message: update documentation of packs
diff: -
  -// a pack file is:
  +// a pack file is a collection of blobs, prefixed by their 32 bits size in BE:
  +//
  +// SIZE (4 bytes BE)
  +// DATA (SIZE bytes)
  +// OPTIONAL ALIGNMENT? (of 0 to 3 bytes depending on SIZE)
  +//
  +// An index file is:
  //
  // MAGIC (8 Bytes)
  // BLOOM SIZE (4 bytes BE)
  // BLOOM FILTER (BLOOM_SIZE bytes)
  // BLOCK HASHES present in this pack ordered lexigraphically (#ENTRIES * 32 bytes)
  // OFFSET of BLOCK in the same order as BLOCK_HASHES (#ENTRIES * 8 bytes)
  +//
  +// The fanout is a cumulative numbers of things stored, ordered by their hash and
  +// group in 256 buckets (first byte of the hash). This give a very efficient
  +// way to "zoom" on the BLOCK HASHES, at it allows to windows only the hash that
  +// start with a specific byte. This improve efficiency when searching inside a pack.
  +//
  +// The bloom filter is an help to the overall pack search, it allows to
  +// efficiently query whether or not a hash is likely to be here or not. By the
  +// nature of a bloom filter, it can only answer with certainty whether it
  +// is present in this pack, there will be false positive in search.
  +//
  use super::{TmpFile};
  