PngDecodingDataStream.java :  » IDE-Eclipse » swt » org » eclipse » swt » internal » image » Java Open Source

/*******************************************************************************
 * Copyright (c) 2000, 2006 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.eclipse.swt.internal.image;


import java.io.*;

import org.eclipse.swt.*;

public class PngDecodingDataStream extends InputStream {
  InputStream stream;
  byte currentByte;
  int nextBitIndex;
  
  PngLzBlockReader lzBlockReader;
  int adlerValue;
  
  static final int PRIME = 65521;
  static final int MAX_BIT = 7;    
  
PngDecodingDataStream(InputStream stream) throws IOException {
  super();
  this.stream = stream;
  nextBitIndex = MAX_BIT + 1;
  adlerValue = 1;
  lzBlockReader = new PngLzBlockReader(this);
  readCompressedDataHeader();
  lzBlockReader.readNextBlockHeader();
}

/**
 * This method should be called when the image decoder thinks
 * that all of the compressed image data has been read. This
 * method will ensure that the next data value is an end of 
 * block marker. If there are more blocks after this one,
 * the method will read them and ensure that they are empty.
 */
void assertImageDataAtEnd() throws IOException {
  lzBlockReader.assertCompressedDataAtEnd();
}

public void close() throws IOException {
  assertImageDataAtEnd();
  checkAdler();
}

int getNextIdatBits(int length) throws IOException {
  int value = 0;
  for (int i = 0; i < length; i++) {
    value |= (getNextIdatBit() << i);
  }
  return value;
}

int getNextIdatBit() throws IOException {
  if (nextBitIndex > MAX_BIT) {
    currentByte = getNextIdatByte();
    nextBitIndex = 0;
  }
  return (currentByte & (1 << nextBitIndex)) >> nextBitIndex++;
}

byte getNextIdatByte() throws IOException {  
  byte nextByte = (byte)stream.read();
  nextBitIndex = MAX_BIT + 1;
  return nextByte;
}

void updateAdler(byte value) {
  int low = adlerValue & 0xFFFF;
  int high = (adlerValue >> 16) & 0xFFFF;
  int valueInt = value & 0xFF;
  low = (low + valueInt) % PRIME;
  high = (low + high) % PRIME;
  adlerValue = (high << 16) | low;
}

public int read() throws IOException {
  byte nextDecodedByte = lzBlockReader.getNextByte();
  updateAdler(nextDecodedByte);
  return nextDecodedByte & 0xFF;
}

public int read(byte[] buffer, int off, int len) throws IOException {
  for (int i = 0; i < len; i++) {
    int b = read();
    if (b == -1) return i;
    buffer[off + i] = (byte)b;
  }
  return len;
}

void error() {
  SWT.error(SWT.ERROR_INVALID_IMAGE);
}

private void readCompressedDataHeader() throws IOException {
  byte headerByte1 = getNextIdatByte();
  byte headerByte2 = getNextIdatByte();
  
  int number = ((headerByte1 & 0xFF) << 8) | (headerByte2 & 0xFF);
  if (number % 31 != 0) error();
  
  int compressionMethod = headerByte1 & 0x0F;
  if (compressionMethod != 8) error();
  
  int windowSizeHint = (headerByte1 & 0xF0) >> 4;
  if (windowSizeHint > 7) error();
  int windowSize = (1 << (windowSizeHint + 8));
  lzBlockReader.setWindowSize(windowSize);
  
  int dictionary = (headerByte2 & (1 << 5));
  if (dictionary != 0) error();
  
//  int compressionLevel = (headerByte2 & 0xC0) >> 6;
}

void checkAdler() throws IOException {
  int storedAdler = ((getNextIdatByte() & 0xFF) << 24)
    | ((getNextIdatByte() & 0xFF) << 16)
    | ((getNextIdatByte() & 0xFF) << 8)
    | (getNextIdatByte() & 0xFF);
  if (storedAdler != adlerValue) error();
}

}