Re: Confusion about the PACK format

[Jeff King <peff@peff.net>]

On Sun, Feb 10, 2019 at 07:35:38PM +0000, Ramsay Jones wrote:

> > Now, the 'n-byte type and length' is a variable length encoding of
> > the object type and length. The number of bytes used to encode this
> > data is content dependant. If the top bit of a byte is set, then we
> > need to process the next byte, otherwise we are done. So, looking
> > at the first 'object entry' byte (at offset 12) '9d', we take the
> > top nibble, remove the top bit, and shift right 4 bits to get the
> > object type. ie. (0x9d >> 4) & 7 which gives an object type of 1
> > (which is a commit object). The lower nibble of the first byte
> > contains the first (or only) 4 bits of the size, here (0x9d & 15)
> > which is 0xd. Given that the top bit of this byte is set, we now
> > process the next byte. After the first byte, each byte contains 7
> > bits of the size field which is combined with the value from the
> > previous byte by shifting and adding (first by 4 bits, then 11, 18,
> > 25 etc.). So, in this case we have (0x13 << 4) + 0xd = 317.
> 
> Sorry, to be clear, I should have said, "mask off the top bit,
> shift and add", so:
> 
>   ((0x13 & 0x7f) << 4) + 0xd = 317

Yes. Also, see the first 10 or so lines of builtin/index-pack.c's
unpack_raw_entry() for real-world example code.

-Peff