Hashed pristine format
======================

(or my understanding thereof).

The file \_darcs/tentative\_pristine contains a pointer to the
"pristine root", in form of a hash reference, like this...
(Actually, there is a reference to pristine root in
\_darcs/hashed\_inventory as well. I currently do not understand
the difference, but there surely is some.)

(Sidenote on tentative\_pristine: this file seems to contain a
"not-yet live" root pointer into pristine.hashed.
`HashedRepo <HashedRepo>`_.revert\_tentative\_changes will take the
pointer from \_darcs/hashed\_inventory and write it over
\_darcs/tentative\_pristine. Likewise,
`HashedRepo <HashedRepo>`_.finalize\_tentative\_changes seems to
take what's in \_darcs/tentative\_hashed\_inventory and
\_darcs/tentative\_pristine and write it over
\_darcs/hashed\_inventory (atomically))

::

    04:43:50 | morn@eri:~/dev/darcs/mainline -> cat _darcs/tentative_pristine 
    pristine:0000001784-5853cb1b7025f159eed27dc8a2dd81f519171bc4aa85cc7ca01c44fded99315e

If you look the referenced file, you will see something like this:

::

    04:44:23 | morn@eri:~/dev/darcs/mainline -> zcat _darcs/pristine.hashed/0000001784-5853cb1b7025f159eed27dc8a2dd81f519171bc4aa85cc7ca01c44fded99315e | head
    file:
    hpc.README
    0000001139-9b29de9930f2e19ab1ef4042919a34c24dfc44bc544ef6fc9c1b20a2d0b6bf32
    directory:
    bugs
    0000000887-7b74bd1cf418f0c82f9af9f50ddfa0a9bb10a075de02403e05b058d5fb2074b0
    directory:
    release
    0000000484-24a4464e3e1f9a56e39247cafcd9e359575bd60cfcacad6ccc766341291ceb05

Basically, this is a directory listing, with filenames accompanied
by their pristine hashes. It is rather simple and somewhat git-ish
(if you know git, the above object looks like a "tree" git
object...). Peeking inside a "file" (git would say blob) object:

::

    04:45:50 | morn@eri:~/dev/darcs/mainline -> zcat _darcs/pristine.hashed/0000001139-9b29de9930f2e19ab1ef4042919a34c24dfc44bc544ef6fc9c1b20a2d0b6bf32 | head
    #!/bin/bash
    
    # To use hpc (Haskell Program Coverage) with darcs you need at least
    # ghc 6.8.3.
    
    # You can build darcs with hpc support using --enable-hpc
    [snip]

Cleanup
-------

When a new root pointer is written, some files under
\_darcs/pristine.hashed may become unreachable.
`HashedRepo <HashedRepo>`_.clean\_pristine takes care of that. It
calls HashedIO.listHashedContents which *seems* (not really
checked) to compute reachability on the hashed.pristine from a
given root and give a flat list of reachable files. (Okey, that's
probably fairly obvious.)