= buf.length - TRAILER_SIZE) {
					writeTrailer(buf, len);
					len = len + TRAILER_SIZE;
					out.write(buf, 0, len);
					return;
				}
				if (len > 0)
					out.write(buf, 0, len);
			}
			byte[] trailer = new byte[TRAILER_SIZE];
			writeTrailer(trailer, 0);
			out.write(trailer);
		}
	}

	private void writeHeader() throws IOException {
		out.write(new byte[] {
			(byte) GZIP_MAGIC,
			(byte)(GZIP_MAGIC >> 8),
			Deflater.DEFLATED,
			0,0,0,0,0,0,0
		});
	}

	private void writeTrailer(byte[] buf, int offset) {
		writeInt((int)crc.getValue(), buf, offset);
		writeInt(def.getTotalIn(), buf, offset + 4);
	}

	private void writeInt(int i, byte[] buf, int offset) {
		writeShort(i & 0xffff, buf, offset);
		writeShort((i >> 16) & 0xffff, buf, offset + 2);
	}

	private void writeShort(int s, byte[] buf, int offset) {
		buf[offset] = (byte)(s & 0xff);
		buf[offset + 1] = (byte)((s >> 8) & 0xff);
	}
}