Re: Piggypack Encoding/Decoding on RandomAccessFile

Eric Sosman <esosman@ieee-dot-org.invalid>
Thu, 03 Nov 2011 20:40:08 -0400
On 11/3/2011 3:50 PM, Jan Burse wrote:

Joshua Cranmer schrieb:

The "standard way" (at least, all of the use cases I've ever had for
RandomAccessFile) effectively uses the methods that are associated with to read data: read(byte[]), and read*().

I would like to use an arbirary encoding/decoding on top of the
byte stream to get a character stream. But since RandomAccessFile
does not implement InputStream/OutputStream, I cannot create
a InputStreamReader/OutputStreamWrite on top.

     For a completely "arbitrary" encoding, I think you're out of luck.
Stateful encodings (where the encoding of byte B[n] is a function of
B[n-1],B[n-2],...) make it difficult to begin in medias res: You cannot
know how to decode the first byte you read without already having seen
all its predecessors.

     To support random access, where you'd like to jump directly to B[n]
without plowing through all that goes before, one usually addresses the
problem by restricting the valid n to multiples of some "block size,"
and encoding each "block" independently. You seek to the next lower
multiple of 32K or whatever, set your decryptor/compressor/decoder to
its initial state, and roll merrily along.

     There's a problem if the encoding does not always map K input bytes
to f(K) output bytes: compressors, for example, output different amounts
of data depending on the values of the bytes compressed. There are two
principal methods for dealing with this difficulty:

     1) Encode the original in blocks of 32K (say), and store each
encoded block in a file region that's sure to be large enough -- 40K,
perhaps. Pad with nulls or other junk values as needed, so long as
your decompressor can recognize and ignore the padding. Then original
byte N is in block number N/32K, whose encoding starts at (N/32K)*40K
in the file; seek to that spot and start decoding.

     2) As before, encode the original in fixed-size blocks, but write
them cheek by jowl to the file. As you do so, also write an index file
that's essentially Map<OriginalByteNumber,EncodedByteNumber> for each
block boundary. Then original byte N is in the block beginning at
theMap.get(N/32K); seek to that spot and start decoding.

     Elsethread you mention that RandomAccessFile provides neither
InputStream nor OutputStream. If you think about this a bit, you'll
see it's a natural consequence of the "Random" part: a Stream provides
the abstraction of a linear sequence of things, and does not admit of
leaping forward or backward to unrelated positions. Yes, there are
skip() and mark() and reset(), but I think you'll agree these are of
a different character than "read bytes 3000-3999, then 10000-10999,
then 936-22728." Streams are sequential; Random isn't.

Eric Sosman

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