Android: how to have random access from an InputStream?

Question

I have an InputStream, and the relative file name and size.

I need to access/read some random (increasing) positions in the InputStream. This positions are stored in an integer array (named offsets).

InputStream inputStream = ...

String fileName = ...
int fileSize = (int) ...

int[] offsets = new int[]{...};  // the random (increasing) offsets array

Now, given an InputStream, I've found only two possible solutions to jump to random (increasing) positions of the file.

The first one is to use the skip() method of the InputStream (note that I actually use BufferedInputStream, since I will need to mark() and reset() the file pointer).

//Open a BufferInputStream:
BufferedInputStream bufferedInputStream = new BufferedInputStream(inputStream);

byte[] bytes = new byte[1];

int curFilePointer = 0;
long numBytesSkipped = 0;
long numBytesToSkip = 0;
int numBytesRead = 0;

//Check the file size:
if ( fileSize < offsets[offsets.length-1] ) {  // the last (bigger) offset is bigger then the file size... 
    //Debug:
    Log.d(TAG, "The file is too small!\n");

    return;
}

for (int i=0, k=0; i < offsets.length; i++, k=0) {  // for each offset I have to jump...
    try {
        //Jump to the offset [i]:
        while( (curFilePointer < offsets[i]) && (k < 10) ) {  // until the correct offset is reached (at most 10 tries)
            numBytesToSkip = offsets[i] - curFilePointer;
            numBytesSkipped = bufferedInputStream.skip(numBytesToSkip);

            curFilePointer += numBytesSkipped;  // move the file pointer forward

            //Debug:
            Log.d(TAG, "FP: " + curFilePointer + "\n");

            k++;
        }

        if ( curFilePointer != offsets[i] ) {  // it did NOT jump properly... (what's going on?!)
            //Debug:
            Log.d(TAG, "InputStream.skip() DID NOT JUMP PROPERLY!!!\n");

            break;
        }

        //Read the content of the file at the offset [i]:
        numBytesRead = bufferedInputStream.read(bytes, 0, bytes.length);
        curFilePointer += numBytesRead;  // move the file pointer forward

        //Debug:
        Log.d(TAG, "READ [" + curFilePointer + "]: " + bytes[0] + "\n");
    }
    catch ( IOException e ) {
        e.printStackTrace();

        break;
    }
    catch ( IndexOutOfBoundsException e ) {
        e.printStackTrace();

        break;
    }
}

//Close the BufferInputStream:
bufferedInputStream.close() 

The problem is that, during my tests, for some (usually big) offsets, it has cycled 5 or more times before skipping the correct number of bytes. Is it normal? And, above all, can/should I thrust skip()? (That is: Are 10 cycles enough to be SURE it will ALWAYS arrive to the correct offset?)

The only alternative way I've found is the one of creating a RandomAccessFile from the InputStream, through File.createTempFile(prefix, suffix, directory) and the following function.

public static RandomAccessFile toRandomAccessFile(InputStream inputStream, File tempFile, int fileSize) throws IOException {
    RandomAccessFile randomAccessFile = new RandomAccessFile(tempFile, "rw");

    byte[] buffer = new byte[fileSize];
    int numBytesRead = 0;

    while ( (numBytesRead = inputStream.read(buffer)) != -1 ) {
        randomAccessFile.write(buffer, 0, numBytesRead);
    }

    randomAccessFile.seek(0);

    return randomAccessFile;
}

Having a RandomAccessFile is actually a much better solution, but the performance are exponentially worse (above all because I will have more than a single file).

---

EDIT: Using byte[] buffer = new byte[fileSize] speeds up (and a lot) the RandomAccessFile creation!

---

//Create a temporary RandomAccessFile:
File tempFile = File.createTempFile(fileName, null, context.getCacheDir());
RandomAccessFile randomAccessFile = toRandomAccessFile(inputStream, tempFile, fileSize);

byte[] bytes = new byte[1];
int numBytesRead = 0;

//Check the file size:
if ( fileSize < offsets[offsets.length-1] ) {  // the last (bigger) offset is bigger then the file size...
    //Debug:
    Log.d(TAG, "The file is too small!\n");

    return;
}

for (int i=0, k=0; i < offsets.length; i++, k=0) {  // for each offset I have to jump...
    try {
        //Jump to the offset [i]:
        randomAccessFile.seek(offsets[i]);

        //Read the content of the file at the offset [i]:
        numBytesRead = randomAccessFile.read(bytes, 0, bytes.length);

        //Debug:
        Log.d(TAG, "READ [" + (randomAccessFile.getFilePointer()-4) + "]: " + bytes[0] + "\n");
    }
    catch ( IOException e ) {
        e.printStackTrace();

        break;
    }
    catch ( IndexOutOfBoundsException e ) {
        e.printStackTrace();

        break;
    }
}

//Delete the temporary RandomAccessFile:
randomAccessFile.close();
tempFile.delete();

Now, is there a better (or more elegant) solution to have a "random" access from an InputStream?

Answer 1

It's a bit unfortunate you have an InputStream to begin with, but in this situation buffering the stream in a file is of no use iff you are always skipping forward. But you don't have to count the number of times you have called skip, that's not really of interest.

What you do have to check if the stream has ended already, to prevent an infinite loop. Checking the source of the default skip implementation, I'd say you'll have to keep calling skip until it returns 0. This will indicate the end of stream has been reached. The JavaDoc was a bit unclear about this for my taste.

Answer 2

You can't. An InputStream is a stream, that is to say a sequential construct. Your question embodies a contradiction in terms.