Subject: Re: [PATCH 09/16] documentation: add documentation for the bitmap format

From: Thomas Rast <[email-masked]>
Date: Tue, 25 Jun 2013 08:58:54 -0700

Vicent Marti <[email-masked]> writes:

> This is the technical documentation and design rationale for the new
> Bitmap v2 on-disk format.

Hrmpf, that's what I get for reading the series in order...

> +			The folowing flags are supported:
                              ^^

typos marked by ^

> +		By storing all the hashes in a cache together with the bitmapsin
                                                                             ^^

> +		The obvious consequence is that the XOR of all 4 bitmaps will result
> +		in a full set (all bits sets), and the AND of all 4 bitmaps will
                                           ^

> +		- 1-byte XOR-offset
> +			The xor offset used to compress this bitmap. For an entry
> +			in position `x`, a XOR offset of `y` means that the actual
> +			bitmap representing for this commit is composed by XORing the
> +			bitmap for this entry with the bitmap in entry `x-y` (i.e.
> +			the bitmap `y` entries before this one).
> +
> +			Note that this compression can be recursive. In order to
> +			XOR this entry with a previous one, the previous entry needs
> +			to be decompressed first, and so on.
> +
> +			The hard-limit for this offset is 160 (an entry can only be
> +			xor'ed against one of the 160 entries preceding it). This
> +			number is always positivea, and hence entries are always xor'ed
                                                 ^

> +			with **previous** bitmaps, not bitmaps that will come afterwards
> +			in the index.

Clever.  Why 160 though?

> +		- 2 bytes of RESERVED data (used right now for better packing).

What do they mean?

> +  With an index at the end of the file, we can load only this index in memory,
> +  allowing for very efficient access to all the available bitmaps lazily (we
> +  have their offsets in the mmaped file).

Is there anything preventing you from mmap()ing the index also?

> +== Appendix A: Serialization format for an EWAH bitmap
> +
> +Ewah bitmaps are serialized in the protocol as the JAVAEWAH
> +library, making them backwards compatible with the JGit
> +implementation:
> +
> +	- 4-byte number of bits of the resulting UNCOMPRESSED bitmap
> +
> +	- 4-byte number of words of the COMPRESSED bitmap, when stored
> +
> +	- N x 8-byte words, as specified by the previous field
> +
> +		This is the actual content of the compressed bitmap.
> +
> +	- 4-byte position of the current RLW for the compressed
> +		bitmap
> +
> +Note that the byte order for this serialization is not defined by
> +default. The byte order for all the content in a serialized EWAH
> +bitmap can be known by the byte order flags in the header of the
> +bitmap index file.

Please document the RLW format here.

-- 
Thomas Rast
trast@{inf,student}.ethz.ch