Re: offset zero stored in catalog

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Wesley Leggette wrote:
| For the offsets stored in the catalog, where is the zero point?
|
| +---------+---------+--------------------------------+---------+------+
| | slice   | archive | data                           | catalog | term |
| | header  | header  |                                |         |      |
| +---------+---------+--------------------------------+---------+------+
|           ^         ^
|           A         B
|
| Is it at point A or B?

point A, but that's not exactly this way it works...



~              +----+--+----+-...........+---------+
archive       |file|EA|file|            |catalogue|
layout        |data|  |data|            |         |
~              +----+--+----+-...........+---------+
~                   |           |              |
~                   |           |              |
~                   V           V              V
~              +-----------------------------------+
compression   |         (compressed)  data        |
~              +-----------------------------------+
~                    |                      |
~                    |                      |          / Terminateur
~                    |                      |          |
~                    |                      |          V
elastic  +---+      |                      |       +----+---+
buffers  |EEE|      |                      |       |TTTT|EEE|
~         +---+      |                      |       +----+---+
~           |        |                      |              |
~           V        V                      V              V
~         +--------------------------------------------------+
cipher   |        (encrypted) data                          |
~         +--------------------------------------------------+
header\               |                         |
version|              |                         |
~       |              |                         |
~       |              |                         |
~       V              V                         V
~     +------------------------------------------------------+
sar  |VVV|                  data                            |
~     +------------------------------------------------------+
~        |         |  |         |   |        |   |        |  |
slice   |         |  |         |   |        |   |        |  |
headers |         |  |         |   |        |   |        |  |
~ |  |   |         |  |         |   |        |   |        |  |
~ |  +---|------\  |  |         |   |        |   |        |  |
~ V      V      V  V  V         V   V        V   V        V  V
+---------+  +---------+  +---------+  +---------+  +-------+
|HH| data |  |HH| data |  |HH| data |  |HH| data |  |HH|data|
+---------+  +---------+  +---------+  +---------+  +-------+
~  slice 1      slice 2      slice 3      slice 4      slice 5


is used either "sar" or "tuyau" (for writing/reading from/to pipes)  or
"fichier" (for single slice archive) or "null_file" (when doing dry-run
execution, in that later case there is no compression layer nor there is
encryption layer even if specified on command-line).

The offset in catalogue defines the position of the first bytes
of a file's data as given by the underlying layer (the compression
object, if present, for example).

Compression offset returns the position of the underlying layer in any
case (if you write 10 bytes than get compressed to 5 bytes, the new
offset reported by compressor will be 5 bytes further, while you have
wrote 10 to it!), so position zero is position zero of the underlying
layer (which is cypher, if present).

The cypher layer alters the offset when the encrypted data takes more
place than the clear data, the offset that reported by the cypher object
is always the number of clear bytes written to it, even if more are
written to the underlying layer due to encryption, but the offset of the
first byte written matches the offset of the underlying layer (sar), so
the first byte written in a cypher object (scrambler or blowfish for
example) is not usually reported at position 0, because the
header_version structure has already been written to the underlying
layer (sar object), before the cypher object has been attached over it.

The sar object reports as offset the number of bytes written (or the
number of bytes read). The first byte is at position zero. The sar
object emulates a giant file splitting the written data to several
'fichier' objects, or when reading, gather the data from several
'fichier' object.

Note that all classes like scrambler, compressor, blowfish, fichier,
sar, tuyau, null_file and some other inherit from a same pure virtual
class called "generic_file", that provides standard method for reading,
writing, skipping, or getting the offset in a "file". This way, if for
example encryption is not used, the compression object writes directly
to the sar object, which is also a "generic_file" object, same remark if
compression is not used, of if archive splitting is not used. In the
most simple case (no encryption, so slicing, no compression) the archive
layout goes directly to a "fichier" which is in fact a plain file
wrapped in a object having the "generic_file" interface.


| Thanks.
|

Cheers,
Denis.
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