The old hash table with chained buckets has been replaced by a new one
using Cuckoo hashing, which offers guaranteed constant lookup times.
This should improve speeds on most use cases, since hash tables in
libgit2 are usually used as caches where the objects are stored once and
queried several times.

The Cuckoo hash implementation is based off the one in the Basekit
library [1] for the IO language, but rewritten to support an arbritrary
number of hashes. We currently use 3 to maximize the usage of the nodes pool.

[1]: https://github.com/stevedekorte/basekit/blob/master/source/CHash.c

Signed-off-by: Vicent Marti <tanoku@gmail.com>

Signed-off-by: Vicent Marti <tanoku@gmail.com>

Signed-off-by: Vicent Marti <tanoku@gmail.com>

Disable sqlite support when ./waf configure is run with --without-sqlite

The new `git_filebuf` structure provides atomic high-performance writes
to disk by using a write cache, and optionally a double-buffered scheme
through a worker thread (not enabled yet).

Writes can be done 3-layered, like in git.git (user code -> write cache
-> disk), or 2-layered, by writing directly on the cache. This makes
index writing considerably faster.

The `git_filebuf` structure contains all the old functionality of
`git_filelock` for atomic file writes and reads. The `git_filelock`
structure has been removed.

Additionally, the `git_filebuf` API allows to automatically hash (SHA1)
all the data as it is written to disk (hashing is done smartly on big
chunks to improve performance).

Signed-off-by: Vicent Marti <tanoku@gmail.com>

Fixed several issues with path joining and bare repos.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

The `prev` and `next` pointers were not being updated after popping one
of the list elements.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

The interlocking on the write threads was not being done properly (index
entries were sometimes written out of order). With proper interlocking,
the threaded write is only marginally faster on big index files, and
slower on the smaller ones because of the overhead when creating
threads.

The threaded index writing has been temporarily disabled; after more
accurate benchmarks, if might be possible to enable it again only when
writing very large index files (> 1000 entries).

Signed-off-by: Vicent Marti <tanoku@gmail.com>

Signed-off-by: Vicent Marti <tanoku@gmail.com>

64-bit types stored in memory have to be truncated into 32 bits when
writing to disk. Was causing warnings in MSVC.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

In response to issue #60 (git_index_write really slow), the write_index
function has been rewritten to improve its performance -- it should now
be in par with the performance of git.git.

On top of that, if Posix Threads are available when compiling libgit2, a
new threaded writing system will be used (3 separate threads take care
of solving byte-endianness, hashing the contents of the index and
writing to disk, respectively). For very long Index files, this method
is up to 3x times faster than git.git.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

git_tag_lookup() and git_tag_new() changed to cast GIT_OBJ_TAG to
git_otype in order to compile lib in xcode

The priority value for different backends has been removed from the
public `git_odb_backend` struct. We handle that internally. The priority
value is specified on the `git_odb_add_alternate`.

This is convenient because it allows us to poll a backend twice with
different priorities without having to instantiate it twice.

We also differentiate between main backends and alternates; alternates have
lower priority and cannot be written to.

These changes come with some unit tests to make sure that the backend
sorting is consistent.

The libgit2 version has been bumped to 0.4.0.

This commit changes the external API:

CHANGED:
	struct git_odb_backend
		No longer has a `priority` attribute; priority for the backend
		in managed internally by the library.

	git_odb_add_backend(git_odb *odb, git_odb_backend *backend, int priority)
		Now takes an additional priority parameter, the priority that
		will be given to the backend.

ADDED:
	git_odb_add_alternate(git_odb *odb, git_odb_backend *backend, int priority)
		Add a backend as an alternate. Alternate backends have always
		lower priority than main backends, and writing is disabled on
		them.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

Signed-off-by: Vicent Marti <tanoku@gmail.com>

The alternates file is now parsed, and the alternate ODB folders are
added as separate backends. This allows the library to efficiently query
the alternate folders.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

The `git__joinpath` function has been changed to use a statically
allocated buffer; we assume the buffer to be 4096 bytes, because fuck
you.

The new method also supports an arbritrary number of paths to join,
which may come in handy in the future.

Some methods which were manually joining paths with `strcpy` now use the
new function, namely those in `index.c` and `refs.c`.

Based on Emeric Fermas' original patch, which was using the old
`git__joinpath` because I'm stupid. Thanks!

Signed-off-by: Vicent Marti <tanoku@gmail.com>

Unfortunately previous commit was only a partial fix, because it broke
SQLite support on platforms w/o pkg-config, e.g. Windows. To be honest
I just forgot about messy Windows.

Now if there is no pkg-config, then user must provide two variables:
SQLITE3_INCLUDE_DIRS and SQLITE3_LIBRARIES if (s)he wants to use SQLite
backend. These variables are added to cmake-gui for her/his convenience
unless they are set by FindPkgConfig module.

pkg-config should work also now in Cygwin.

FindPkgConfig obviously uses pkg-config's output for setting convenient
variables such as <PREFIX>_LIBRARIES or <PREFIX>_INCLUDE_DIRS. It also
sets <PREFIX>_FOUND to 1 if <PREFIX> module exists.

So why checking for SQLITE3_FOUND is better than (SQLITE3_LIBRARIES AND
SQLITE3_INCLUDE_DIRS)? Apart from obvious readability factor, latter
condition has strong assumption that both variables are filled with
appropriate paths, which is unjustifiable unless you add another
assumptions...

pkg-config by default strips -I/usr/include from Cflags and -L/usr/lib
from Libs if some environment variables are not set,
PKG_CONFIG_ALLOW_SYSTEM_CFLAGS and PKG_CONFIG_ALLOW_SYSTEM_LIBS
respectively. This behavior is sane, because it prevents polluting the
compilation and linking commands with superfluous entries.

In debian SQLITE3_INCLUDE_DIRS is empty for instance.

Remark for developers:
Always check commands invoked by CMake after changing CMakeLists.txt.

    VERBOSE=1 cmake --build .

Added tests exercising git_reference_write() to create a new symbolic reference and a new object id reference.

Removed `git_tree_add_entry_unsorted`. Now the `git_tree_add_entry`
method doesn't sort the entries array by default; entries are only
sorted lazily when required. This is done automatically by the library
(the `git_tree_sort_entries` call has been removed).

This should improve performance. No point on sorting entries all the time, anyway.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

Signed-off-by: Vicent Marti <tanoku@gmail.com>

We now have proper sonames in Mac OS X and Linux, proper versioning on
the pkg-config file and proper DLL naming in Windows.

The version of the library is defined exclusively in 'src/git2.h'; the build scripts
read it from there automatically.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

Conflicts:
	CMakeLists.txt

Use pkg-config to find the library in Unix systems. In Win32, just set
manually the path to your libraries.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

The git_odb_backend_* symbols were being redefined as external.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

It can now be run twice in a row without failing.

Configure again the build system to look for SQLite3. If the library is
found, the SQLite backend will be automatically compiled.

Enjoy *very* fast reads and writes.

MASTER PROTIP: Initialize the backend with ":memory" as the path to the
SQLite database for fully-hosted in-memory repositories. Rejoice.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

Signed-off-by: Vicent Marti <tanoku@gmail.com>

 - Added a test to ensure that a nested symbolic reference is properly resolved.
 - Added comparisons of object ids.

Signed-off-by: Vicent Marti <tanoku@gmail.com>