Converting byte to BitSet

Hi

I want to write a program that can read the header properties of a .mp3 file. If you don't know what I'm talking about, visit: MPEG AUDIO FRAME HEADER

The problem is that FileInputStream only allows you to read byte by byte. I need to convert that byte into something like a BitSet so I can read each bit in the byte.

The solution I came up with was:
1. Read the byte
2. Convert it to String using Integer.toBinaryString(byte)
3. Pad the String with zeros so there is 8 bits that can be read from it.

I have not tested this part since I'm still hoping there is a better way to do it (and it probably won't work).

Here is what I've done so far to retrieve the header and tag parts of the file:

File file; long fileSize; byte[] header; byte[] tag; FileInputStream inputStream; header = new byte[4]; tag = new byte[128]; file = new File("file.mp3"); int read = 0; if(file.exists()){ fileSize = file.length(); System.out.println("File: \n" + file.getAbsolutePath() + "\nwas successfully opened.\n\n"); }else{ System.out.println("File does not exist. Exiting..."); System.exit(0); } inputStream = new FileInputStream(file); read = inputStream.read(header); System.out.println("Read " + read + " bytes to header."); if(read != 4){ throw new IOException(); } read = (int)inputStream.skip(fileSize - 132L); System.out.println("Skipped " + read + " bytes."); if(read != fileSize - 132L){ throw new IOException(); } read = inputStream.read(tag); System.out.println("Read " + read + " bytes to tag."); if(read != 128){ throw new IOException(); } int i = 0; while(inputStream.read() != -1){ i++; System.out.print("\r"); System.out.print(i + "bytes left."); } System.out.print("\r"); System.out.print(i + "bytes left.\n"); inputStream.close();

Thanks

I'm pretty sure that this is a lot faster:
byte b = 3; // replace with input from stream BitSet bitset = new BitSet(8); for (int i=0; i<8; i++) { if ((b & (1 << i)) > 0) { bitset.set(i); } }

Or you could look at the bits the old fashioned way using Java's bitwise operators.

Bill

Wouter Oet wrote:I'm pretty sure that this is a lot faster:

Yes I would think so too. I guess it's time to start studying bitwise operators. It sucks that they took it out of SCJP 6.

Thanks

hi

any ideas how to convert back?

thanks.

Loop over the length of the BitSet, take each bit value, and bitwise or it: value |= bit << index. Note that BitSet is not as limited as int or long so you can get larger results out of them. The int / long will then be truncated.

like this?

public static byte[] fromBitSet(BitSet bits) { byte[] b = new byte[1]; for(int i = 0; i<bits.length(); i++) { b[0] |= 1 <<i ; } return b; }

still not familiar whith how | / & works with bit opperations
also not quite sure what was meant by value and bit.

thanks.

Why use a one-element byte array? What's wrong with just a byte?

That code is almost what I meant. You use b[0] for the value. The thing is, you don't test if the bit is set or not. Use the result of bits.get(i). If this returns false you don't need to do anything.

William Brogden wrote:Or you could look at the bits the old fashioned way using Java's bitwise operators.

Agreed++. Why bother converting to a bit set, when it is just as easy to check a bit, set a bit, and clear a bit with the bitwise operators. Heck, with the bitwise operators, you can check, set, or clear multiple bits simultaneously.

Henry

so this then;

public static byte[] fromBitSet(BitSet bits) { byte[] b = new byte[1]; for(int i = 0; i<bits.length(); i++) { if(bits.get(i) == true) b[0] |= 1 <<i ; } return b; }


the reason i am using an array instead of a byte is beacuse i am then using System.arraycopy() to copy each byte into a larger byte array for writing
(its part of a WAVE file steganography project)
does it work just the same as if it were a non array single byte? (im assuming it would because its still a byte)

thanks.

Henry Wong wrote:

William Brogden wrote:Or you could look at the bits the old fashioned way using Java's bitwise operators.

Agreed++. Why bother converting to a bit set, when it is just as easy to check a bit, set a bit, and clear a bit with the bitwise operators. Heck, with the bitwise operators, you can check, set, or clear multiple bits simultaneously.

Henry

by this do you mean for example to read bits from an input stream rarther than bytes?

Stephen Longville wrote:the reason i am using an array instead of a byte is beacuse i am then using System.arraycopy() to copy each byte into a larger byte array for writing
(its part of a WAVE file steganography project)
does it work just the same as if it were a non array single byte? (im assuming it would because its still a byte)

Sure. Instead of using System.arraycopy(b, 0, destination, offset, b.length) (with b being a byte[]) you use destination[offset] = b (with b being a byte).

hehe there i go over complicating things again thanks for clearing that up Rob

now im sure my byte to bit (and vice versa) functions are correct my problem still exists, if you feel like taking a look its HERE on the ranch.

thanks.