Re: infinint, libdar

I thought I had replied to this mail, but cannot find my answer in the 
mailing-list archive, so if I didn't replied sorry to reply so late !

The infinint when saved in file (in archive) are a variable byte field 
representing the integer in network byte order. To determine how much 
byte it has of length, this field is preceded by a header of variable 
length, which is called "preamble" in the comments of the code (see 
real_infinint.cpp and the infinint::dump() method for details).

This preamble contains only one bit set to one. This bit is located in 
the last byte of the preamble. So while we read 0x00 the preamble is not 
finished, when we reach a non null byte, this is the end of the header, 
and this byte must have only one bit set to one. If we count the number 
of bit  (not byte) that have been read and which are set to zero before 
this bit set to 1, we get the size of the infinint field that follows 
the preamble in TG unit. A TG is actually fixed to 4 byte.

I don't remember well where from I took "TG" for, G for Group, and T 
maybe for Tributary ? TG = Tributary Group ? OK It was quite three years 
ago (July 2001), and I had not to fix any bug in this part since that 
time ! By the way, there is room to have the preamble of infinint 
changed its structure, by giving a first byte containing two or more 
bits set to one... actual code detects this condition and consider this 
is a format it does not know, or a data corruption.

Let's take an example, in memory we have an infinint of 5 bytes (0x 
FA.79.00.12.34, in network order), so we need two TG, to store it, (8 
bytes), the preamble is then 0x40 (it would have been 0x80 for 1 TG, or 
  0x00.80 for 9 TG (for a 36 bytes integer!) for example) We must add 
tree bytes of padding to have the integer field be exactly 2 TG (=8 
bytes) and not 5 bytes only, as it is in memory. So the resulting string 
written in file is:

0x40.00.00.00.FA.79.00.12.34
----+--------+--------------+
Pre.| Padding| integer field|
----+-----------+-----------+
Pre.|    1 TG   |   1 TG    |

the dot '.' in this notation separate each byte.

Wesley Leggette wrote:
> In libdar, it's my impression that infinint's are dumped like this:
> 
> 0x00 + bit pattern - header indicating how long the infinint is
> byte fields - actual data in network byte order
> 
> I'm having a hard time trying to figure out how exactly the header is
> formed.
> 
> As a school project, I'm working on implementing a dar library in java,
> hence the above question.
> 
> Also, I'd like to be able to use individual parts of libdar (to compare
> libdar output with what I write in java). Is it best to set up a
> generic_file and then call various dump() functions, or is there an
> easier way?
> 
> 
> 
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