Hi, I have a school assignment, to send a jpg image,split it into groups of 100 bytes, corrupt it, use a CRC check to locate the errors and re-transmit until it eventually is built back into its original form. It's practically ready, however when I check out the new images, they appear with errors.. I would really appreciate if someone could look at my code below and maybe locate this logical mistake as I can't understand what the problem is because everything looks ok :S For the file with all the data needed including photos and error patterns one could download it from this link:http://rapidshare.com/#!download|932tl2|443122762|Data.zip|739 Thanks in advance, Stefan p.s dont forget to change the paths in the code for the image and error files package networks; import java.io.*; // for file reader import java.util.zip.CRC32; // CRC32 IEEE (Ethernet) public class Main { /** * Reads a whole file into an array of bytes. * @param file The file in question. * @return Array of bytes containing file data. * @throws IOException Message contains why it failed. */ public static byte[] readFileArray(File file) throws IOException { InputStream is = new FileInputStream(file); byte[] data=new byte[(int)file.length()]; is.read(data); is.close(); return data; } /** * Writes (or overwrites if exists) a file with data from an array of bytes. * @param file The file in question. * @param data Array of bytes containing the new file data. * @throws IOException Message contains why it failed. */ public static void writeFileArray(File file, byte[] data) throws IOException { OutputStream os = new FileOutputStream(file,false); os.write(data); os.close(); } /** * Converts a long value to an array of bytes. * @param data The target variable. * @return Byte array conversion of data. * @see http://www.daniweb.com/code/snippet216874.html */ public static byte[] toByta(long data) { return new byte[] { (byte)((data >> 56) & 0xff), (byte)((data >> 48) & 0xff), (byte)((data >> 40) & 0xff), (byte)((data >> 32) & 0xff), (byte)((data >> 24) & 0xff), (byte)((data >> 16) & 0xff), (byte)((data >> 8) & 0xff), (byte)((data >> 0) & 0xff), }; } /** * Converts a an array of bytes to long value. * @param data The target variable. * @return Long value conversion of data. * @see http://www.daniweb.com/code/snippet216874.html */ public static long toLong(byte[] data) { if (data == null || data.length != 8) return 0x0; return (long)( // (Below) convert to longs before shift because digits // are lost with ints beyond the 32-bit limit (long)(0xff & data[0]) << 56 | (long)(0xff & data[1]) << 48 | (long)(0xff & data[2]) << 40 | (long)(0xff & data[3]) << 32 | (long)(0xff & data[4]) << 24 | (long)(0xff & data[5]) << 16 | (long)(0xff & data[6]) << 8 | (long)(0xff & data[7]) << 0 ); } public static byte[] nextNoise(){ byte[] result=new byte[100]; // copy a frame's worth of data (or remaining data if it is less than frame length) int read=Math.min(err_data.length-err_pstn, 100); System.arraycopy(err_data, err_pstn, result, 0, read); // if read data is less than frame length, reset position and add remaining data if(read<100){ err_pstn=100-read; System.arraycopy(err_data, 0, result, read, err_pstn); }else // otherwise, increase position err_pstn+=100; // return noise segment return result; } /** * Given some original data, it is purposefully corrupted according to a * second data array (which is read from a file). In pseudocode: * corrupt = original xor corruptor * @param data The original data. * @return The new (corrupted) data. */ public static byte[] corruptData(byte[] data){ // get the next noise sequence byte[] noise = nextNoise(); // finally, xor data with noise and return result for(int i=0; i<100; i++)data[i]^=noise[i]; return data; } /** * Given an array of data, a packet is created. In pseudocode: * frame = corrupt(data) + crc(data) * @param data The original frame data. * @return The resulting frame data. */ public static byte[] buildFrame(byte[] data){ // pack = [data]+crc32([data]) byte[] hash = new byte[8]; // calculate crc32 of data and copy it to byte array CRC32 crc = new CRC32(); crc.update(data); hash=toByta(crc.getValue()); // create a byte array holding the final packet byte[] pack = new byte[data.length+hash.length]; // create the corrupted data byte[] crpt = new byte[data.length]; crpt = corruptData(data); // copy corrupted data into pack System.arraycopy(crpt, 0, pack, 0, crpt.length); // copy hash into pack System.arraycopy(hash, 0, pack, data.length, hash.length); // return pack return pack; } /** * Verifies frame contents. * @param frame The frame data (data+crc32). * @return True if frame is valid, false otherwise. */ public static boolean verifyFrame(byte[] frame){ // allocate hash and data variables byte[] hash=new byte[8]; byte[] data=new byte[frame.length-hash.length]; // read frame into hash and data variables System.arraycopy(frame, frame.length-hash.length, hash, 0, hash.length); System.arraycopy(frame, 0, data, 0, frame.length-hash.length); // get crc32 of data CRC32 crc = new CRC32(); crc.update(data); // compare crc32 of data with crc32 of frame return crc.getValue()==toLong(hash); } /** * Transfers a file through a channel in frames and reconstructs it into a new file. * @param jpg_file File name of target file to transfer. * @param err_file The channel noise file used to simulate corruption. * @param out_file The name of the newly-created file. * @throws IOException */ public static void transferFile(String jpg_file, String err_file, String out_file) throws IOException { // read file data into global variables jpg_data = readFileArray(new File(jpg_file)); err_data = readFileArray(new File(err_file)); err_pstn = 0; // variable that will hold the final (transfered) data byte[] out_data = new byte[jpg_data.length]; // holds the current frame data byte[] frame_orig = new byte[100]; byte[] frame_sent = new byte[100]; // send file in chunks (frames) of 100 bytes for(int i=0; i<Math.ceil(jpg_data.length/100); i++){ // copy jpg data into frame and init first-time switch System.arraycopy(jpg_data, i*100, frame_orig, 0, 100); boolean not_first=false; System.out.print("Packet #"+i+": "); // repeat getting same frame until frame crc matches with frame content do { if(not_first)System.out.print("F"); frame_sent=buildFrame(frame_orig); not_first=true; }while(!verifyFrame(frame_sent)); // usually, you'd constrain this by time to prevent infinite loops (in // case the channel is so wacked up it doesn't get a single packet right) // copy frame to image file System.out.println("S"); System.arraycopy(frame_sent, 0, out_data, i*100, 100); } System.out.println("\nDone."); writeFileArray(new File(out_file),out_data); } // global variables for file data and pointer public static byte[] jpg_data; public static byte[] err_data; public static int err_pstn=0; public static void main(String[] args) throws IOException { // list of jpg files String[] jpg_file={ "C:\\Users\\Stefan\\Desktop\\Data\\Images\\photo1.jpg", "C:\\Users\\Stefan\\Desktop\\Data\\Images\\photo2.jpg", "C:\\Users\\Stefan\\Desktop\\Data\\Images\\photo3.jpg", "C:\\Users\\Stefan\\Desktop\\Data\\Images\\photo4.jpg" }; // list of error patterns String[] err_file={ "C:\\Users\\Stefan\\Desktop\\Data\\Error Pattern\\Error Pattern 1.DAT", "C:\\Users\\Stefan\\Desktop\\Data\\Error Pattern\\Error Pattern 2.DAT", "C:\\Users\\Stefan\\Desktop\\Data\\Error Pattern\\Error Pattern 3.DAT", "C:\\Users\\Stefan\\Desktop\\Data\\Error Pattern\\Error Pattern 4.DAT" }; // loop through all jpg/channel combinations and run tests for(int x=0; x<jpg_file.length; x++){ for(int y=0; y<err_file.length; y++){ System.out.println("Transfering photo"+(x+1)+".jpg using Pattern "+(y+1)+"..."); transferFile(jpg_file[x],err_file[y],jpg_file[x].replace("photo","CH#"+y+"_photo")); } } } }